An I2C Matrix Keypad

The completed I2C Matrix Keypad

In a previous post this month I introduced my 4×4 matrix keypad. That keypad was designed to be directly interfaced to a microcontroller’s GPIO pins or alternatively to an IO expander chip like the PCF8574. That design, while working very well had the problem of requiring 8 GPIO pins to function correctly.

GPIO pins on a microcontroller can be considered very precious resources, and it should then be logical to assume that we should find a way to use these GPIO pins in a more conservative way, to allow us to interface more peripherals.

I solved this problem by integrating the keypad with an IO Expander on the same PCB. That will allow us to get away with using only 2 GPIO pins, and also open up the option of adding more keypads to the I2C bus, in the event that we need that many keys for a particular project.

The Schematic

I2C 4×4 Matrix Keypad Schematic

Looking closely at the schematic, we can see that it is exactly the same basic keypad circuit that I used in the initial design. The only difference is that in this design, I have integrated a PCF8574 directly onto the PCB.

Some additional features include selectable I2C Pullup resistors ( usually my microcontroller development boards already include those) that can be activated with a jumper when needed. There are also a set of address selection jumpers, making it possible to stack keypads together into a bigger keyboard if you require something like that. Note that, in this version of the hardware, I did not include headers for stacking.

The keypad can be powered by a DC power source of 3.3v to 5v.


I2C Keypad PCB
3D Render of the I2C Keypad

The PCB is a double-layer board of 68.8mm x 50.8mm. Male header pins provide access to the connections as well as address and pullup resistor jumpers. In my build, I have mounted these male headers on the back of the PCB. That makes it possible to mount the Keypad in an enclosure without having the jumpers “stick out” and get in the way.

The top layer of the I2C Keypad PCB
Bottom Layer


I choose PCBWay for my PCB manufacturing.
This month, PCBWay is also celebrating its 9th anniversary, and that means that there are quite a lot of very special offers available.

What makes them different from the rest?

PCBWay‘s business goal is to be the most professional PCB manufacturer for prototyping and low-volume production work in the world. With more than a decade in the business, they are committed to meeting the needs of their customers from different industries in terms of quality, delivery, cost-effectiveness and any other demanding requests. As one of the most experienced PCB manufacturers and SMT Assemblers in China, they pride themselves to be our (the Makers) best business partners, as well as good friends in every aspect of our PCB manufacturing needs. They strive to make our R&D work easy and hassle-free.

How do they do that?

PCBWay is NOT a broker. That means that they do all manufacturing and assembly themselves, cutting out all the middlemen, and saving us money.

PCBWay’s online quoting system gives a very detailed and accurate picture of all costs upfront, including components and assembly costs. This saves a lot of time and hassle.

PCBWay gives you one-on-one customer support, that answers you in 5 minutes ( from the Website chat ), or by email within a few hours ( from your personal account manager). Issues are really resolved very quickly, not that there are many anyway, but, as we are all human, it is nice to know that when a gremlin rears its head, you have someone to talk to that will do his/her best to resolve your issue as soon as possible.

Find out more here


The assembly of this PCB was quite easy and quick. A stencil is not required. All SMD components are 0805 or bigger. It would thus be quite easy to solder them all by hand with a fine-tipped soldering iron.

I have however used soldering paste and hot air to reflow the components, as it is the fastest, in my opinion, and definitely looks neater than hand soldering.

After placing SMD components onto solder paste – ready for reflow soldering
After Reflow soldering with Hot Air

The board is now ready to solder the switches and header pins in place. As already mentioned above, I chose to assemble the headers on the back of the PCB to prevent them from interfering with any enclosure that I may later use with the keypad.

Final Assembly
Note that I assembled the headers onto the back of the PCB.

Testing and Coding

Testing the keypad consisted of a few steps, the first of which was ensuring that there were no short circuits, as well as that all the momentary switches worked.
This was done with a multimeter in continuity as well as diode mode, with probes alternatively on each column and row in turn, while pressing the buttons.

The next stage was testing the I2C IO Expander. This was done with a simple I2C Scanning sketch on an Arduino Uno. It did not do a lot, but, I could see that the PCF8574 is responding to its address and that the pullup resistors work when enabled. This test was repeated with my own ESP8266 and ESP32 boards, this time with pullup resistors disabled, as these boards already have them onboard.

Coding came next, and it was another case of perspectives. It seems like all commercial keypads do not have diodes. This affects the way that they work with a given library. It seems that software developers and hardware developers have different understandings of what a row and a column is.

This meant that, due to the fact that I have diodes on each switch, and the way that the library work – which pins are pulled high and which are set as inputs -, I had to swap around my rows and columns in the software to get everything to work. On a keypad with the diodes replaced with 0-ohm links, that was not needed.

A short test sketch follows below:

Note that with was run on an ESP8266-12E, therefore the Wire.begin() function was changed to Wire.begin(4,5); in order to use GPIO 4 and GPIO 5 for I2C

Another point to note is that the keypad Layout will seem strange. Remember that this is due to the diodes in series on each switch. That forces us to swap around the Rows and the Columns in the software, resulting in a mirrored and rotated left representation of the keypad. It looks funny, but believe me, it actually still works perfectly.

#include <Wire.h>
#include "Keypad.h"
#include <Keypad_I2C.h>

const byte n_rows = 4;
const byte n_cols = 4;

char keys[n_rows][n_cols] = {
    {'1', '4', '7', '*'},
    {'2', '5', '8', '0'},
    {'3', '6', '9', '#'},
    {'A', 'B', 'C', 'D'}};

byte rowPins[n_rows] = {4, 5, 6, 7};
byte colPins[n_cols] = {0, 1, 2, 3};

Keypad_I2C myKeypad = Keypad_I2C(makeKeymap(keys), rowPins, colPins, n_rows, n_cols, 0x20);

String swOnState(KeyState kpadState)
    switch (kpadState)
    case IDLE:
        return "IDLE";
    case PRESSED:
        return "PRESSED";
    case HOLD:
        return "HOLD";
    case RELEASED:
        return "RELEASED";
    } // end switch-case
    return "";
} // end switch on state function

void setup()
    // This will be called by App.setup()
    while (!Serial)
    { /*wait*/
    Serial.println("Press any key...");

char myKeyp = NO_KEY;
KeyState myKSp = IDLE;
auto myHold = false;

void loop()

    char myKey = myKeypad.getKey();
    KeyState myKS = myKeypad.getState();

    if (myKSp != myKS && myKS != IDLE)
        Serial.print("myKS: ");
        myKSp = myKS;
        if (myKey != NULL)
            myKeyp = myKey;
        String r;
        r = myKeyp;
        Serial.println("myKey: " + String(r));
        if (myKS == HOLD)
            myHold = true;
        if (myKS == RELEASED)
            if (myHold)
                r = r + "+";
            myHold = false;
        myKey == NULL;
        myKS = IDLE;


This project once again delivered what I set out to achieve. It has some quirks, but nothing serious. Everything works as expected, both in the Arduino IDE/platform IO realm, as well as in ESPHome. It is worth noting that in ESPHome, we do not need to swap the rows and columns to use the Keypad component. Do remember to leave the has_diodes flag to false though…

Remaking the STM32 Blue-Pill – Part 1

Why did I do this?
Of all the various development boards in my electronics lab, surely the most versatile, but most neglected must be the STM32 Blue Pill. Not that there is anything wrong with it, It is a very very nice board, but unfortunately, there are a few issues ( mostly personal ) that make it not jump out at me for use in a new project.

Of these, and quite similar to the Arduino Nano, and original Raspberry Pi Pico is the fact that it was designed to be mostly used on a breadboard, issues with using the USB port, wrong components installed at the factories, and needing magnification to read port numbers, made me decide to solve it in my own way.

It is also not possible to use it (the original Blue-Pill) with any of the ready-made shields for the Arduino Nano, due to different pin-outs etc. Thus requiring a mess of wires and more modules on a breadboard, or lying free on the bench, to add sensors and other devices to this particular development board…

It can be argued that its small size does make it attractive, with which I definitely do not disagree, but unlike the bigger STM32 development boards with a lot of included peripherals, or the Arduino Ecosystem, where it is possible to develop your hardware prototypes on an Arduino Uno, using various shields, and then move on to a custom PCB later, the blue pill’s development cycle seems to be very much tied to the use of a breadboard and/or modules connected with jumper wires etc…

My idea was thus to take some inspiration from the bigger STM32 Nucleo and Discovery type platforms, add very detailed, and readable pin numbers, and peripheral descriptions, as well as lay a foundation onto which I can in future design stackable shields to expand the functionality of the device.

