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.
So at 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. The next bit AKC/ NACK is used by the slave device to indicate whether it has successfully received the previous sequence of bits. The device addressing sequence stars with the most significant bit (MSB) first and ends with the least significant bit (LSB) and it’s actually composed of 7 bits because the 8 th bit is used for indicating whether the master will write to the slave (logic low) or read from it (logic high). After this the clock starts and each data bit is transferred during each clock pulse. The start condition occurs when data line drops low while the clock line is still high. Let’s take even closer look at these events. Right after the addressing sequences follows the data sequences as many until the data is completely sent and it ends with a special stop condition. After the first Acknowledge bit in most cases comes another addressing sequence but this time for the internal registers of the slave device. After each 8 bits sequence follows a bit called Acknowledge. So after a special start condition occurs comes the first 8 bits sequence which indicates the address of the slave to which the data is being sent. The data signal is transferred in sequences of 8 bits. Commonly used values for the resistors are from 2K for higher speeds at about 400 kbps, to 10K for lower speed at about 100 kbps. 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. The other line is the SDA line which carries the data.
The SCL line is the clock signal which synchronize the data transfer between the devices on the I2C bus and it’s generated by the master device.
Two wire interface arduino serial#
The two wires, or lines are called Serial Clock (or SCL) and Serial Data (or SDA). How is it possible, a communication between so many devices with just to wires? Well each device has a preset ID or a unique device address so the master can choose with which devices will be communicating. The easy implementations comes with the fact that only two wires are required for communication between up to almost 128 (112) devices when using 7 bits addressing and up to almost 1024 (1008) devices when using 10 bits addressing. 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 a master and multiple slave devices or even multiple master devices. 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. You can watch the following video or read the written tutorial below.
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