On Wed, May 27, 1998 at 08:42:23AM +1200, Paul Gaastra wrote:
> I thought the allowable % error for the baud rate was worked out from
> how far out the clock can be out after all the bits have been clocked
> in. Can anyone point to a document on the web that explains this?

It's pretty simple; after all the bits have been seen, there should
be no more than 1/2 bit phase difference (because after syncing on the
start bit, the data bits are sampled at mid-bit time). If you have 10 bits
(1 start, 8 data, one stop) then the error per bit can only be 1/10th of
1/2 a bit, i.e. 5% of the baud rate. It's wise to aim for a little better
than that for reliability. 4% or less should be quite ok.

> 9600 and 19200 baud).  Why does the book have those crystals when the
> crystals mentioned by contributors to the list, eg 3.6864MHz give

Probably because the crystal frequencies mentioned are readily available
for other purposes, e.g. the 3.579545 something mentioned is a
color-burst crystal for NTSC TV, so they're manufactured in the millions.

Try Farnell for crystals - they have surprisingly cheap crystals that you
can buy in quantity 1.


--
Clyde Smith-Stubbs               |            HI-TECH Software
Email: clyde@htsoft.com          |          Phone            Fax
WWW:   http://www.htsoft.com/    | USA: (408) 490 2885  (408) 490 2885
PGP:   finger clyde@htsoft.com   | AUS: +61 7 3354 2411 +61 7 3354 2422
---------------------------------------------------------------------------
HI-TECH C: compiling the real world.