set_adc_channel, undocumented second parameter?

[nrieper]

Hi,

In general I measured the analog voltages by setting f.e. set_adc_channel(1), set_adc_channel(2) .....

Now I want to measure 5 channels this way and one analog input I want to measure differential voltage across two channels.
In PIC18F67K22.h (thats the actual device I'm using) I found:

// Constants used for second parameter of SET_ADC_CHANNEL() (first parameter is Analog Positive Channel)

[Ttelmah]

The reason that set_adc_channel, is only shown with one parameter, is simply that 99.9% of PICs only have a single input to the ADC selectable. Yours is one of a very few (new) chips that are appearing with the ability to setup differential inputs.

Now, first comment. 'Odds' of a new feature like this actually working for the next few compiler versions are low.....
What you have found, is a 'sign' that CCS are working on it.
New features like this are usually documented in the 'readme' with the compiler, when they first expect people to actually start using them, before they appear in the manual. At present nothing there....

Realistically, I'd simply make a #byte definition for ADCON1, and try setting the negative channel number myself.
I'd also try putting a value into the second parameter, and look at the assembler generated.

The value seems somewhat 'odd', since the Avss selection involves writing zeros to the low bits of the register. 0xFF, doesn't make a lot of sense here...

Best Wishes

[nrieper]

Thank you Ttelmah,

I just tried to use the second parameter. The compiler didn't complain and when I checked the ADCON1 register, CHSN is set to 110, thats the value for AN5, the channel I was setting. So the function seems to work with the second parameter. Next I have to check the result with a signal to the inputs.

[nrieper]

So I tried to set the adc to differential input by using the second parameter:
set_adc_channel(ANALOG_SHUNT_H, ANALOG_SHUNT_L), where ANALOG_SHUNT_H = 11 and ANALOG_SHUNT_L = 5.

Everything works fine, so the undocumented second parameter works for this pic.