>  I am still trying to get this timer code working. Thanks to those
> that helped last time. I have gone over the datasheet again, but
> still don't understand how the three bits PS0:PS2 set the prescaler
> value. Could someone please explain this too me?

I'll have a go.

Page 9 of the data sheet is useful. It would have been logical if this
table appeared in the timer section of the data sheet as well, but it
doesn't.

To use the prescaler you need only be concerned with bits PSA,
PS2, PS1, and PS0. (PS means PreScaler not PlayStation!).

First you tell it you want the prescaler to work on the timer, so you
clear bit PSA (PSA = 0). The prescaler can be considered a divider
that divides the number of clock pulses into the timer so you can
make it count slower.

The 3 bits PS2:PS0 set the amount of division. There are 8 division
ratios to choose from ranging from 1:2 when PS2=0, PS1=0, PS0=0
up to 1:256 when PS2=1, PS1=1, PS0=1. The table makes this
easy to follow:-

000     1 : 2
001     1 : 4
010     1 : 8
011     1 : 16
100     1 : 32
101     1 : 64
110     1 : 128
111     1 : 256

For example, with PS2:PS0 set to 101 the timer will increment by
one every 64th clock pulse. If in timer mode (T0CS=0) the clock
pulse will be the oscillator frequency divided by 4 (this is 1
instruction cycle). Total division would then be 64 x 4 = 256, so for
example with a 4MHz crystal the timer would run at 15.625kHz. In
counter mode (T0CS=1) the clock pulse comes from an external
signal (RA4/T0CKI).

Remember that you can use the prescaler for Timer0 or for the
Watchdog timer, but not for both at the same time. Bit PSA selects
which is assigned.

Hope this helps a bit.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brown@clear.net.nz

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