How I2C Communication Works and How To Use It with Arduino

Hello, Dejan Nedelkovski here from HowToMechatronics.com. In this tutorial we will learn how the I2C communication protocol works and also we will make a practical example of it with the Arduino board and a sensor which uses this protocol. The I2C communication bus is very popular and broadly used by many electronic devices because it can be easily implemented in many electronic designs which require communication between amaster and multiples slave devices or even multiple master devices. The easy implementation comes with the fact that only two wires are required for communication between up to almost 128 (112) devices when using 7-bit addressing and up to almost 1024 (1008) devices when using 10 - bit addressing. How is that possible? Well each device has a preset ID or a unique device address so the master can choose with which device will be communicating. The two wires or lines are called Serial Clock (SCL) and Serial Data (SDA). The SCL line is the clock signal which synchronize data transfer between the devices on the I2C bus and it's generated by the master device. The other line is SDA line which carries the data. The two lines are "open-drain" which means that pull-up resistors needs to be attached to them so that the lines are high, because the devices on the I2C bus are active low. Commonly used values for the resistors are from 2K for a higher speeds at about 400 kbps second, up to 10 K for lower speeds at about 100 kbps. Okay now let's see the data protocol of the I2C bus. The data signal is transferred in sequences of 8 bits. So after special start condition occurs comes the first 8 bits sequence which indicates the address of the slave to which the data is being sent. After each 8 bits sequence follows a bit called Acknowledge. After the first Acknowledge bit in most cases comes another addressing sequence but this time for the internal registers of the of slave device. After the addressing sequences follows the data sequences as many until the data is completely and it ends with a speacial stop condition. Okay let's take even closer look at these events. The start condition occurs when the data line drops low while the clock line is still high. After this, the clock starts and each data bit is transferred during each clock pulse. The device addressing sequence starts with the most significant bit first and end with the least significant bit and it's actually composed of 7bits because the 8th bit is used for indicating whether the master will write to the slave (logic low) or read from it (logic high). The next bit Acknowledge is used by the slave device to indicate whether it has successfully received the previous sequence of bits. So this time the master device hands the control of the SDA line over to the slave device and if the slave device has successfully received the previous sequence it will pull the SDA line down to the condition called Acknowledge. If the slave does not pull the SDA line down, the condition is called Not Acknowledge and means that it didn't successfully received the previous sequence which can be caused by several reasons. For example, the slave might be busy, might not understand the received data or cannot receive any more data and so on. In such a case the master device decides how it will proceed. Next is the internal registers addressing. The internal registers are locations in the slave's memory containing various information or data. For example, the ADXL345 accelerometer has a unique device address and additional internal registers addresses for the X, Y and Z axes. So if we want to read the data of the x-axis first we need to send the device address and then the particular internal register address for the x axis. This addresses can be found from the datasheet of the sensor. After the addressing the data transfers sequences begin either from the master or the slave depending on the selected mode at the Read / Write bit. After the data is completely sent the transfer will end with a stop condition which occurs when the SDA line goes from low to high while the SCL line is high. That's how the I2C communication protocol works and now let'a make an example and demonstrate it using the Arduino Board and some sensors. So as an example I will use the GY - 80 breakout board which consists five differences sensors and the GY - 521 breakout board which consists three different sensors. So we can get data from eaight different sensors with just two wires with the I2C bus. Here's how we will connect the boards. The Serial Clock pin of the Arduino board will be connected to the Serial Clock pins of the two breakout boards and the same goes for the Serial Data pins and also we will power the boards with the Ground and 5V pins from the Arduino board. Note here that we are not using pull-up resistors because the breakout boards already have. Now in order to communicate with these chips, or sensors we need to know their unique addresses. We can find them from the datasheets of the sensors. For the GY-80 breakout board we have the following four addresses: a hexadecimal 0x53 for the three-axis accelerometer, a hexadecimal 0x69 for 3 Axis Gyro, a hexadecimal 0x1E for the 3 Axis Magnetometer and a hexadecimal 0x77 for the barometer and thermometer sensor. For the GY-521 breakout board we have only one address and that's the hexadecimal 0x68. We can also get or check the addresses using I2C Scanner sketch which can be found from the Arduino official website. So if we upload and run that sketch we will get the addresses of the connected devices on the I2C bus. Ok so after we have found the addresses of the devices we also need to find the addresses of their internal registers in order to read the data from them. For example if we want to read the data for the X axis from the 3 Axis accelerometer sensor of the GY-80 breakout board we need to find the internal register address where the data of the X axis is stored. From the datasheet of the sensor we can see that the data for the X Axis is actually stored in two registers, DATAX0 with a hexadecimal address 0x32 and DATAX1 a hexadecimal address 0x33. Now let's make the code that will get the data for the X axis. So we will use the Arduino Wire library which has to be included in the sketch. Here first we have to define the sensor address and the two internal register addresses that we previously found. The Wire.begin() function will initiate the Wire library and we also need to initiate the serial communication because we will use the serial monitor to show the data from the sensor. In the loop() we will start with the Wire.beginTransmission() function which will begin the transmission to the particular sensor the 3 Axis accelerometer in our case. Then with the Wire.write() function we will ask for the particular data from the two registers of the X axis. The Wire.endTransmission() function will end transmission and transmit the data from the registers. Now with the Wire.requestFrom() function we will requires the transmitted data or the two bytes from the registers. Wire.available() function will return the number of bytes available for retrieval and if that number match with our requested bytes, in our case two bytes, using the Wire.read() function we will read the bytes from the registers of the X axis. At the end we will print the data in the serial monitor. Here's that data but keep in mind that these is raw data and some Math is needed to be done in order to get the right values of the X axis. You can find more details for that in my next tutorial for using accelerometers with the Arduino Board because I don't want to overload this tutorial because its main goal was to explain the I2C communication works.

Channel: How To Mechatronics

Views: 1,517,922

Keywords: I²C, Arduino (Brand), How-to (Website Category), i2c, how it works, i2c communication, protocol, bus, master, slave, address

Id: 6IAkYpmA1DQ

Length: 9min 57sec (597 seconds)

Published: Mon Oct 05 2015