| ??? 04/04/05 21:27 Read: times |
#91002 - Baud rate errors Responding to: ???'s previous message |
Andy Neil said:
Note that Keil recommends a error of less that 1% in the actual baud rate:
http://www.keil.com/c51/baudrate.asp So, for a example, for a nominal 9600 baud, your actual value needs to be between 9504 and 9696. Sasha Jevtic said:
1% seems pretty conservative; some Dallas documentation states 3% tolerance Which Dallas doc was that? Did it justify this figure? Can you give a link? I can't justify the 1% - I just quoted it straight off that Keil page! I think the receivers in UARTs tend to use a clock at 16x the baud rate to sample the incoming data; they take 3 samples where they think the centre of the bit should be, then do a "majority decision" to decide the bit value. So you should be able to work out the effects of baud rate error by considering: 1. The effects of errors on determining the Start bit, and 2. The cumulative effects of errors over all 10 bits (Start + 8 Data + Stop). Don't forget that a Start bit may immediately follow a Stop bit (especially from a fast PC with FIFOs in its UART) - so, if too much error has accumulated by the Stop bit of one character, the UART might miss the Start bit of the next... |
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