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SOFTWARE IMPLEMENTATION OF I²C PROTOCOL

by Dariusz Caban

Start ý The Bus ý The Protocol ý The Process ý Conclusions ý Sources and PDF

THE PROTOCOL

The I²C protocol is level-sensitive. The data must be stable when SCL is high. Except for two situations, the state of the SDA line can only change when SCL is low. The exceptions have special meanings (see Figure 2). A 1-to-0 transition signals the beginning of a transfer and is termed as a start condition. A 0-to-1 transition signals the end of a transfer and is termed as a stop condition. The data is transferred in bytes, with the most significant bit sent first. Note that the byte transfer requires nine clock pulses. The transfer of a byteýs bits takes eight pulses, and the ninth is used for acknowledgment. Between start and stop conditions, an unrestricted number of bytes can be transferred.

After a start condition, the byte containing the slave address (or part of the address, when 10-bit addressing is used) and a data direction bit is always sent first (see Figure 3). A start condition can be repeated without first generating a stop condition. This is used to change transfer direction or to address another slave. If there is no acknowledgment from the addressed slave (because it is not connected or performs some internal operation), the master can abort the transfer. Next, if the slave is being written to, it must acknowledge each byte received. Lack of acknowledgment indicates that it cannot accept data. While reading from the slave, the master is also obliged to acknowledge each byte, except the last byte.

The master can communicate with the slave according to several scenarios called transfer formats. For example, there are three possible formats when 7-bit addressing is used. Their descriptions can be found in "The I²C-Bus SpecificationýVersion 2.1," by Philips. [1]

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