On Fri, 2 Sep 2011 21:26:10 +0200
Uwe Bonnes <bon@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >>>>> "Jim" == Jim Paris <jim@xxxxxxxx> writes:
>
> ...
> >> results. However with the RS232 pattern you can't easily measure
> >> exact frequency with the scope.
> ^^^^^
> Jim> If you send a continuous string of UUUUU then you'll see the base
> Jim> frequency as a simple square wave.
>
> But the square wave is disturbed by the start/stop bits and the
> turnaround between bytes. So you can estimate the frequency, but not measure
> exact.
>
Well, yes, but... every UART I've ever tested will send the start bit
of an already-queued byte immediately at the end of the requested
number of stop bits. The ASCII value for 'U' is 0x55, so its bit
pattern is <start0>10101010<stop1>. If you set up no parity, 8 bit, one
stop and send UUUUUUUUU then this really does produce a perfect
square-wave. Of course, the scope will report the frequency in terms of
full cycles, so that's half the Baud rate.
Cheers,
Rob
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