REPOST w/ topic tag and updates:


I've looked in the archives, and found a little bit of info.  I've pored
over the datasheet, and can't figure out the problem.

So, I'm trying to use the comparators in the 3:1 mode (001) so three
different levels can be compared against a single input level.

All I really want is a simple way to poll the damn thing to know the
status of the comparator.

Am I right in thinking that, once set up, I can poll the CMCON,C1OUT bit
at will in order to determine the output of comparator 1?  Do I have to do
anything "special" to "initiate" a comparison (as I might expect to do
with an ADC)?

When I set it all up like I think I'm supposed to, and poll the comparator
output bit, and toggling an output accordingly, I can't get the output to
toggle like I would expect.  I am testing the input pins with a meter.  I
can easily verify that the comparator "should" be toggling (based on the
input voltages), but it doesn't.

I don't even understand my problem well enough to explain it, so basically
I'm saying:  "Are there any tricks?  Does anyone have any ideas of what to
look for? " etc.

It looked so simple when I read the datasheet - boy was I wrong.  :)

Thanks

-Steve



UPDATE:

I read the errata for this device, and "mode 1" of the comparator module
is broken in a way that sucks for me.  When CIS=1, AN3 is supposed to be
connected to the - input of comparator one.  According to the errata,

"AN3 is not completely connected to the comarator, resulting in incorrect
behavior."

I'm not sure what "not completely connected" means, but I sure do know
what "incorrect behavior" means.  :)

At any rate, I think this is my problem, and I'm pretty bummed out.  Is
there a drop in replacement (narrow DIP package, 18 pins, w/ comparator
inputs from RA0-RA3) for the 628?  Is there an updated version?

Sniff, sniff.

ALSO:  I noticed that "input impedence should be < 10kOhm for comparator
inputs".  I'm trying to compare voltage levels that change very
slowly.  The input levels are generated from a voltage divider consisting
of a (for example) 10K resistor to ground, 8k resistor to +5, and a 5k pot
in between.  The center of the pot going to the analgo input.

Is this going ot be a problem?

Maybe I should start over.

-Steve

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