8.5.1.4 Protocol

Serial data transfer using the UART interface is performed in byte increments. For each byte that is sent by either the host or the device, a start bit (logic low) is transmitted first, followed by eight bits of data in LSB-first format. A stop bit (logic high) is transmitted at the end of each byte. By using a start and stop bit for each byte, the ADS122U04 can latch each byte and maintain synchronous communication throughout the process.

The ADS122U04 actively synchronizes to the baud rate of the host each time the host transmits a command. Baud rate synchronization occurs when the host transmits the synchronization word (55h) preceding any command sent to the ADS122U04. The host must always transmit the synchronization word first followed by the command byte or bytes. Each byte begins with a start bit and ends with a stop bit, including the synchronization word.

Figure 58 shows the timing sequence for the UART communication. In Figure 58, as an example, there is only one byte for the command and one byte for the readback data. There may be multiple bytes for a command or the data that is read from the device. The protocol takes the 8-N-1 format: eight (8) data bits, no (N) parity bit, and one (1) stop bit.

Figure 58. Example ADS122U04 UART Protocol