James Fitzsimons wrote:
> Hi all,
> I am about to start playing with 16F628's after so far only using 16F84's.
>
> I have a couple of initial questions:
>
> 1) I have a home made parallel port type programmer that I have been using
> for the 16F84's will this be ok to use with the 16F628's (As I recall the
> 16F84 uses high voltage programming of the flash, will this fry my '628?)

That will work just fine. The 16F628 can use either high voltage
programming (the usual 13V applied to the MCLR* pin) or low voltage
programming (5V applied to the PGM pin).

One thing to watch out for: the low voltage programming pin should stay
at ground during high voltage programming; if it changes state, the chip
will exit programming mode. (This is mentioned in the errata.) If you're
using some sort of simple programmer, you may want to modify it to tie
RB4 to ground. (It's a bit safer to do this with a pulldown resistor,
just in case the PIC actually starts running and makes RB4 an output. If
you're using the INTRC oscillator, that can happen.)

By the way, both the 16F84 and the 16F628 just use the high voltage as a
signal to enter programming mode, unlike EPROM parts, which actually use
a significant amount of current at the programming voltage.

> 2) I am intending to run it with a 20Mhz crystal. Should I be using the HS
> or XT fuses?

Use HS for crystal frequencies above 4 MHz, XT for lower frequencies,
and LP for really low frequencies (<200 KHz). One of the neat things
about the 628, though, is that you can also use the internal RC
oscillator (INTRC), giving you additional I/O pins. It's not terribly
accurate, though; the spec sheet says that the frequency can vary by as
much as 9%, though the actual parts I've used aren't nearly that bad.

The one other big "gotcha" to watch out for when moving from the 16F84
to the 16F628: using RA0-RA3. When the chip powers up, these pins are
set up as analog inputs for the comparator module; if you want to use
them for digital I/O, you have to write 0x07 to CMCON. All the other new
stuff in the 628 defaults to "off", so you don't have to worry about it
until you want it.

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