Here is a real-life story about how PC serial port baud rates are _not_
exact!

I had an application that required receiving an async serial data stream of
an unusual structure: 1 start bit + 240 data bits + 2 stop bits. Obviously
no hardware UART in the world is going to handle that. Also obviously, this
will be about 30 times as sensitive to timing error as 8N1 communications.

Well, I built a bit-banged UART based on a PC platform (actually an embedded
PC product). I needed a high-accuracy clock for sampling the input signal
(which I brought in through the CTS signal on COM1). Instruction cycle
counting was out of the question (with caches, poor documentation, etc.).

After a bit of thinking, I realized that if I programmed the UART to operate
at 9600 baud, 6 data bits, 1 start bit and 1 stop bit, that it would be
ready for a new character every 8/9600 of a second, or 1/1200 of a second
(my desired baud rate). So I just kept the transmitter supplied with dummy
characters (which I just let fall of the end of the TX wire), and every time
it was ready for a new one, I sampled the input on the CTS signal.

Well, everything worked pretty well on a desktop machine I started with, but
when I tried using the real hardware I started getting all kinds of comms
problems. I tried a bunch of different machines with varying levels of
success.

Well, to make a long story short, the problem is that the various PC's were
using various clocks for the baud rate. In fact, it seems that common design
practice these days is to find some 'close' frequency on the motherboard
that can be divided down to something approaching the correct input
frequency for the UART chip.  I way errors of as much as 0.2%, which were
more than sufficient to kill my communications.

I solved my problem by recoding, basing my timing on the system timer chip
instead of the UART.

Bob Ammerman
RAm Systems
(contract developer of high performance, high function, low-level software)

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