The bus master first must send the PCA9554 address with the least-significant bit set to a logic 0 (see Figure 8-6 for device address). The command byte is sent after the address and determines which register is accessed. After a restart, the device address is sent again but, this time, the least-significant bit is set to a logic 1. Data from the register defined by the command byte then is sent by the PCA9554 (see Figure 8-10 and Figure 8-11). After a restart, the value of the register defined by the command byte matches the register being accessed when the restart occurred. Data is clocked into the register on the rising edge of the ACK clock pulse. There is no limitation on the number of data bytes received in one read transmission, but when the final byte is received, the bus master must not acknowledge the data

Figure 8-10 Read From Register

This figure assumes the command byte has previously been programmed with 00h.

Transfer of data can be stopped at any moment by a Stop condition.

This figure eliminates the command byte transfer, a restart, and slave address call between the initial slave address call and actual data transfer from P port. See Figure 8-10 for these details.

Figure 8-11 Read From Input Port Register