In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Peter Van der Zee wrote:

Hi Tony/PeterM

Your suggestion of un-needed UART baud clock accuracy is bang-on!

In all my software UARTs I run the clock from a multiple of interrupt based ticks, generally based on 20 uSec for 9600 baud. The receiver generally samples every 20 uSec to find the leading edge of the start bit, and then 2 ticks later (the middle fifth of the start bit) locks the data sampler to take a reading every 5th consecutive tick. This samples the data in the 3rd (middle) tick of every data bit.

The transmitter sends a bit every 5 ticks, giving 100 uSec bits instead of the standard 104 usec. To date this has worked perfectly and all UARTs I have connected to have not complained about this discreptancy.

Tighter timing can of course be provided at the expense of some greater complexity and processor overhead.

Using 20 uSec as a base tick gives all other events a nice timing reference.

Cheers,

Peter (pjv)

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