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'External Clocking of 16F84'
1999\04\13@232215 by Rob

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Hi all,

I need to externally clock a 'F84-04 with a 4 MHz clock. Does the duty
cycle of this clock need to be exactly 50%. I cannot find any reference
to Duty Cycle requirements in the data sheet, and my feeling is that it
is not critical????

Also when programming the PIC I assume I still select "XT" for the
crystal mode when  supplying an external clock into the CLKIN pin??

thanks for your help
Rob

Perth
Western Australia

1999\04\14@010638 by Gerhard Fiedler

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At 11:23 04/14/99 +0800, Rob wrote:
>I need to externally clock a 'F84-04 with a 4 MHz clock. Does the duty
>cycle of this clock need to be exactly 50%. I cannot find any reference
>to Duty Cycle requirements in the data sheet, and my feeling is that it
>is not critical????

they specify a minimum high and low time. as the minimum frequency can be 0
(dc), ie. the maximum high or low time approach infinity, i guess there
can't be a duty cycle req. beyond the minimum hi and lo times.

ge

1999\04\14@102424 by Matt Bonner

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Rob wrote:
>
> I need to externally clock a 'F84-04 with a 4 MHz clock. Does the duty
> cycle of this clock need to be exactly 50%. I cannot find any reference
> to Duty Cycle requirements in the data sheet, and my feeling is that it
> is not critical????

Look for the spec in a table labelled "External Clock Timing
Requirements" or something similar.  There is no "duty cycle" spec, but
one for a minimum "external clock in high or low time".
>
> Also when programming the PIC I assume I still select "XT" for the
> crystal mode when  supplying an external clock into the CLKIN pin??
>
You can set it to either XT, LP, or HS.  All this fuse does is set the
value of the internal feedback resistor between OSC1 and OSC2.  Since an
external clock is only fed into OSC2, the feedback resistor is not
used.  However, you'll get lower power operation with the fuse set to LP
(higher feedback resistor value).  The fuse setting *will* affect the
minimum external clock input high/low time spec, though.

--Matt

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