Tag: ESP32

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

Introduction

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’.

The ACK/NACK Bit

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

Conclusion

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



Using MQTT with IoT Devices – A Short Summary

You are excited, you have just finished your new IoT device, and have many excellent ideas on how you will use it remotely, on your smartphone, or even from your computer at the office. You now start thinking about how you will send and receive data to this new device of yours…

There are many ways that you can do this, but today, I would like to suggest a very easy and lightweight data transfer protocol. It is called MQTT. Some of you may already be using it, or you may at least have heard about it. Read on if you would like to know more about what it is, as well as how it works.

The History of MQTT

MQTT was designed by Andy Stanford-Clark (IBM) and Arlen Nipper (Cirrus Link, then Eurotech) in 1999. It was first used to monitor an oil pipeline running through the desert. The goal was to design and implement a protocol that is bandwidth-efficient, lightweight and uses very little battery power, because the devices were connected via satellite link which, at that time, was extremely expensive.

In 2013, IBM submitted the MQTT v3.1 protocol to the OASIS specification body with a charter that ensured that only minor changes to the specification could be accepted.MQTT-SN is a variation of the main protocol aimed at embedded devices on non-TCP/IP networks, such as Zigbee.

Historically, the “MQ” in “MQTT” came from the IBM MQ (then ‘MQSeries’) MQ product line. The protocol, however, provides publish-and-subscribe messaging (no queues, in spite of the name) and was specifically designed for resource-constrained devices and low bandwidth, high latency networks. This makes it an excellent candidate for data transmission on IoT and other low resource devices.

The Protocol Architecture (How does it work)

MQTT uses a client-server architecture, where the server is called the broker, and the client, called a client. The broker typically functions like a post office, in the sense that it doesn’t use the client’s address, but rather the subject of the topic that a client is subscribed to, to determine which client should receive a certain message.

This enables many clients to subscribe to the same subject, with each receiving the same message. Clients can also publish a topic, thus transmitting or sending a message to other clients. This concept make bidirectional communication possible between clients, and it also ensures that it is extremely easy to use.

What do theses topics look like?

An MQTT Topic is a text string, delimited by a /
for example, let us say you have a device in your kitchen, that monitors the temperature, and controls the lights and microwave oven.

This device may use the following topics to publish (send data) or subscribe to (receive data)

@msg/myhouse/kitchen/temp
@msg/myhouse/kitchen/microwave
@msg/myhouse/kitchen/lights
@msg/myhouse/livingroom/lights
@msg/myhouse/bedroom/lights

As you can see, we can easily group topics by their location. It would thus be very easy for a smart home controller, like OpenHab or similar to get status or set a particular state in a certain room or area.

This can be done by using wildcard operators, of which there are two, the + and #

If we want to subscribe to all the lights in the house, we can subscribe to the following topic:

@msg/myhouse/+/lights

This will give us the data from the following topics:
@msg/myhouse/kitchen/lights
@msg/myhouse/livingroom/lights
@msg/myhouse/bedroom/lights

You can also subscribe to a multilevel wildcard topic, for example:

@msg/myhouse/kitchen/#

This will subscribe you to all the topics related to the kitchen.

How do I get access to MQTT

There are quite a few online MQTT brokers available that allow you to apply for a free account. These services have many limits on the amounts of messages you may send or receive, but generally, they are quite useful.

Adafruit.io should be well known. It is stable, and easy to use. They do however have a few limitations on how many devices and messages you can add to the service.

NetPie.io is a fairly unknown provider outside of Thailand. They limit you to 3 Projects, with 10 devices per project, but their messaging limits are extremely liberal and permissive. This service will also always be free (or at least that is according to NetPie themselves). On the negative side, all the configuration is available in English, but all the documentation is in Thai. You can however use Google Translate to translate it into a language that you can understand.

If you are not comfortable with running your data on an online broker, you can also download and install one of the many MQTT brokers that are available for installation on Linux and MS-Windows. This will, however, mean that you should also have permanent Internet access, as well as a public IP address, to enable your devices to connect to your broker.

Microprocessor and other Device support

MQTT Libraries are available for use on Arduino, STM32, ESP32 as well as Raspberry PI and Python. You can also get clients and brokers for the Linux and MS-Windows platforms.

Smartphone Access

Various applications exist on the google playstore, as well as the Apple App Store, that allow you to connect to an MQTT broker, subscribe to topics, and also publish topics. Many of these are free to use, and also provide you with a nice user interface.

You can also write your own smartphone app, if you are skilled enough, to do exactly what you want it do do.