The I²C bus needs to be free before any communication starts. The bus is free when both the SDA and the SCL lines are high. The master will detect this before it begins communication. A START bit is a high-to-low transition on the SDA line. Note that the SCL line is still kept high. The SCL line is then pulled low to send out the slave address information bit-by-bit. During data transfer, every bit is put on the bus when the SCL line (clock) is low. The SDA line must be stable when SCL is high. With these signaling fundamentals in mind, let’s look at a complete communication cycle.