Matrix user forums

Hello,

I want two comparators with internal reference voltage for e.g. PIC16F877A
http://ww1.microchip.com/downloads/en/D ... 39582b.pdf
The CVRON Control register gives me the wanted reference voltage (see chapter 13) and the comparator register CMCON (CIS=CM3) tells me which 2 inputs I can use (=0 for RA3 and RA2 or =1 RA0 and RA1). The output of the comparators can be read from CMCON bit 6 and 7 (CM6 and CM7).

Now I want to use these outputs in the software only and preventing them to come on the outputs RA4 and RA5.

Now my question: How do I prevent that the the comparator-outputs comes on RA4 and RA5? Just by setting trisa?
But if I set RA4 and RA5 as output, can I use RA4 and RA5 as ouptput for other reasaons? Or will RA4 and RA5 as output always comparator outputs?

With kind regards

Jan Lichtenbelt

Hi Jan,

I read the datasheet (chapter 13) and they don't seem to say that when using the comparator you can't use RA4 and RA5 as 'normal' I/O.

I'd suggest you to leave RA4 and RA5 as inputs (with trisa) and when you need to output something on those I/O, turn them back to output, but also, disabling the comparator.
Once you're done, set RA4 and RA5 back to input and enable the comparator.

Nicolas

Dear Nicolas

The problem is that I want RA4 and RA5 to use as outputs for other purposes and at the same time I need the 2 comparators (form which I read outputs from CMCON register).

Kind regrads

Jan

The selection of RA4 and 5 as comparator outputs is based on the CM2:0 mode bits.

The modes listed as 'with Outputs' in the Comparator I/O Operating Modes table (Figure 12-1 in my copy of the 877 data sheet) control the I/O. The remaining modes leave the I/O unaffected.

The multiplexers that controls the I/O can be seen in the schematics for RA4 and 5 in the I/O Ports section of the data sheet. This is more informative than the comparator table.

Hi,
I haven't checked that chapter yesterday, but it seems that setting CM2:0 to 010b will allow you tu use the comparator without outputs.
Thanks Sean .

Nicolas