In order to make it easier for Makers and Hobby Electronics Enthusiasts, the new PCB was designed with 0805 SMD components. These are still quite easy to solder by hand, and usually do not require the use of any special equipment (That most beginners just starting out with the hobby won’t necessarily own or have access to by default). All of this was done with the hope that more people will start using this form factor, as well as to stimulate the development of shields and add-on cards to truly bring the blue pill to the same level regarding ease-of-use that the Arduino Ecosystem currently occupies.

What changes did I make?

Blue Dev Board Schematic Diagram – MakerIot2020

The schematic diagram was not changed at all. All ports and pin numbers are 100% compatible with the original Blue Pill. Components sizes were changed from 0402 to 0805. I also made sure that the USB port would be functional, and would not require any modifications to be performed to make it useable (unlike some of the blue pill PCB’s available in Asia, which ships with a wrong value resistor at R10.

The PCB form factor has been changed to be the same size as the Arduino Uno, but with two rows of 20-pin 2.54mm headers. A more powerful 3.3v LDO regulator and additional smoothing capacitors were added. The main changes are to the silkscreen, where the pin numbering was enlarged to be more easily readable, and detailed labelling of the various possible peripherals were added. The board is also a single layer PCB, whereas the original was a double layer PCB, due to its small size.

Where and when can you get one?

The PCB is currently in production at PCBWay. When I have completed all testing of the PCB, it will be available as a PCBWay shared project, which means that you can order your own, or freely download the Manufacturing files to produce your own if you so choose. You can also contact me directly.

I plan to have completed all testing and to make available the manufacturing files, by the end of February 2022. Please be patient.

This PCB was manufactured at PCBWAY. The Gerber files and BOM, as well as all the schematics, will soon be available as a shared project on their website. If you would like to have PCBWAY manufacture one of your own, designs, or even this particular PCB, you need to do the following…
1) Click on this link
2) Create an account if you have not already got one of your own.
If you use the link above, you will also instantly receive a $5USD coupon, which you can use on your first or any other order later. (Disclaimer: I will earn a small referral fee from PCBWay. This referral fee will not affect the cost of your order, nor will you pay any part thereof.)
3) Once you have gone to their website, and created an account, or login with your existing account,

4) Click on PCB Instant Quote

5) If you do not have any very special requirements for your PCB, click on Quick-order PCB

6) Click on Add Gerber File, and select your Gerber file(s) from your computer. Most of your PCB details will now be automatically selected, leaving you to only select the solder mask and silk-screen colour, as well as to remove the order number or not. You can of course fine-tune everything exactly as you want as well.

7) You can also select whether you want an SMD stencil, or have the board assembled after manufacturing. Please note that the assembly service, as well as the cost of your components, ARE NOT included in the initial quoted price. ( The quote will update depending on what options you select ).

8) When you are happy with the options that you have selected, you can click on the Save to Cart Button. From here on, you can go to the top of the screen, click on Cart, make any payment(s) or use any coupons that you have in your account.

Then just sit back and wait for your new PCB to be delivered to your door via the shipping company that you have selected during checkout.

Free PCB Files available on Request

Everyone likes some free stuff once in a while. I have decided to release 3 recent PCB projects for free, no strings attached. The PCB’s include a STM32F103C8T6 ( BluePill ) in Arduino Uno Form Factor, as well as an additional two I2C IO Extender Shields, 8 DI and 8 DO, Both optically isolated, in a stack-able, addressable format.

Both I2C Shields are configured to use PB11 as SDA, PB10 as SCL and PB1 for interrupt.
All other “Blue Pill” Pins are broken out on Headers, completely pin for pin compatible.

Blue Base Board, STM32 “Blue Pill” Clone in Uno Form Factor


8 Digital Input, Stack-able I2C Shield, Opto Isolated inputs.
8 Digital Output, Stack-able I2C Shield, Opto Isolated.

Using the MCP23017 to increase your GPIO’s

Today I will show you another useful IO Expander chip, The MCP23017. This chip, although similar to the PCF8475, which I have already covered in a previous article, has many additional features that may make it a very attractive solution when you need some more extra GPIO pins for a big project…


Let us look at some of the features of this chip

  • 16-Bit Remote Bidirectional I/O Port:
  • I/O pins default to input
    • High-Speed I2C Interface (MCP23017):
  • 100 kHz
  • 400 kHz
  • 1.7 MHz
    • High-Speed SPI Interface (MCP23S17):
  • 10 MHz (maximum)
    • Three Hardware Address Pins to Allow Up to
    Eight Devices On the Bus
    • Configurable Interrupt Output Pins:
  • Configurable as active-high, active-low or
    • INTA and INTB Can Be Configured to Operate
    Independently or Together
    • Configurable Interrupt Source:
  • Interrupt-on-change from configured register
    defaults or pin changes
    • Polarity Inversion Register to Configure the
    Polarity of the Input Port Data
    • External Reset Input
    • Low Standby Current: 1 µA (max.)
    • Operating Voltage:
  • 1.8V to 5.5V @ -40°C to +85°C
  • 2.7V to 5.5V @ -40°C to +85°C
  • 4.5V to 5.5V @ -40°C to +125°C
MCP23017 Pinout Diagram

The sixteen I/O ports are separated into two ‘ports’ – A (on the right) and B (on the left. Pin 9 connects to 5V, 10 to GND, 11 isn’t used, 12 is the I2C bus clock line (Arduino Uno/Duemilanove analogue pin 5, Mega pin  21), and 13 is the I2C bus data line (Arduino Uno/Duemailnove analogue pin 4, Mega pin 20).

External pull-up resistors should be used on the I2C bus – in our examples we use 4.7k ohm values. Pin 14 is unused, and we won’t be looking at interrupts, so ignore pins 19 and 20. Pin 18 is the reset pin, which is normally high – therefore you ground it to reset the IC. So connect it to 5V!

Finally we have the three hardware address pins 15~17. These are used to determine the I2C bus address for the chip. If you connect them all to GND, the address is 0x20. If you have other devices with that address or need to use multiple MCP23017s, see figure 1-2 in the datasheet.

You can alter the address by connecting a combination of pins 15~17 to 5V (1) or GND (0). For example, if you connect 15~17 all to 5V, the control byte becomes 0100111 in binary, or 0x27 in hexadecimal.

It is also available on a convenient breakout PCB, for about $USD0.80 from AliExpress

MCP23017 on Breakout PCB – Back
MCP23017 on Breakout PCB – Front


As you can see, the pins are however very clearly labelled, and thus easy to use. I have also purposely soldered my header pins “the wrong way round” to prevent using it on a breadboard, as this will short out Vcc to Ground!

Having interrupt outputs is one of the most important features of the MCP23017, since the microcontroller does not have to continuously poll the device to detect an input change. Instead an interrupt service routine can be used to react quickly to an input change such a key press…

To make life even easier each GPIO input pin can be configured with an internal pullup (~100k) and that means you won’t have to wire up external pull up resistors for keyboard input. You can also mix and match inputs and outputs the same as any standard microcontroller 8 bit port.


The 23017 has three input pins to allow you to set a different address for each attached MCP23017.

The above corresponds to a hardware address for the three lines A0, A1, A2 corresponding to the input pin values at the IC. You must set the value of these hardware inputs as 0V or (high) volts and not leave them floating otherwise they will get random values from electrical noise and the chip will do nothing!

The four left most bits are fixed a 0100 (specified by a consortium who doles out address ranges to manufacturers).

So the MCP23017 I2C address range is 32 decimal to 37 decimal or 0x20 to 0x27 for the MCP23017.

Please note: The addresses are the same as those for the PCF8475. You must thus be careful if you use these two devices on the same i2c bus!

MCP23017 Non interrupt registers

IODIR I/O direction register

For controlling I/O direction of each pin, register IODIR (A/B) lets you set the pin to an output when a zero is written and to an input when a ‘1’ is written to the register bit. This is the same scheme for most microcontrollers – the key is to remember that zero (‘0’) equates to the ‘O’ in Output.

GPPU Pullup register

Setting a bit high sets the pullup active for the corresponding I/O pin.

