Comparator and unused pin?

[hmmpic]

Hi

When using a chip with 2 internal opamp as comparators and I only use one of them I have understanding problem. I only use 18F4550 for illustrate my confusing.

How to handle unused pin, is they disconnected from the real pin?
If using internal VRef as CVREF, will the VIN+ on both comp be disconnect from the input pin?

I use the config to the right on the picture (CM2:CM0=110), internal VREF as CVREF.

I have confusion to understand how to handle the pin on the PIC. Normally I just use an external opamp. This time I will use one of the new smart chip with included hardware as opamp.

http://s000.tinyupload.com/index.php?file_id=00461571666128813305

[Ttelmah]

You start with some confusion. The 4550, does not have an internal op-amp. It only has comparators. There are similarities between the devices (you can make an op-amp behave as a slightly less than optimal comparator), but the key is that the comparator output, is designed to be digital, not to 'linger' in the transition area, while the op-amp is designed to be linear across this region dependant on it's gain and what the input is doing. The comparator is also designed to recover quickly from being driven to the rails.
Have a look at:
<rohmfs.rohm.com/en/.../amp.../opamp_comparator_tutorial_appli-e.pdf>

So, ignoring this, and talking about the comparators, start by looking at the port I/O summary (Table 10-1). Note the order of priority. Note that a TRIS setting of '0' on any of the pins to the comparator inputs (setting it as a digital output), _overrides_ it's use as an analog input. However also note that if you try to use the pin as a digital input, or if the pin is set as an analog output, the digital TRIS settings are overridden.

Then step to figure 22-1. This shows all the comparator configurations. The only ones that allow CVref to be used as one input, are 000, 010, 011, and 110. The one of interest is 110, with CIS=0. If you use this, and use AN0 as the input to comparator 0. Then you can still use RA1, _as a digital output_ only. The TRIS will then override it's analog function.

If you want an internal op-amp, then there are chips with these (mainly a few of the newer PIC16's). The configurations on these are very different from those on the comparators. If it's an op-amp you want, then you need to be looking at these, rather than the comparators. If you look at a chip like the PIC16F1703, this has two genuine op-amps, and you can separately choose between the external pin, and the voltage reference on the +ve input, and between the external pin, or the output on the -ve input. Very different....

[hmmpic]

Just a fast reply to correct some confusion. At the moment I only want the Comparator function! I call it an opamp...I mean Comparator.
I will read and try to understand the newer PIC hardware later, thanks. As you suggest, it is the 16Fxxxx I looking at, but have not made a conclusion.

[hmmpic]

In the data sheet for respective comparator interrupt there is noted that

Direct from the data sheet.

[Ttelmah]

All interrupts involving an event like this, require you to clear the event. After all unless you have, the even is still waiting to be serviced.
So in serial RX interrupts, you must read the byte received.
In 'interrupt on change' interrupts, you need to read port B.
etc. etc..

[hmmpic]

Of cause, don't know what I was thinking...