Hi, I'm just trying to understand a few things from your code in the first =
link.

What's the significance of 100Hz with 1 second counting? :)

What is the rb part? I can't find any reference to it in the MPASM help fil=
e. :)

ticks  rb                  ;count IRQs (200 per sec)
dg1    rb                  ;digit1 data
dg2    rb
dg3    rb
dg4    rb
temp   rb

loop   rb
count  rb                  ;digit to show (1 2 4 8)

On Mon, Nov 7, 2011 at 11:34 PM, IVP <joecolquitt@clear.net.nz> wrote:
>> 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 th=
e
> interrupt counter
>
>> 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
> --
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