On Fri, 2001-11-23 at 12:15, Bob Barr wrote:
> On Fri, 23 Nov 2001 15:40:10 +0100, you wrote:
>
> >
> >Note that there is no law that you must send the bits in a loop: with some
> >calculation (and enough code space) you can unroll the loop and be up to 1
> >instruction accurate.
> >
>
> True, but it doesn't make much difference whether you write your code
> as a loop or unroll it. Either way, the baud rate cannot be made any
> more accurate than the 1 instruction time granularity. Whether your 1
> instruction accuracy is in a loop or in-line code, the instruction
> time determines how close you can get to the desired baud rate.
>
> Using a 10 MHz or 20 MHz crystal, by definition, introduces some
> amount of baud rate error.
>
> Using the 'oddball' 9.216 MHz crystal (or 18.432 MHz as someone else
> suggested. How did I forget that one?) will allow you to minimize the
> error.
>
> Regards, Bob
>
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>
>

By unrolling the loop, you can make the character timing accurate within
1 instruction cycle vs the bit times being accurate within one
instruction cycle ( at the expense of some non cummulative bit timing
jitter ). In this users case, with 1 start, 16 data bits and 1 stop bit,
the character timing can be made 18 times better by unrolling the loop.
This makes just about any crystal usable ( faster is better ).
cheers
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