OLAT Output Latch register

This is exactly the same as the I/O port in 18F series PIC chips where you can read back the “desired” output of a port pin whether or not the actual state of that pin is reached. i.e. consider a strong current LED attached to the pin – it is easily possible to pull down the output voltage at the pin to below the logic threshold i.e. you would read back a zero if reading from the pin itself when in fact it should be a one. Reading the OLAT register bit returns a ‘one’ as you would expect from a software engineering point of view.

IPOL pin inversion register

The IPOL(A/B) register allows you to selectively invert any input pin. This reduces the glue logic needed to interface other devices to the MCP23017 since you won’t need to add inverter logic chips to get the correct signal polarity into the MCP23017.

It is also very handy for getting the signals the right way up e.g. it is common to use a pull up resistor for an input so when a user presses an input key the voltage input is zero, so in software you have to remember to test for zero.

Using the MCP23017 you could invert that input and test for a 1 (in my mind a key press is more equivalent to an on state i.e. a ‘1’) however I use pullups all the time (and uCs in general use internal pullups when enabled) so have to put up with a zero as ‘pressed’. Using this device would allow you to correct this easily.Note: The reason that active low signals are used everywhere is a historical one: TTL (Transistor Transistor Logic) devices draw more power in the active low state due to the internal circuitry, and it was important to reduce unnecessary power consumption – therefore signals that are inactive most of the time e.g. a chip select signal – were defined to be high. With CMOS devices either state causes the same power usage so it now does not matter – however active low is used because everyone uses it now and used it in the past.

SEQOP polling mode : register bit : (Within IOCON register)

If you have a design that has critical interrupt code e.g. for performing a timing critical measurement you may not want non critical inputs to generate an interrupt i.e. you reserve the interrupt for the most important input data.

In this case, it may make more sense to allow polling of some of the device inputs. To facilitate this “Byte mode” is provided. In this mode, you can read the same set of GPIOs using clocks but not needling to provide other control information. i.e. it stays on the same set of GPIO bits, and you can continuously read it without the register-address updating itself. In non-byte mode, you either have to set the address you read from (A or B bank) as control input data.

Now to examine how to use the IC in our sketches.

As you should know by now most I2C devices have several registers that can be addressed. Each address holds one byte of data that determines various options. So before using we need to set whether each port is an input or an output. First, we’ll examine setting them as outputs. So to set port A to outputs, we use:

Wire.write(0x00); // IODIRA register
Wire.write(0x00); // set all of port A to outputs

Then to set port B to outputs, we use:

Wire.write(0x01); // IODIRB register
Wire.write(0x00); // set all of port B to outputs

So now we are in void loop()  or a function of your own creation and want to control some output pins. To control port A, we use:

Wire.write(0x12); // address port A
Wire.write(??);  // value to send

To control port B, we use:

Wire.write(0x13); // address port B
Wire.write(??);  // value to send

… replacing ?? with the binary or equivalent hexadecimal or decimal value to send to the register.

To calculate the required number, consider each I/O pin from 7 to 0 matches one bit of a binary number – 1 for on, 0 for off. So you can insert a binary number representing the status of each output pin. Or if binary does your head in, convert it to hexadecimal. Or a decimal number.

So for example, you want pins 7 and 1 on. In binary that would be 10000010, in hexadecimal that is 0x82, or 130 decimal. (Using decimals is convenient if you want to display values from an incrementing value or function result).

For example, we want port A to be 11001100 and port B to be 10001000 – so we send the following (note we converted the binary values to decimal):

Wire.write(0x12); // address port A
Wire.write(204); // value to send
Wire.write(0x13); // address port B 
Wire.write(136);     // value to send

A complete Example

// pins 15~17 to GND, I2C bus address is 0x20
#include "Wire.h"
void setup()
 Wire.begin(); // wake up I2C bus
// set I/O pins to outputs
 Wire.write(0x00); // IODIRA register
 Wire.write(0x00); // set all of port A to outputs
 Wire.write(0x01); // IODIRB register
 Wire.write(0x00); // set all of port B to outputs
void binaryCount()
 for (byte a=0; a<256; a++)
 Wire.write(0x12); // GPIOA
 Wire.write(a); // port A
 Wire.write(0x13); // GPIOB
 Wire.write(a); // port B
void loop()

Using the pins as inputs

Although that may have seemed like a simple demonstration, it was created show how the outputs can be used. So now you know how to control the I/O pins set as outputs. Note that you can’t source more than 25 mA of current from each pin, so if switching higher current loads use a transistor and an external power supply and so on.

Now let’s turn the tables and work on using the I/O pins as digital inputs. The MCP23017 I/O pins default to input mode, so we just need to initiate the I2C bus. Then in the void loop() or other function all we do is set the address of the register to read and receive one byte of data.

// pins 15~17 to GND, I2C bus address is 0x20
#include "Wire.h"
byte inputs=0;
void setup()
 Wire.begin(); // wake up I2C bus
void loop()
 Wire.write(0x13); // set MCP23017 memory pointer to GPIOB address
 Wire.requestFrom(0x20, 1); // request one byte of data from MCP20317; // store the incoming byte into "inputs"
 if (inputs>0) // if a button was pressed
 Serial.println(inputs, BIN); // display the contents of the GPIOB register in binary
 delay(200); // for debounce

Other Libraries

You can also download and install the MCP23017 Library from Adafruit for the Arduino IDE.
This library will make using this chip even easier… I will discuss this library in another post

I hope this will be useful to somebody.

SPI between Maker Nano ( Arduino Nano Clone ) and STM32 “Blue Pill” – Part 2

Sometimes it is necessary to send data between two microprocessors.
The SPI bus is a very fast serial data-bus that is normally used to interface with various peripherals like OLED Screens, Radios and various sensors. In today’s very short post, I will show you how to interface the STM32F103C8T6 “Blue Pill” with an Arduino Nano to send bidirectional data via the SPI Interface between the two microprocessors. You will need the following to experiment with this by yourself.

1) An STM32F103C8T6 ” Blue Pill”
2) An Arduino Compatible or Original Uno or Nano
3) Two Breadboards
4) Some Linkup wires (at least 4 male-to-male DuPont wires)

Let us look at the pin configuration on the two boards

PIN NAME“Blue Pill”Arduino Nano or Uno

Connections for the “Blue Pill” are shown above

Connections for Maker NANO are shown above

You can now type in the code for the Master ( The Blue Pill ) into your Arduino IDE


// Define Constants and Variables

#define SS PA4

#define led PC13

unsigned char MasterSend;;

unsigned char MasterReceive;

void setup() {



// SPI Init



digitalWrite(SS,HIGH); // set as master

MasterSend = 0xFF;


void loop() {

Serial.print(“Sent to Slave “);

Serial.print(” [0x”);


MasterReceive = SPI.transfer(MasterSend);

Serial.print(“Received from Slave “);

Serial.print(” [0x”);






And in ANOTHER INSTANCE of the Arduino IDE, the code for the SLAVE (Maker NANO)

//SPI Slave Code for Arduino

//SPI Communication between STM32F103C8 & Arduino


volatile boolean received;

volatile unsigned char SlaveReceived;

volatile unsigned char SlaveSend;

void setup()





received = false;


SlaveSend = 0xAA;




SlaveReceived = SPDR;

received = true;


void loop() {

Serial.print(“Received from Master”);

Serial.print(” [0x”);


Serial.println(“] “);

SPDR = SlaveSend;

Serial.print(“Sent to Master”);

Serial.print(” [0x”);





Upload the code to the boards, and open the serial monitors on both instances of the Arduino IDE.
Set the Baud Rate to 115200
You will see that the Master sends a byte to the Slave, and the Save replies with a byte of it’s own.

Master sends data to Slave, Receives Data Back

Slave received data from Master, and replies with data of its own

This sketch can now very easily be modified to send reading from sensors, or instructions to control other peripherals between the two microprocessors. It is limited only by your imagination, and your ability to code.

I hope you found this interesting and useful.

What is SPI? Serial Peripheral Interface – Part 1


The Serial Peripheral Interface is a synchronous serial communication interface for short-distance communication, it is typically used in embedded systems. The interface was developed by Motorola in the mid 1980’s and has become a very popular standard.

It is used with many kinds of sensors, LCD’s and also SD-Cards. SPI operates in a Master-Slave model, with a possibility of multiple slave devices, each selected in turn by a SS (slave select) or CS (chip select) pin that is usually pulled low by the master.

