> Hello, I'm looking for some ideas on how I can start building (the
> software) a race timer. This timer has to count at least 20 seconds
> and be accurate enough to 100th of a second.

First thing to do is choose a crystal or method that will give you a
100Hz timebase

For the timers I build I generally use a 'hex' crystal, something which
divides evenly by 256. For example, 3.276800MHz (=3D 0x320000)

(Sorry about the colours, I really should update these pages)

http://home.clear.net.nz/pages/joecolquitt/2wireled.html

3276800 / 256 =3D 12800 =3D 128 Timer0 interrupts per 10ms. The timer
can be free-running, and the test for 128 is simply looking at bit7 of the
interrupt counter
=20
> An LCD would display the time to 3 decimal places

3 decimal places is better than 0.01, unless you want to measure to
1/1000ths internally and round to the 1/100th

Common alpha-numeric displays are fast enough for 1/100ths but
I prefer to use panel LCDs or LEDs, as they're bigger and faster

This circuit, with a faster crystal, has been used as a 1/1000ths
gated timer

http://home.clear.net.nz/pages/joecolquitt/0sp505.html

This is a counter and 1/10,000ths timer. 10MHz is adequate

http://home.clear.net.nz/pages/joecolquitt/0counter.html

Software for a timer is very simple, simpler than a clock, as it's all
based on decades, ie each digit is 10x the one preceeding it

You might want to think about using a 0 - 20,000 counter and
converting it to 5-digit ASCII for display, rather than work out
each digit individually

http://www.piclist.com/techref/microchip/math/radix/index.htm

Can help you more if you have specific questions

Joe
--=20
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