Rob Stockley <PICLIST@mitvma.mit.edu> wrote:

> PIC:    16f648a with 20MHz xtal
>
> Assuming no prescaling does one tick occur for every cycle?
>
> t(s) = 1 / xtal

    Almost.  The first thing that the PIC does with the 20 MHz clock
    is divide it by 4... So with a 20 MHz clock, each timer tick
    (and each instruction cycle) is:

        t(s) = 1 / (xtal/4)
             = 1 / 5 MHz
             = 200 nanoseconds

> So with prescaler 1:8...
>
> t(s) = ( 8 / 20 * 10^6 ) = 0.4us

    Same thing; you need to divide the clock speed by 4:

        t(s) = 8 / 5 MHz = 1.6 uS

    -Andy

=== Andrew Warren -- aiw@cypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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