AN635 Datasheet, PDF (1/6 Pages) Silicon Laboratories – AUTOMATIC BAUD RATE DETECTION

AN635
Si8900 AUTOMATIC BAUD RATE DETECTION
1. Introduction
Universal Asynchronous Receiver Transmitters (UARTs) are a decades-old technology used at relatively low baud
rates for serial data communication in many applications. The Silicon Labs Si890x Monitoring ADC family includes
the Si8900, which has a simple Rx/Tx UART interface. The Si8900 does not have dedicated baud rate selection
pins, but instead uses an adaptive baud rate detection algorithm to synchronize the baud rate between the Si8900
and the master controller. The Si8900 automatically detects and adapts to the host's baud rate to a maximum of
520 kbps. This application note explains how the adaptive baud rate detection is implemented and includes proven
firmware routines for the user's host to establish initial communication.
2. Si8900 Auto Baud Process
2.1. Operation
The synchronization process (Figure 1) begins with the master processor transmitting (at its own standard or non-
standard baud rate) a series of hex bytes to the Si8900, each with a value of 0xAA. Upon receiving the first byte,
the Si8900 defaults to its highest baud rate (520 kbps) and compares the received byte to 0xAA. A received byte
other than 0xAA indicates that the Si8900 baud rate setting is different from that of the master, causing the Si8900
to incrementally reduce its baud rate, then wait for the next 0xAA host byte. This process continues until a valid
0xAA byte is received, indicating the upper boundary of the correct baud rate. The same process is used to
determine the lower boundary of the host's baud rate. With the upper and lower baud rate boundaries known, the
Si8900 then places its baud rate setting in the middle of the high and low boundaries. The Si8900 then sends two
continuous 0x55 bytes back to the host to terminate the master/slave baud rate synchronization process, and
communication between the host and the Si8900 is established. Note that this iterative process also has the
benefit of finding the optimal baud rate, thereby minimizing bit error rate.
Rev. 0.1 10/11
Copyright © 2011 by Silicon Laboratories
AN635