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Rev. 0.1 10/11 Copyright © 2011 by Silicon Laboratories AN635 AN635 Si8900 A UTOMATIC B AUD R ATE D ETECTION 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, ea ch with a value of 0xAA. Upon receiving the first byte, the Si8900 defaults to its highest baud rate (520 kbps) and compares the received by te 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 ne xt 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 estab lished. Note that this iterative process also has the benefit of finding the optimal baud rate, thereby minimizing bit error rate.
Figure 1. State Diagram for Auto Baud Rate Detection and Setting The upper and lower baud rate limits are discovered through the iterative process described in “2.1. Operation”. the RESET state, which clears both limit settings and restarts the process.
- Programming the Master Controller
master to re-send a command that was not properly received by the Si8900. ensure proper communication with the Si8900. Figure 2. Master Controller Auto Baud Rate Detection and Setting Routine
Rev. 0.1 5 4. Related Documents Silicon Laboratories, Inc., Si8900/1/2 data sheet AN637: Configuring the Si890x Master Controller AN638: Isolated ADC Facilitates Low Noise, Precision Measurements
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