Typical connection between two SPI devices

Typical configuration

SPI is a four-wire interface, with the different lines being
– MOSI [Master Out Slave In]
-MISO [Master In Slave Out]
-SCLK [Serial Clock OUT – generated by the master]
-SS/CS [Slave Select or Chip Select, sometimes also labelled CE – Chip Enable]

SPI is a FULL DUPLEX interface, where the master initiates the communication frames between the various slave devices. This is usually done by pulling the particular device’s SS/CS pin low. Data is then shifted simultaneously into and out of the devices by means of the MOSI and MISO lines on the bus. The frequency of the serially shifted data is controlled by the SCLK line. This clock signal is generated by the master device.

It is important to note that MOST of the slave devices have a tri-state (HIGH IMPEDANCE) mode on their MISO pins. This electrically disconnects the MISO pin from the bus when the device is not selected via the SS/CS pin.

You should also note the SPI slave devices that do not have a tri-state mode on their MISO pins, should not be used on the same bus as devices that have without using an external tri-state buffer circuit between the non-tristate device and the rest of the devices on the MISO bus.

Typical connection between an SPI Master and three Slave devices

It is possible to connect multiple SPI slave devices to on Master device if you remember that each slave device will need its own dedicated SS/CS pin on the master. This can however quickly use a lot of IO pins on a microcontroller, thus being one of the disadvantages of SPI versus I2C. SPI is however quite a bit faster than I2C.

Data Transmission

To begin communication, the bus master configures the clock, using a frequency supported by the slave device, typically up to a few MHz. The master then selects the slave device with a logic level 0 on the select line. If a waiting period is required, such as for an analog-to-digital conversion, the master must wait for at least that period of time before issuing clock cycles.

During each SPI clock cycle, full-duplex data transmission occurs. The master sends a bit on the MOSI line and the slave reads it, while the slave sends a bit on the MISO line and the master reads it. This sequence is maintained even when only one-directional data transfer is intended.

A typical hardware setup using two shift registers to form an inter-chip circular buffer

Transmissions normally involve two shift registers of some given word-size, such as eight bits, one in the master and one in the slave; they are connected in a virtual ring topology. Data is usually shifted out with the most significant bit first. On the clock edge, both master and slave shift out a bit and output it on the transmission line to the counterpart. On the next clock edge, at each receiver the bit is sampled from the transmission line and set as a new least-significant bit of the shift register. After the register bits have been shifted out and in, the master and slave have exchanged register values. If more data needs to be exchanged, the shift registers are reloaded and the process repeats. Transmission may continue for any number of clock cycles. When complete, the master stops toggling the clock signal, and typically deselects the slave.

Transmissions often consist of eight-bit words. However, other word-sizes are also common, for example, sixteen-bit words for touch-screen controllers or audio codecs, such as the TSC2101 by Texas Instruments, or twelve-bit words for many digital-to-analogue or analogue-to-digital converters.

Every slave on the bus that has not been activated using its chip select line must disregard the input clock and MOSI signals and should not drive MISO (I.E. must have a tri-state output) although some devices need external tri-state buffers to implement this.

Clock polarity and phasing

In addition to setting the clock frequency, the master must also configure the clock polarity and phase with respect to the data. Motorola SPI Block Guide names these two options as CPOL and CPHA (for clock polarity and phase) respectively, a convention most vendors have also adopted.

The timing diagram is shown below. The timing is further described below and applies to both the master and the slave device.

  • CPOL determines the polarity of the clock. The polarities can be converted with a simple inverter.
  • CPOL=0 is a clock which idles at 0, and each cycle consists of a pulse of 1. That is, the leading edge is a rising edge, and the trailing edge is a falling edge.
  • CPOL=1 is a clock which idles at 1, and each cycle consists of a pulse of 0. That is, the leading edge is a falling edge, and the trailing edge is a rising edge.
  • CPHA determines the timing (i.e. phase) of the data bits relative to the clock pulses. Conversion between these two forms is non-trivial.
  • For CPHA=0, the “out” side changes the data on the trailing edge of the preceding clock cycle, while the “in” side captures the data on (or shortly after) the leading edge of the clock cycle. The out-side holds the data valid until the trailing edge of the current clock cycle. For the first cycle, the first bit must be on the MOSI line before the leading clock edge.
  • An alternative way of considering it is to say that a CPHA=0 cycle consists of a half cycle with the clock idle, followed by a half cycle with the clock asserted.
  • For CPHA=1, the “out” side changes the data on the leading edge of the current clock cycle, while the “in” side captures the data on (or shortly after) the trailing edge of the clock cycle. The out-side holds the data valid until the leading edge of the following clock cycle. For the last cycle, the slave holds the MISO line valid until slave select is de-selected.
  • An alternative way of considering it is to say that a CPHA=1 cycle consists of a half cycle with the clock asserted, followed by a half cycle with the clock idle.
A timing diagram showing clock polarity and phase. Red lines denote clock leading edges, and blue lines, trailing edges.

The MOSI and MISO signals are usually stable (at their reception points) for the half cycle until the next clock transition. SPI master and slave devices may well sample data at different points in that half cycle.

This adds more flexibility to the communication channel between the master and slave.

Mode numbers

The combinations of polarity and phases are often referred to as modes which are commonly numbered according to the following convention, with CPOL as the high order bit and CPHA as the low order bit:

For “Microchip PIC” / “ARM-based” microcontrollers (note that NCPHA is the inversion of CPHA):

SPI modeClock polarity
Clock phase
Clock edge
For PIC32MX: SPI mode configure CKP, CKE and SMP bits. Set SMP bit and CKP, CKE two bits configured as above table.
For other microcontrollers:

Another commonly used notation represents the mode as a (CPOL, CPHA) tuple; e.g., the value ‘(0, 1)’ would indicate CPOL=0 and CPHA=1.

Note that in Full Duplex operation, the Master device could transmit and receive with different modes. For instance, it could transmit in Mode 0 and be receiving in Mode 1 at the same time.

Independent Slave Configuration

In the independent slave configuration, there is an independent chip select line for each slave. This is the way SPI is normally used. The master asserts only one chip select at a time.

Pull-up resistors between the power source and chip select lines are recommended for systems where the master’s chip select pins may default to an undefined state. When separate software routines initialize each chip select and communicate with its slave, pull-up resistors prevent other uninitialized slaves from responding.

Since the MISO pins of the slaves are connected together, they are required to be tri-state pins (high, low or high-impedance), where the high-impedance output must be applied when the slave is not selected. Slave devices not supporting tri-state may be used in independent slave configuration by adding a tri-state buffer chip controlled by the chip select signal. (Since only a single signal line needs to be tri-stated per slave, one typical standard logic chip that contains four tristate buffers with independent gate inputs can be used to interface up to four slave devices to an SPI bus.)

Typical SPI configuration

Daisy chain configuration

Some products that implement SPI may be connected in a daisy chain configuration, the first slave output being connected to the second slave input, etc. The SPI port of each slave is designed to send out during the second group of clock pulses an exact copy of the data it received during the first group of clock pulses. The whole chain acts as a communication shift register; daisy chaining is often done with shift registers to provide a bank of inputs or outputs through SPI. Each slave copies input to output in the next clock cycle until the active low SS line goes high. Such a feature only requires a single SS line from the master, rather than a separate SS line for each slave.

Note that not all SPI devices support this. You should thus check your datasheet before using this configuration!

SPI Daisy Chain configuration

Valid Communications

Some slave devices are designed to ignore any SPI communications in which the number of clock pulses is greater than specified. Others do not care, ignoring extra inputs and continuing to shift the same output bit. It is common for different devices to use SPI communications with different lengths, as, for example, when SPI is used to access the scan chain of a digital IC by issuing a command word of one size (perhaps 32 bits) and then getting a response of a different size (perhaps 153 bits, one for each pin in that scan chain).


SPI devices sometimes use another signal line to send an interrupt signal to a host CPU. Examples include pen-down interrupts from touchscreen sensors, thermal limit alerts from temperature sensors, alarms issued by real-time clock chips, SDIO, and headset jack insertions from the sound codec in a cell phone. Interrupts are not covered by the SPI standard; their usage is neither forbidden nor specified by the standard. In other words, interrupts are outside the scope of the SPI standard and are optionally implemented independently from it.

Bit Banging a SPI Master – Example code

Below is an example of bit-banging the SPI protocol as an SPI master with CPOL=0, CPHA=0, and eight bits per transfer. The example is written in the C programming language. Because this is CPOL=0 the clock must be pulled low before the chip select is activated. The chip select line must be activated, which normally means being toggled low, for the peripheral before the start of the transfer, and then deactivated afterwards. Most peripherals allow or require several transfers while the select line is low; this routine might be called several times before deselecting the chip.

