Bob.A wrote:
>Mike,
>
>In many respects the F628 is just a better F84. There have already been a
>few threads on the gotcha's when subbing the former for the latter.
>
..........
>> place....  Anyways, I've been gearing up for quite some time on a project
>> that will use a digital frequency readout.  I've seen alot of existing
>> plans/code  for frequency counters with displays that use pre-scalers.
.......


Mike, the biggest advantage of the new 'F628 over the old '84 is
the presence of Timer1, which can be used for external counting with
16-bit resolution. The '84 contains only 8-bit Timer0, which can
be used in conjunction with the prescaler to do 16-bit counts,
but you have to play some games to do it. See Mchp appnote AN592.

With the 'F628, Timer1 in conjunction with the prescaler can give
you much more range and resolution.

However, if you look in the datasheet for the 'F628, Table 17-7,
you will find info that is almost totally inscrutable. It indicates
that Timer1 will count to about 16 mhz, 1/60nsec, in aynchronous
mode. In fact, when using the prescaler, it will count much faster
than this. Personally, I think the datasheet is wrong, and the
prescaler can go at least 50 mhz, 1/20nsec, like for Timer0.

At any rate, if you use an external prescaler ahead of the PIC,
you can really fly. I use a 74AC74 [which is good to ~180 mhz,
IIRC] in one app, and count to over 120 mhz.

- dan michaels
www.oricomtech.com
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