On Sunday 02 February 2003 09:22 pm, you wrote:
> At 02:04 PM 2/3/2003 +1100, you wrote:
> >I did post an answer yesterday, the problem is NOT the
> >comparator, it's the fact that you are doing successive
> >approximation using a PIC OUTPUT pin! The output pin
> >is not capable of going rail to rail, there is a sat
> >voltage loss of 0.3v to 0.8v or so depending on current
> >and which PIC and which type of output, TTL or ST.
>
> I don't know his circuit, but the outputs of a PIC should swing to
> within a few millivolts of either rail if not loaded- and there's no
> such thing as a ST output.

I believe that Roman is misunderstanding my problem.  I'm not having=20
trouble with the output voltage of the PIC, that's working as expected. =20
At the bottom of this message is an ascii-art schematic, use a fixed=20
width font to view.

I have RA3 (output) connected thru a 4.7K resistor to a 1uF cap.  One=20
comparator input is connected to the junction of the resistor and the=20
cap, to monitor the DC charge present.  There is no further load on=20
that part of the circuit. =20

RA3 is used to charge/discharge the cap based upon feedback from the=20
comparator.  RA3 has no trouble pushing the cap voltage to very near 5=20
volts, nor does it have any problem lowering the cap voltage to almost=20
zero.  IOW, I'm not having any trouble getting rail to rail voltages=20
out of the pic pin for charging the cap thru the resistor.

The other comparator input is connected to a pot wiper.  When the pot=20
wiper's voltage is below 300mV, the comparator seems to percieve that=20
as being less than the other input which is almost 0V.  This causes my=20
program to stop charging the cap completely, by not turning on RA3 at=20
all, even though the comparator output should be a 1.  It's as if the=20
comparator can't handle the inputs being driven near the rail voltages. =20

If the pot wiper is above about 4.5 volts, the opposite problem occurs. =20
The cap voltage suddenly increases to 5V as my program (which simply=20
copies the comparator output to RA3 starts writing nothing but ones to=20
the pin, apparently never seeing the fact that RA2 is now < RA1.  The=20
datasheet does say that I should keep the input below 3.5V (Vdd -=20
1.5V), but it also says that I should be able to go to the lower rail.

If I were using a revision A chip, would it be marked with an actual=20
letter 'A'?  The chip I'm using is not so marked.

Here is some ugly ascii art of the schematic.  Obviously it has been=20
simplified for my convenience, but it shows all the components of the=20
feedback loop.=20


+5V     |-------------
|       V            |            16F628
|____/\/\/\/\___     |         __________________
                |    |___RA2__| Comp +   Comp - |___RA1___
       10K      V             |                 |        |
               Gnd     __RA3__| Charge Pin      |        |
                       |      |                 |        |
                       |      |_________________|        |
                       |                                 |
                       |                _________________|
                       |                |
                       |____/\/\/\/\____|___|(______
                                                   |
                              4.7K         1uF     V
                                                  Gnd

Thanks for any advice you may have.

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