 * Simultaneously transmit and receive a byte on the SPI.
 * Polarity and phase are assumed to be both 0, i.e.:
 *   - input data is captured on rising edge of SCLK.
 *   - output data is propagated on falling edge of SCLK.
 * Returns the received byte.
uint8_t SPI_transfer_byte(uint8_t byte_out)
    uint8_t byte_in = 0;
    uint8_t bit;

    for (bit = 0x80; bit; bit >>= 1) {
        /* Shift-out a bit to the MOSI line */
        write_MOSI((byte_out & bit) ? HIGH : LOW);

        /* Delay for at least the peer's setup time */

        /* Pull the clock line high */

        /* Shift-in a bit from the MISO line */
        if (read_MISO() == HIGH)
            byte_in |= bit;

        /* Delay for at least the peer's hold time */

        /* Pull the clock line low */

    return byte_in;

This concludes part 1 of my series on SPI. I hope you found it interesting and useful.

The OLED Display


Adding a display to any project can instantly increase its visual appeal, as well as make the project easier to control. Displays available to Electronic enthusiasts mostly include some sort of LCD or even TFT display. LCD displays are usually bulky and very limited in their ability to display a lot of information, whereas TFT type displays are still a bit on the expensive side, and not very easy to interface with for the beginner.

Today, I would like to introduce a different type of display, which is available in an I2C as well as SPI version. These displays are very easily readable in almost any light conditions, lightweight, and most importantly, they are extremely cheap. I am talking about the OLED display of course… Many of us may already have one of them in our mobile phones, or even TV screen…

128×32 I2C OLED Display (40mmx10mm) [0.91″] Front view

Some Technical Data

An organic light-emitting diode (OLED or Organic LED), also known as an organic EL (organic electroluminescent) diode,[1][2] is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This organic layer is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as smartphoneshandheld game consoles and PDAs. A major area of research is the development of white OLED devices for use in solid-state lighting applications.[3][4][5]

There are two main families of OLED: those based on small molecules and those employing polymers. Adding mobile ions to an OLED creates a light-emitting electrochemical cell (LEC) which has a slightly different mode of operation. An OLED display can be driven with a passive-matrix (PMOLED) or active-matrix (AMOLED) control scheme. In the PMOLED scheme, each row (and line) in the display is controlled sequentially, one by one,[6] whereas AMOLED control uses a thin-film transistor backplane to directly access and switch each individual pixel on or off, allowing for higher resolution and larger display sizes.

An OLED display works without a backlight because it emits visible light. Thus, it can display deep black levels and can be thinner and lighter than a liquid crystal display (LCD). In low ambient light conditions (such as a dark room), an OLED screen can achieve a higher contrast ratio than an LCD, regardless of whether the LCD uses cold cathode fluorescent lamps or an LED backlight. OLED displays are made in the same way as LCDs, but after TFT (for active matrix displays), addressable grid (for passive matrix displays) or ITO segment (for segment displays) formation, the display is coated with hole injection, transport and blocking layers, as well with electroluminescent material after the 2 first layers, after which ITO or metal may be applied again as a cathode and later the entire stack of materials is encapsulated. The TFT layer, addressable grid or ITO segments serve as or are connected to the anode, which may be made of ITO or metal.[7][8] OLEDs can be made flexible and transparent, with transparent displays being used in smartphones with optical fingerprint scanners and flexible displays being used in foldable smartphones.

The full article is available here if you are interested.

128×32 I2C OLED Display (40mmx10mm) [0.91″] Back view

Connecting the circuit

This display is once again extremely easy to connect, as it uses the very versatile I2C protocol. (An SPI version is also available).

Connecting 128×32 OLED display to an Arduino Uno Clone

Connect the following wires to the Arduino / ESP32
+5v (red) to the VCC pin on the display
Gnd to Gnd
SDA (A4 on Uno) to SDA, and SCL (A5 on Uno) to SCL

The Software Libraries

The 128×32 OLED display that we will be using today, is based on the SSD1306. We will thus be using a library suplied by Adafruit to interface with this chip. There are various other libraries available, but I have found the Adafruit library the most stable.

To load this, start by opening the Arduino IDE, and go to the Sketch->Include Library->Manage Libraries option on the menu

The Library Manager will now open

We need to install two (2) Libraries

– Adafruit GFX ( this is for graphics)
– Adafruit SSD1306 ( to control the actual display )

Click on “Close” after installation is completed.

Using the display

We will use one of the standard Adafruit examples to show you the capabilities of the tiny little screen. The example are so straight forward to use, that I find it unnecessary to say anything else about it 🙂

Open the ssd1306_128x32_ic2 Example from the Examples menu in the Arduino IDE and upload it to your Arduino, making sure that you set the dimensions of your screen first (in my case 128×32 )

 This is an example for our Monochrome OLEDs based on SSD1306 drivers

 Pick one up today in the adafruit shop!

 This example is for a 128x32 pixel display using I2C to communicate
 3 pins are required to interface (two I2C and one reset).

 Adafruit invests time and resources providing this open
 source code, please support Adafruit and open-source
 hardware by purchasing products from Adafruit!

 Written by Limor Fried/Ladyada for Adafruit Industries,
 with contributions from the open source community.
 BSD license, check license.txt for more information
 All text above, and the splash screen below must be
 included in any redistribution.

#include <SPI.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 32 // OLED display height, in pixels

// Declaration for an SSD1306 display connected to I2C (SDA, SCL pins)
#define OLED_RESET     4 // Reset pin # (or -1 if sharing Arduino reset pin)

#define NUMFLAKES     10 // Number of snowflakes in the animation example

#define LOGO_HEIGHT   16
#define LOGO_WIDTH    16
static const unsigned char PROGMEM logo_bmp[] =
{ B00000000, B11000000,
  B00000001, B11000000,
  B00000001, B11000000,
  B00000011, B11100000,
  B11110011, B11100000,
  B11111110, B11111000,
  B01111110, B11111111,
  B00110011, B10011111,
  B00011111, B11111100,
  B00001101, B01110000,
  B00011011, B10100000,
  B00111111, B11100000,
  B00111111, B11110000,
  B01111100, B11110000,
  B01110000, B01110000,
  B00000000, B00110000 };

void setup() {

  // SSD1306_SWITCHCAPVCC = generate display voltage from 3.3V internally
  if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) { // Address 0x3C for 128x32
    Serial.println(F("SSD1306 allocation failed"));
    for(;;); // Don't proceed, loop forever

  // Show initial display buffer contents on the screen --
  // the library initializes this with an Adafruit splash screen.
  delay(2000); // Pause for 2 seconds

  // Clear the buffer

  // Draw a single pixel in white
  display.drawPixel(10, 10, SSD1306_WHITE);

  // Show the display buffer on the screen. You MUST call display() after
  // drawing commands to make them visible on screen!
  // display.display() is NOT necessary after every single drawing command,
  // unless that's what you want...rather, you can batch up a bunch of
  // drawing operations and then update the screen all at once by calling
  // display.display(). These examples demonstrate both approaches...

