>Could someone please give me some help on hoe to work out TMR1H and TMR1L
>so
>that I get an interupt on rollover every 2.275ms on a 16F877.
>
>Does someone have a formula that you can 'plug' values into.

I don't have a formula, but if you break it down into time per instruction,
you can figure out how many cycles you need to roll over.  It is helpful to
do this anyways instead of using a formula, because the old "plug-and-chug"
method rarely leads to an *understanding* of what you're really trying to
do.

At 4MHz, each instruction cycle is 1us.  TMR1L will overflow in 256us.
TMR1H will overflow in 256*256us = 65.535ms, which will be enough for your
purpose.  (If you're using 20MHz, you can use the same theory, except that
TMR1L will overflow in 51.2us and TMR1H will overflow in 13.1ms.  Etc. for
any other frequency.)

2.275ms/256us = 8.8875.....  So, you need a TMR1H of 8 and some extra value
(.8875.....*256) for TMR1L.

8*256us = 2.048ms    2.275ms-2.048ms = 227us.  So, you need a TMR1L of 227us
in addition to your 8 for TMR1H.

Preload TMR1L with 256-227= 29 (decimal) and preload TMR1H with 256-8= 248
decimal to begin your timer.  Then just check for TMR1H.  When TMR1H rolls
over to 0x00, then 2.275ms has passed since you set your TMR1 values.

--Andrew
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