18F2520 ADC issue

[Acetronics2]

Hi,

A little thing I do not understand :

I have a 4.096 voltage ref for ADC
Input sees a LM335 , through a 10k/.22µ L.P.F., powered from 5 v with a 2.2k series resistor.
Wiring to lm335 is ground shielded.

Everything has worked fine for 25+ hours ... ( reading Ok ... etc,etc ...)


Now,

An ADC input overflow has occured ( logged by the system ) that should NEVER had happend ... and the ADC result now is DIVIDED by 2 !!!

a ( say many ! ) system reset did not change anything ...


I took the chip off to read its logging values and just rewrote the "wrong logged" data. ( NO change to the program ).


Pic placed Back to its place ...

ISSUE has disappeared... everything fine again.



Soooo ...

Is there some limitation on ADC input voltage relative to the ADC Vref ??? ...

Yess ... Parameter A25 ( Midrange data ) says ... Vref + 0.3v !!! , that seems logical not to input more than what can be read, ... but no such issue specified ...

Alain

[skimask]

I am just wondering if there are some one know how to change c code to picbasic.
Or if some one can help me to write PICBASIC code for LCD03
I also use BASIC.

How did this go from Ace's ADC issue to converting code from C to PicBasic?
If you know C (cut and paste from some website somewhere?), then reading your code in Post#3 and converting that to a similar PicBasicPro program should be easy enough. Looks easy enough to me...but that's just me...


ADC issue - maybe a spike on the A/D input, CMOS latchup or something similar?

[mister_e]

What about if you add some clamping diode, but instead of sending them to Vdd, send them to Vref... well a buffered Vref.

[Acetronics2]

Hi, Steve

Good idea ... I've placed a low-leakage BAW62 ... hope my TL431 ref. will agree !!!

Alain

[Acetronics2]

Hi, pros

One more 18F2520 Out of use after 30 Hours of good work, like the others, and still the same AN2 ADC input leakage current tremendously increasing. ( I repeat here AN1 has EXACTLY the same input circuit : a LM335 fully filtered and clamped : a 10k resistor in series ...).

EVERYTHING has been carefully verified ... Even Microchip gurus didn't have anything to reply ...

another " damaged " circuit has been continuously running for more than a year ... all other analog inputs are still Ok !!!

Has anyone a little idea ???

Date Markings are 07204AY ...

Alain

[Jerson]

Alain

I'm not sure if just powering off and on the PIC would solve your problem. Most likely it would if the input protection got clamped on a spike and stayed stuck(latched). I've seen similar behaviour in other hardware with input protections and the usual way to recover was a power cycle. Is it possible for you to rule that out?

Jerson

[Darrel Taylor]

... and still the same AN2 ADC input leakage current tremendously increasing.

Is there any possibility that the CVRCON register is getting modified somewhere along the way?

Either intentionally ... CVRCON = %11000000

or Unintentionally ... if you have ASM in your program and are in the wrong bank when writing to address B5.

If the CVref is turned on, and the CVROE bit is set, the CVref will output on RA2/AN2/VREF-/CVREF.
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