  testdrawline();      // Draw many lines

  testdrawrect();      // Draw rectangles (outlines)

  testfillrect();      // Draw rectangles (filled)

  testdrawcircle();    // Draw circles (outlines)

  testfillcircle();    // Draw circles (filled)

  testdrawroundrect(); // Draw rounded rectangles (outlines)

  testfillroundrect(); // Draw rounded rectangles (filled)

  testdrawtriangle();  // Draw triangles (outlines)

  testfilltriangle();  // Draw triangles (filled)

  testdrawchar();      // Draw characters of the default font

  testdrawstyles();    // Draw 'stylized' characters

  testscrolltext();    // Draw scrolling text

  testdrawbitmap();    // Draw a small bitmap image

  // Invert and restore display, pausing in-between

  testanimate(logo_bmp, LOGO_WIDTH, LOGO_HEIGHT); // Animate bitmaps

void loop() {

void testdrawline() {
  int16_t i;

  display.clearDisplay(); // Clear display buffer

  for(i=0; i<display.width(); i+=4) {
    display.drawLine(0, 0, i, display.height()-1, SSD1306_WHITE);
    display.display(); // Update screen with each newly-drawn line
  for(i=0; i<display.height(); i+=4) {
    display.drawLine(0, 0, display.width()-1, i, SSD1306_WHITE);


  for(i=0; i<display.width(); i+=4) {
    display.drawLine(0, display.height()-1, i, 0, SSD1306_WHITE);
  for(i=display.height()-1; i>=0; i-=4) {
    display.drawLine(0, display.height()-1, display.width()-1, i, SSD1306_WHITE);


  for(i=display.width()-1; i>=0; i-=4) {
    display.drawLine(display.width()-1, display.height()-1, i, 0, SSD1306_WHITE);
  for(i=display.height()-1; i>=0; i-=4) {
    display.drawLine(display.width()-1, display.height()-1, 0, i, SSD1306_WHITE);


  for(i=0; i<display.height(); i+=4) {
    display.drawLine(display.width()-1, 0, 0, i, SSD1306_WHITE);
  for(i=0; i<display.width(); i+=4) {
    display.drawLine(display.width()-1, 0, i, display.height()-1, SSD1306_WHITE);

  delay(2000); // Pause for 2 seconds

void testdrawrect(void) {

  for(int16_t i=0; i<display.height()/2; i+=2) {
    display.drawRect(i, i, display.width()-2*i, display.height()-2*i, SSD1306_WHITE);
    display.display(); // Update screen with each newly-drawn rectangle


void testfillrect(void) {

  for(int16_t i=0; i<display.height()/2; i+=3) {
    // The INVERSE color is used so rectangles alternate white/black
    display.fillRect(i, i, display.width()-i*2, display.height()-i*2, SSD1306_INVERSE);
    display.display(); // Update screen with each newly-drawn rectangle


void testdrawcircle(void) {

  for(int16_t i=0; i<max(display.width(),display.height())/2; i+=2) {
    display.drawCircle(display.width()/2, display.height()/2, i, SSD1306_WHITE);


void testfillcircle(void) {

  for(int16_t i=max(display.width(),display.height())/2; i>0; i-=3) {
    // The INVERSE color is used so circles alternate white/black
    display.fillCircle(display.width() / 2, display.height() / 2, i, SSD1306_INVERSE);
    display.display(); // Update screen with each newly-drawn circle


void testdrawroundrect(void) {

  for(int16_t i=0; i<display.height()/2-2; i+=2) {
    display.drawRoundRect(i, i, display.width()-2*i, display.height()-2*i,
      display.height()/4, SSD1306_WHITE);


void testfillroundrect(void) {

  for(int16_t i=0; i<display.height()/2-2; i+=2) {
    // The INVERSE color is used so round-rects alternate white/black
    display.fillRoundRect(i, i, display.width()-2*i, display.height()-2*i,
      display.height()/4, SSD1306_INVERSE);


void testdrawtriangle(void) {

  for(int16_t i=0; i<max(display.width(),display.height())/2; i+=5) {
      display.width()/2  , display.height()/2-i,
      display.width()/2-i, display.height()/2+i,
      display.width()/2+i, display.height()/2+i, SSD1306_WHITE);


void testfilltriangle(void) {

  for(int16_t i=max(display.width(),display.height())/2; i>0; i-=5) {
    // The INVERSE color is used so triangles alternate white/black
      display.width()/2  , display.height()/2-i,
      display.width()/2-i, display.height()/2+i,
      display.width()/2+i, display.height()/2+i, SSD1306_INVERSE);


void testdrawchar(void) {

  display.setTextSize(1);      // Normal 1:1 pixel scale
  display.setTextColor(SSD1306_WHITE); // Draw white text
  display.setCursor(0, 0);     // Start at top-left corner
  display.cp437(true);         // Use full 256 char 'Code Page 437' font

  // Not all the characters will fit on the display. This is normal.
  // Library will draw what it can and the rest will be clipped.
  for(int16_t i=0; i<256; i++) {
    if(i == '\n') display.write(' ');
    else          display.write(i);


void testdrawstyles(void) {

  display.setTextSize(1);             // Normal 1:1 pixel scale
  display.setTextColor(SSD1306_WHITE);        // Draw white text
  display.setCursor(0,0);             // Start at top-left corner
  display.println(F("Hello, world!"));

  display.setTextColor(SSD1306_BLACK, SSD1306_WHITE); // Draw 'inverse' text

  display.setTextSize(2);             // Draw 2X-scale text
  display.print(F("0x")); display.println(0xDEADBEEF, HEX);


void testscrolltext(void) {

  display.setTextSize(2); // Draw 2X-scale text
  display.setCursor(10, 0);
  display.display();      // Show initial text

  // Scroll in various directions, pausing in-between:
  display.startscrollright(0x00, 0x0F);
  display.startscrollleft(0x00, 0x0F);
  display.startscrolldiagright(0x00, 0x07);
  display.startscrolldiagleft(0x00, 0x07);

void testdrawbitmap(void) {

    (display.width()  - LOGO_WIDTH ) / 2,
    (display.height() - LOGO_HEIGHT) / 2,
    logo_bmp, LOGO_WIDTH, LOGO_HEIGHT, 1);

#define XPOS   0 // Indexes into the 'icons' array in function below
#define YPOS   1
#define DELTAY 2

void testanimate(const uint8_t *bitmap, uint8_t w, uint8_t h) {
  int8_t f, icons[NUMFLAKES][3];

  // Initialize 'snowflake' positions
  for(f=0; f< NUMFLAKES; f++) {
    icons[f][XPOS]   = random(1 - LOGO_WIDTH, display.width());
    icons[f][YPOS]   = -LOGO_HEIGHT;
    icons[f][DELTAY] = random(1, 6);
    Serial.print(F("x: "));
    Serial.print(icons[f][XPOS], DEC);
    Serial.print(F(" y: "));
    Serial.print(icons[f][YPOS], DEC);
    Serial.print(F(" dy: "));
    Serial.println(icons[f][DELTAY], DEC);

  for(;;) { // Loop forever...
    display.clearDisplay(); // Clear the display buffer

    // Draw each snowflake:
    for(f=0; f< NUMFLAKES; f++) {
      display.drawBitmap(icons[f][XPOS], icons[f][YPOS], bitmap, w, h, SSD1306_WHITE);

    display.display(); // Show the display buffer on the screen
    delay(200);        // Pause for 1/10 second

    // Then update coordinates of each flake...
    for(f=0; f< NUMFLAKES; f++) {
      icons[f][YPOS] += icons[f][DELTAY];
      // If snowflake is off the bottom of the screen...
      if (icons[f][YPOS] >= display.height()) {
        // Reinitialize to a random position, just off the top
        icons[f][XPOS]   = random(1 - LOGO_WIDTH, display.width());
        icons[f][YPOS]   = -LOGO_HEIGHT;
        icons[f][DELTAY] = random(1, 6);

I hope that you find this useful and inspiring.
Thank you

What exactly is I2C?

In this post, I will tell you all the basics of the I2C protocol. What it is, where it comes from and also how it is configured and setup. We will also look at how data is transferred and received

Table of contents
1. Introduction
2. The Features of I2C
3. The Hardware
3.1 The physical I2C Bus
3.2 The Master and Slave devices on the bus
4. The data transfer protocol
4.1 The Start Condition
4.2 The Address Block
4.3 The Read/Write Bit
4.4 The ACK/NACK Bit
4.5 The Data Block
4.6 The Stop Condition
5. How does I2C work in practice
5.1 Sending data to a Slave Device
5.2 Reading data from a Slave Device
5.3 The Clock stretching concept


I2C communication is the short form name for inter-integrated circuit protocol. It is a communication protocol developed by Philips Semiconductors for the transfer of data between a central processor and multiple integrated circuits on the same circuit board by using just two common wires.

Due to its simplicity, it is widely adopted for communication between microcontrollers and sensor arrays, displays, IoT devices, EEPROMs etc.

This is a synchronous serial communication protocol. It means that data bits are transferred one by one at regular intervals of time set by a reference clock line.

The Features of I2C

The I2C protocol has the following important features

  • Only two common bus lines (wires) are required to control any device/IC on the I2C network.
  • There is no need for a prior agreement on data transfer rate like in UART communications. The data transfer speed can thus be adjusted whenever it is required.
  • It has a simple mechanism for validating the transferred data.
  • It uses a 7-bit addressing system to target a specific device/IC on the I2C bus.
  • I2C networks are extremely easy to scale. New devices can simply be connected to the two common I2C bus lines.

