Weak pullups failing?

[Melanie]

There is no problem with weak pull-ups on switches...

How are you enabling them?

Apart from OPTION_REG you also have to set them in the WPUA register in this chip.

[flotulopex]

Reading the data sheet of my 16F88, I can't find the way how to enable pull-ups on PORTB one by one.

I don't understand what "latch" means (in french, latch is translated as a kind of lock)?

Bit 7 of OPTION_REG sais: "PORTB pull-ups are enabled by individual port latch values".

Does it that mean you have to enable the pull-ups for all PORTBs and then, if you'd like to disable it for one or another port, to set this port as an output?

There are no pull-ups for PORTA. Is this correct?

[Dave]

flotulopex, Yes that is correct. There are no pullups available for porta..

Dave Purola,
N8NTA

[Melanie]

OPTION_REG.7=0 will enable ALL PortB weak pull-up's for this PIC. You cannot enable or disable pull-up's on an individual basis on this device. You are correct that this PIC doesn't have pull-up's on PortA.

[ardhuru]

There is no problem with weak pull-ups on switches...

How are you enabling them?

Apart from OPTION_REG you also have to set them in the WPUA register in this chip.

Hi Melanie,

(this is a 16F676)
OPTION_REG.7 = 0 ' ENABLE WEAK PULLUPS
WPUA = 255 ' ALL PULLUPS ON

Since a fresh chip works flawlessly, I do not think the issue is to do with the code or configuration. From your mail and the earlier ones, I gather that this is an extremely rare happenning. Just how rare is my worry; often enough to justify external pullups?

Anand

[Melanie]

If this is anything other than a random happening then I'm in deep sh*t, because my top ten best selling designs all use weak pull-up's. Never, ever had a single return that has a fault on a pull-up input - and that's after years of sales...

[mister_e]

mmm, i don't know but you really have a PIC with a faulty internal pull-up?
If you add a external pull-up it works?

Is there any chance somewhere in your code some pin change from input to output? in this case it could disable some internal pull-up but...

Maybe some electrostatic discharge burned it? but if so... i guess it should go deeper than only the internal weak pull-up no? Strange...

[ardhuru]

mmm, i don't know but you really have a PIC with a faulty internal pull-up?
If you add a external pull-up it works?

Is there any chance somewhere in your code some pin change from input to output? in this case it could disable some internal pull-up but...

Maybe some electrostatic discharge burned it? but if so... i guess it should go deeper than only the internal weak pull-up no? Strange...

Yes, I think that last part should be it. I have an ICSP arrangement, with +5 to the target being supplied exclusively from an external supply. At times, when I'm not working on the project, this supply is switched off. And the programmer (an Epic clone), still connected to the PC, keeps the Vcc (not an issue in this case) *and* Vpp high, unless the programmer s/w (WinPic800) is initialised (loaded). The Pic therefore is exposed to Vpp without getting Vcc.

I think this kind of abuse might have caused the damage.

And yes, I confirm that external pullups work.

[malc-c]

Do you have a 0.1uF capacitor across the supply lines as near to the PIC. I found that without this I would sometimes have strange issues with the way a PIC would behave. Maybe this has some bearing on the problem here ?