My 12lf1822 ADC won't read zero (lowest reading is 008)

[HankMcSpank]

well, I got to the bottom of it...something whacky going on here!

It's something to do with the way I'm setting the weak pullup register....

Firstly, I noticed that when I disabled weak pullups (I just took a flier!)

OPTION_REG.7 = 1 ' disable weak pullups

My ADC started reading down to zero as it should!

Sure enough when I enabled weak pullups again, the problem returned.

But this is where I’m obviously doing something wrong, this way actually works…

(I want weak pullups of RA3)

WPUA  = %00001000 
IOCAN = %00001000 

…ie even with weak pullups enabled my ADC reads down to zero.

But if I do this….

WPUA.3 = 1
IOCAN.3 = 1

My ADC won’t read down below 008 again!

So to get ADC reading down to zero *with* weak pullups enabled on a 12LF1822, it must be set like this....

OPTION_REG.7 = 0
WPUA  = %00001000 
IOCAN = %00001000 

Not sure if it makes any difference but my weak pullup is on the MCLR pin.

Anyway, I'm happy - it's sorted!

[mister_e]

That's interesting. I also thought the WeakPullups could do something but never had this problem before. Nice finding.

So I guess doing the ADC while sleeping would also have do the trick.

Since it's working... don't fix it

[Darrel Taylor]

Nothing whacky there.

The WPUA register defaults to %00111111 on power-up.

If you do WPUA.3 = 1
nothing changes, because that bit is already a 1.
You have to disable pull-ups on the pins you don't want pulled-up.

The numbers over each bit show the POR/BOR defaults.

[HankMcSpank]

For once you' re wrong (& I've got you on a minor technicality!) - there was something whacky - ME!

I guess (me aside) the other whacky thing was even with weak pullups enabled ...my ADC pin still functioned.....albeit only down to a reading of 008 (whenever I've set an ADC pin wrong prior...I've had very "something oviously wrong with my config" type results ...this one was more obscure!)

Anyway, apologies for the schoolboy error....

[mister_e]

must do mistake from time to time... this is the best way to learn