The Hardware

The physical I2C Bus

The I2C Bus (Interface wires) consists of just two wires and are named Serial Clock Line (SCL) and Serial Data Line (SDA). The data to be transferred is sent through the SDA wire and is synchronized with the clock signal from SCL. All the devices/ICs on the I2C network are connected to the same SCL and SDA lines as shown in the image below:

The physical I2C Bus. All devices are connected to the same 2 wired on the bus, namely SDA and SCL

Both the I2C bus lines (SDA, SCL) are operated as in open-drain driver mode. It means that any device/IC on the I2C network can drive(pull) SDA and SCL low, but they cannot drive them high. So, a pull-up resistor is used on each bus line, to keep them high (at positive voltage) by default.

This is to prevent the bus from shorting, which might happen when one device tries to pull the line high and some other device tries to pull the line low.

The Master and Slave Devices on the I2C Bus

The devices connected to the I2C bus are categorized as either masters or slaves. At any instant of time, only a single master stays active on the I2C bus. It controls the SCL clock line and decides what operation is to be done on the SDA data line.

All the devices that respond to instructions from this master device are slaves. For differentiating between multiple slave devices connected to the same I2C bus, each slave device is physically assigned a permanent 7-bit address.

When a master device wants to transfer data to or from a slave device, it specifies this particular slave device address on the SDA line and then proceeds with the transfer. So effectively communication takes place between the master device and a particular slave device.

All the other slave devices don’t respond unless their address is specified by the master device on the SDA line.

The Master and Slave Devices on the I2C Bus. Note that each Slave device has it’s own address.

The Data Transfer Protocol

The protocol (set of rules) that is followed by the master device and slave devices for the transfer of data between them works as follows:

Data is transferred between the master device and slave devices through the SDA data line, via patterned sequences of 0’s and 1’s (bits). Each sequence of 0’s and 1’s is called a transaction and each data transaction is structured as in the image below:

The structure of an I2C Data transaction

The Start Condition

Whenever a master device/IC decides to start a transaction, it switches the SDA line from a high level to a low level before the SCL line switches from high to low.

Once a start condition is sent by the master device, all the slave devices get active even if they were in sleep mode, and wait for the address bits to see which device should respond.

The I2C Start Condition. Note that SDA Switches LOW before SCL. All slave devices on the bus will now listen for an address bit to decide which device should respond.

The Address Block

The Address block is comprised of 7 bits and are filled with the address of slave device (in binary) to/from which the master device needs to send/receive data. All the slave devices on the I2C bus will compare these address bits with their own address.

The Read/Write Bit

This bit specifies the direction that the data must be transferred in. If the master device/IC needs to send data to a slave device, this bit is set to ‘0’. If the master device/IC needs to receive data from the slave device, it is set to ‘1’.


This is the Acknowledged/Not-Acknowledged bit. If the physical address of any slave device is the same as the address that was broadcasted by the master device, that slave device will set the value of this bit to ‘0’ . If there are no slave device(s) with the broadcasted address, this bit will remain at logic ‘1’ (default). This will tell the master that the data/command has been received and/or acknowledged by a slave device.

The Data Block

The data block is comprised of 8 bits and they are set by the transmitter,wheather this be the master or the slave, depending on wheather a read or a write operation was requested, with the data bits that needs to transfered to the receiver. This block is followed by an ACK/NACK bit that is set to ‘0’ by the receiver if it successfully receives data. Otherwise it stays at logic ‘1’.

This combination of data blocks followed by an ACK/NACK bit is repeated until all the data is completely transferred.

The Stop Condition

After all the required data blocks are transferred through the SDA line, the master device switches the SDA line from low to high before the SCL line switches back from high to low.

The I2C Stop condition. This signals the end of a transaction. Note SDA returns to High BEFORE the SCL line is pulled High.

How does I2C work in practice

When an I2C transaction is initiated by a master device either to send or receive data to/from a slave device, all of the processes mentioned above will happen at least one.
Let us look at a typical scenario for each of the different type of scenarios.

Sending Data to a Slave Device

The following sequence of operations will take place when a master device tries to send data to a particular slave device through I2C bus:

  • The master device sends a start condition
  • The master device sends the 7 address bits which correspond to the slave device to be targeted
  • The master device sets the Read/Write bit to ‘0’, which signifies a write
  • Now two scenarios are possible:
    • If no slave device matches with the address sent by the master device, the next ACK/NACK bit stays at ‘1’ (default). This signals the master device that the slave device identification is unsuccessful. The master clock will end the current transaction by sending a Stop condition or a new Start condition
    • If a slave device exists with the same address as the one specified by the master device, the slave device sets the ACK/NACK bit to ‘0’, which signals the master device that a slave device is successfully targeted
  • If a slave device is successfully targeted, the master device now sends 8 bits of data which is only considered and received by the targeted slave device. This data means nothing to the remaining slave devices
  • If the data is successfully received by the slave device, it sets the ACK/NACK bit to ‘0’, which signals the master device to continue
  • The previous two steps are repeated until all the data is transferred
  • After all the data is sent to the slave device, the master device sends the Stop condition which signals all the slave devices that the current transaction has ended

The image below represents the transaction with the data bits sent on the SDA line and the device that controls each of them:

I2C Master sending data to a slave device

Reading Data from a Slave Device

The sequence of operations remain the same as in previous scenario except for the following:

  • The master device sets the Read/Write bit to ‘1’ instead of ‘0’ which signals the targeted slave device that the master device is expecting data from it
  • The 8 bits corresponding to the data block are sent by the slave device and the ACK/NACK bit is set by the master device
  • Once the required data is received by the master device, it sends a NACK bit. Then the slave device stops sending data and releases the SDA line

If the master device to read data from specific internal location of a slave device, it first sends the location data to the slave device using the steps in previous scenario. It then starts the process of reading data with a repeated start condition.

The below figure represents the overall data bits sent on the SDA line and the device that controls each of them:

Reading data from a Slave device on the I2C bus

The Clock Stretching concept

Let say the master device started a transaction and sent address bits of a particular slave device followed by a Read bit of ‘1’. The specific slave device needs to send an ACK bit, immediately followed by data.

But if the slave device needs some time to fetch and send data to master device, during this gap, the master device will think that the slave device is sending some data.

To prevent this, the slave device holds the SCL clock line low until it is ready to transfer data bits. By doing this, the slave device signals the master device to wait for data bits until the clock line is released


This concludes this tutorial. In a future post, I will show you how to use I2C to transfer data between two micro-controllers.

Extending Arduino/Esp32/STM32 GPIO Pins – PART 2

In the first part of this series, I showed you how to extend the available output pins on your microprocessor by using a SIPO (Serial In, Parallel Out) Shift Register. These work great to extend your outputs, but they do tend to involve a bit of extra work and organisation in your code. They are also a bit slower than the normal GPIO pins, because data has to be serially shifted into them, and then latched out onto the parallel port.

I have also mentioned that there are I2C devices available that can make this much easier… In today’s article, I will show you how to use one of these I2C devices, the PCF8574.

These little modules have some quite impressive features, for one, allowing you to cascade up to 8 of them together, giving you a quite impressive 64 GPIO ports ! I am also happy to tell you, that if you can find the PCF8574A variant, as well, you can increase the total amount of ports to 128! ( If you chain 8x PCF8574 as well as 8x PCF8574A together) This is possible because the I2C addresses of the two series of chips are different. Thus allowing us to add a total of 16 of them to the I2C bus.

It must however be said that you should calculate your bus resistance very carefully if you plan on doing that. For most of us, I do not believe we will need that much GPIO on a single microprocessor!

Enough introduction, let us start by looking closely at the chip, as well as the modules that you can purchase for around 1 USD each…

A word of caution, there is also another version of these available, which is specifically designed to be used with LCD screens. You should thus be careful when you buy a premade module, that you choose the io-extender version, and not the LCD controller version.

PCF8574 I2C IO Extender module – Front View
PCF8574 I2C IO Extender – Back View

As we can see, the GPIO ports are clearly labeled, from P0 to P7, with the INT (Interrupt Pin) on the very right.

As I have said before, you can cascade up to 8 of these onto the I2C bus. This is done by setting the I2C address of the module. This is done by setting the jumpers as seen in the picture below.

Address Jumpers on the PCF8574 I2C IO Extender Module

The Address can be set by using the following table to lookup the address and set the jumpers accordingly.

