AN637

4

Rev. 0.1

3. Si8900 Auto Baud Master Controller Algorithm

Figure 3 shows the master/Si8900 adaptive automatic baud rate detection (“auto baud”). The Si8900 baud rate

detection algorithm uses the master's UART baud rate to determine the upper and lower baud rate boundaries and

centers the operating baud rate between these two limits. Adaptive baud rate detection begins with the Si8900

setting its baud rate to its maximum value of 520 kbps and waiting for the master to send a stream of bytes (each

having a hex value of 0xAA) to the Si8900. The Si8900 compares each received byte to 0xAA and decreases the

baud rate when the received byte is other than 0xAA. This process continues until 0xAA is successfully received by

the Si8900, at which time the upper baud rate limit value is stored in Si8900 memory.

Figure 3. Si8900 Auto Baud Process Functional Flow Diagram

The Si8900 then discovers its lower baud rate limit by setting the baud rate to its minimum value of 58 kbps and

again comparing each incoming byte to 0xAA, increasing the baud rate until 0xAA is successfully received and

stored in Si8900 memory. The Si8900 then optimizes the baud rate by centering the operating baud rate half-way

between the upper and lower baud rate limits. The Si8900 then terminates the adaptive baud rate sequence by

transmitting two sequential 0x55 bytes to the master. Master controller C-code firmware examples for Figure 3 are

available in the application note, “AN635: Si8900 Automatic Baud Rate Detection” available for download at

www.silabs.com/isolation.

Master

Transmit

Start

Si8900 (Slave)

Serial Hex Byte (0xAA) Transmission from Master

Decrease 

Baud Rate

?0xAA

Increase 

Baud Rate

?0xAA

No Data

=0xAA

Upper BR 

Detect

Upper BR 

Limit Setting

Maximum 

BR Setting

Lower BR 

Detect

Lower BR 

Limit Setting

=0xAA

Re‐

Transmit

Exit 

Process

Center BR 

? 0x55, 

0x55

Receive

Minimum 

BR Setting

No Data

Exit 

Process

=0x55, 

0x55

Process 

Complete

=0xAA

Store 

Final Setting

Validate

BR

Baud Rate 

Valid

Baud Rate 

Invalid

Reset

Clear Registers, 

Start Over

Terminate

Process

No Data