I have had no trouble with 3 samples per bit when the baud rate error is
nominally zero and the communication link is clean.

I require two consecutive samples to detect the start bit.
I then sample the other bits every three samples after the second start bit
detection sample.

Bob Ammerman
RAm Systems

----- Original Message -----
From: "M. Adam Davis" <adampic@UBASICS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, June 06, 2003 12:55 PM
Subject: Re: Software UARTs (was Zilog Encore series microprocessors)


> Tim McDonough wrote:
>
> >2) If you did not have an external interrupt available to sense the start
of
> >a byte, how many times the bit rate would you need to sample/poll for
> >reliable receive? 4X?
> >
> As with nearly everything else, the correct answer is "It depends".  On
> my last stint in bit banging a full duplex rs-232 port I used 4.  It is
> possible to do it with three, but I ran the numbers and found that my
> maximum bit rate error coupled with the maximum error introduced by
> using three may result in missing the last bit (stop bit - reading too
> early or too late).
>
> I'm certian that with a crystal that could be divided down correctly you
> could get great reliability with three.  You would receive the start
> bit, wait four cycles for the first read, and then read every three
> cycles thereafter.  The earliest the reads would occur is 1/3 into each
> bit, and the latest is 2/3 into each bit.
>
> So the correct answer is - run a simulation at the maximum possible
> error and see how much error there is at your given N.
>
> -Adam
>
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