A2A1A0I2C Device Address
Available I2C Addresses for PCF8574 selected by setting the jumpers

Connecting the device is very easy. You only have to supply 5v and Ground, as well as connect it to the SCL and SDA Pins on your microprocessor. For Arduino Uno / Nano that is A4 (SDA) and A5 (SCL)

As far as the coding is concerned, you have two options. You can either use the built-in Wire library, or you can download a special library. Both works equally well, but I do believe that the built-in Wire library might be a little bit faster.

Another point to make is that there are a lot of “fake” modules on the market these days. These modules work, but some of them have extremely weak current sourcing abilities. ( I recently bought a pair online, and they are unable to properly light an LED even without a current limiting resistor. I fixed that issue by driving the LED through a small BJT transistor, like the 2n2222a.

You should also take note that the ports will start up in a weak HIGH state when the module is powered up. This should be taken into consideration when designing your circuit to drive external devices through the outputs. In other words, you should take precautions to prevent the devices from switching on before the microprocessor takes control of the module.

Let us start to look at the coding that you will need to do to use this device.
I will start with the built-in wire Library that is included with the Arduino IDE.

#include <Wire.h> // Wire.h provide access to I2C functions

void setup()
Wire.begin(); // Start I2C

void loop()

Wire.beginTransmission(0x20); // Our device is on Address 0x20
Wire.write(0x0F); // This is equal to 0b00001111, meaning it will switch ports P0 to P3 High
Wire.write(0xF0); // This is equal to 0b11110000, meaning it will switch ports P4 to P7 High

The code above will alternate between switching 4 ports high and low every one second.
You can observe this by connecting 8 LEDs through 330ohm resistors to ports P0 through P7

Reading the status of a port (meaning that you configured it as an input) can be done using the following code


void setup()
  Wire.write(0x00);  //LED1 is OFF

void loop()
  Wire.requestFrom(0x20, 4); // Read the state of P4
  byte x =;
  if (bitRead(x, 4) == LOW)
    Wire.write(0x01);  //LED1 is ON
    Wire.write(0x00);  //LED1 is OFF

This code assumes that you have connected a LED throught a resistor to P0, and that you have connected a pullup resistor of 10k to P4, with a pushbutton to GROUND.

The LED should switch on when you press the switch, and go off again once you release it.

If you want to use the special library, you can download it below:

Install this into your Arduino Libraries, and use the following code:

include “Arduino.h”

include “PCF8574.h

// Set i2c address
PCF8574 pcf8574(0x20);

void setup()

// Set pinMode to OUTPUT

pcf8574.pinMode(P0, OUTPUT);



void loop()
pcf8574.digitalWrite(P0, HIGH);
pcf8574.digitalWrite(P0, LOW);

Reading the status of an Input can be done like this:

include “Arduino.h”

include “PCF8574.h”

// Set i2c address
PCF8574 pcf8574(0x20);

void setup()

pcf8574.pinMode(P0, OUTPUT);
pcf8574.pinMode(P1, INPUT);


void loop()
uint8_t val = pcf8574.digitalRead(P1);
if (val==HIGH) Serial.println(“KEY PRESSED”);


There are also excellent examples included with the library. These include using the interrupt pin.

I hope that this will be useful to somebody.

Extending the Inputs and Outputs of your Arduino/ESP32/STM32

There will come a time that you will run out of available input or output pins on your Arduino/ESP32 or STM32 device. Today, I will show you one way to work around this problem and how to add additional input and/or outputs to your device.

There are many ways to do this, the easiest of them being adding some sort of i2c
chip (the Waveshare PCF8574 IO Expansion Board is a good example).

This particular device can be cascaded to provide up to 64 IO ports on the i2c bus.

At Maker and IoT Ideas, we would however like to show you electronics that you can build yourself, or stuff that have not already been made up into some kind of commercial project.

So, keeping this DIY attitude going, I will teach you how to use a shift register today.

Before we do that, we have to look at what a shift register is, and how they work…

A shift register, at its basic level, is actually just a series of D Type Flip-Flops.

D Type Flip Flop

These flip flops are connected together to give this

Parallel in, Serial out Shift Register

There are three basic types of Shift Register
– Serial In, Serial Out (SISO)
– Serial In, Parallel Out (SIPO) and
– Parallel In, Serial Out (PISO)

We can see that they these type of shift registers are mainly used to convert between an serial and parallel data interface.

Serial In, Serial Out ( SISO)

This is the easiest configuration for a shift register. It is basically just a row of flip flops, with the output of the first, connected to the input of the next… and so on. This type of shift register is mainly used to introduce a delay into the data stream. This means that for a 8 bit SISO Shift register, the first data will only appear at the output after 8 clock cycles !

Serial In, Parallel Out (SIPO)

This kind of Shift Register has the same configuration as the SISO type, but it differs in that there is output after every flip-flop. That makes this type of shift register a good choice to use to extend the outputs on a microcontroller like Arduino or ESP32. The downside of using a shift register as a parallel output device is that the outputs will be slightly slower than if you used the native outputs on the microcontroller.

Parallel In, Serial Out (PISO)

A Parallel In, Serial out shift register can be used to read inputs from buttons or other digital devices. These inputs are then serially shifted into a serial pin on the microcontroller to be processed. It is also slightly slower than if you used the native inputs on the microcontroller.

Universal Shift register

You are also able to use a universal shift register. These chips can be used as both inputs and outputs. They are however only available in a 4 bit configuration.

The Chips (IC’s)

SISO / SIPOTTL74HC5958 bit
SIPOTTL74LS1648 bit
PISOTTL74HC1668 bit
Some common Shift Register Chips

Example using Arduino with 74HC595n SIPO Shift Register

We will look at an example to connect the 74HC5959N to Arduino to drive eight (8) LEDs.
This example can be adapted to drive other loads as well by using a small BJT transistor and current limiting resistor instead of the LED

74hc595n Pin NumberDescription and Connection
Q0…Q7 (15,1,2,3,4,5,6,7)Parallel outputs of the shift register to write up to 8 signals with only 3 pins on your Arduino or ESP32 microcontroller
GND (8)Connect to the ground on the microcontroller
VCC (16)Connect to 3.3V or 5V on the microcontroller
DS (14)Serial data input has to be connected to the microcontroller (in this example D4)
OE (13)Output enable input do we not need and connected to ground
STCP (Latch) (12)Storage register clock input has to be connected to the microcontroller (in this example D5)
SHCP (11)Shift register clock input has to be connected to the microcontroller (in this example D6)
MR (10)Master reset connected to VCC because is active with LOW and we do not want to reset the register
Q7S (9)Serial data output not needed and therefore not connected
Connections for 74HC595n to Arduino

The operation of the 74hc595n SIPO Shift Register can easily be explained in three steps:

  1. The latch (STCP pin) is pulled LOW because the data is only stored in the register on a LOW to HIGH transition of this pin.
  2. Data is shifted out to the register with the data pin (DS) and with a LOW to HIGH transition of the clock signal (SHCP).
  3. The latch (STCP pin) is pulled HIGH to save the data in the register.

Make the following connections to your Arduino and Breadboard ( we will use Arduino Nano today, but you can use the same pins and code for Arduino Uno )

Breadboard Layout for 74HC595 Shift Register as Output Extender with Arduino Nano
Schematic Diagram for using 74HC595 Shift Register as Output Extender with Arduino Nano

Double-check all your connections, and then you can start the coding. The code will be much easier than you may think, as there is already a builtin function in the Arduino C language that we will use to shift the data out to the register.

// Define the Control Pins for the register
int latch-Pin = 5;
int clock-Pin = 7;
int data-Pin = 6;

void setup()
pinMode(latch-Pin, OUTPUT);
pinMode(data-Pin, OUTPUT);
pinMode(clock-Pin, OUTPUT);


void loop()
byte leds = 0;
for (int i = 0; i < 8; i++)
bitSet(leds, i);
for (int i = 7; i >= 0; i–)
bool b = bitRead(leds, i);
Serial.println(” “);

void updateShiftRegister()
digitalWrite(latch-Pin, LOW);
shiftOut(data-Pin, clock-Pin, LSBFIRST, less);
digitalWrite(latch-Pin, HIGH);

In this example, we output a HIGH to each led in turn, shifting from 0 to 7.

I hope that this will make sense to everybody, and be useful.
Questions and comments are welcome. Next time, we will look as using a PISO Shift register to extend the inputs on you Arduino/ESP32/STM32

Thank you.