Post by thread:[PIC] Strange analog behavior

This is going to be another one of those....really wierd problems. OK, so we have a board design, using a 18F4680, I as the developer have a couple while the client has a couple as well. I develop the code, test it best I can, send it off to him where he dumps it down to his target board to look for issues, etc. 99% of it works correctly, but the analog seems to be really wierd. On my boards, I am measuring the Vin (thru a divider of course) and another analog input from an onboard sensor. I test...send to him, he says....no data, or whats even stranger, the analog data is stuck at 7F. The boards are running off a 3.3V DC/DC module so all is well there. So finally I said....send me YOUR board let me look...sure enough, the analog is hosed so I swap out the chip. I cycle once time thru the code, see good (semi valid) data and the next time....it goes bad and stays bad. Hmmmm. So I change to another chip...3rd time, half the analog works fine meaing the voltage meas! urement is OK but the sensor data is not. I take the chips that didnt work correctly, drop into my board...works just fine. I can now see why the client is so frustrated...nothing I sent him would appear to work right. So I am going to send my board to him...etc...so he can continue testing while I try and figure out what the deal is on his. And yes, I did measure the voltage on the pins and its where I would expect it to be for both Vin and sensor data on the "bad" board. I've done all the visual checks on components to make sure nothing was in backwards, wrong part, etc. All the normal things one would do when first diving into a problem like this. So, making a long story short....anyone ever seen where the PIC A/D gets "stuck" on a certain value and doesn't want to change? I'll have to go back and look at unused pins, but pretty sure that they are all tied to something right now. --------------------------------- Yahoo! Music Unlimited - Access over 1 million songs. Try it free.

>-----Original Message----- >From: spam_OUTpiclist-bouncesTakeThisOuTmit.edu [.....piclist-bouncesKILLspam@spam@mit.edu] >Sent: 19 October 2005 13:31 >To: Microcontroller discussion list - Public. >Subject: [PIC] Strange analog behavior > > >So, making a long story short....anyone ever seen where the >PIC A/D gets "stuck" on a certain value and doesn't want to >change? I'll have to go back and look at unused pins, but >pretty sure that they are all tied to something right now. Is the code somehow clearing the TRIS bits of the analog pins, turning them into outputs? Is the PIC itself OK? Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

I'm not discounting firmware yet, other than I have two boards that have NO issues at all, while two others do. And I can take a chip from one board, drop into the other and it works just fine. Thats why I am looking for some sort of external stimuli that would be present on the bad boards and not the good, but the question is....what to look for that might cause such a condition. Olin Lathrop <.....olin_piclistKILLspam.....embedinc.com> wrote:alan smith wrote: > So, making a long story short....anyone ever seen where the PIC A/D > gets "stuck" on a certain value and doesn't want to change? I have never seen this. It sounds like a firmware bug. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com

alan smith wrote: >I'm not discounting firmware yet, other than I have two boards that have NO issues at all, while two others do. And I can take a chip from one board, drop into the other and it works just fine. Thats why I am looking for some sort of external stimuli that would be present on the bad boards and not the good, but the question is....what to look for that might cause such a condition. > >Olin Lathrop <EraseMEolin_piclistspam_OUTTakeThisOuTembedinc.com> wrote:alan smith wrote: > > >>So, making a long story short....anyone ever seen where the PIC A/D >>gets "stuck" on a certain value and doesn't want to change? >> >> > >I have never seen this. It sounds like a firmware bug. > >***************************************************************** >Embed Inc, embedded system specialists in Littleton Massachusetts >(978) 742-9014, http://www.embedinc.com > > When I run into problems like this I usually take a fresh pic and test some code I really know thats working. In your case I suggest you do the same or write up a new simple a/d code and test it in your customers board, just to make sure its not in the firmware.

How much decoupling do you have on the board and have you checked that the voltage regulator is not going unstable? It sounds a lot like a power supply problem of some sort to me. Richard P On 20/10/05, Robert Young <rwybeakerspam_OUThotmail.com> wrote: > You mention a resitive divider. What is the total impedance seen by the PIC > due to the divider? Can you booger in a buffer op-amp just for testing the > "bad" client board? > > > > Rob > > > -

The power supply is a Artesyn POL, with plenty of decoupling. Thats my first thought...power supply....so I made sure it was in range to start with, haven't looked at noise yet...but maybe I will run that board with the good one's supply (jumper over) and see if that makes a difference. Richard Prosser <@spam@rhprosserKILLspamgmail.com> wrote:How much decoupling do you have on the board and have you checked that the voltage regulator is not going unstable? It sounds a lot like a power supply problem of some sort to me. Richard P On 20/10/05, Robert Young wrote: > You mention a resitive divider. What is the total impedance seen by the PIC > due to the divider? Can you booger in a buffer op-amp just for testing the > "bad" client board? > > > > Rob > > > -

Its been done....as I have taken my chip...dropped it into his board....and visa versa....it follows the board, not the chip. Johan Baarman <RemoveMEbaarmanjTakeThisOuTiki.fi> wrote:alan smith wrote: {Quote hidden}>I'm not discounting firmware yet, other than I have two boards that have NO issues at all, while two others do. And I can take a chip from one board, drop into the other and it works just fine. Thats why I am looking for some sort of external stimuli that would be present on the bad boards and not the good, but the question is....what to look for that might cause such a condition. > >Olin Lathrop wrote:alan smith wrote: > > >>So, making a long story short....anyone ever seen where the PIC A/D >>gets "stuck" on a certain value and doesn't want to change? >> >> > >I have never seen this. It sounds like a firmware bug. > >***************************************************************** >Embed Inc, embedded system specialists in Littleton Massachusetts >(978) 742-9014, http://www.embedinc.com > > When I run into problems like this I usually take a fresh pic and test some code I really know thats working. In your case I suggest you do the same or write up a new simple a/d code and test it in your customers board, just to make sure its not in the firmware.

Alan, My 2 cents worth. If you're using a 10 bit AtoD converter and you constantly get 0x7F back every time, and if the reference voltage is 5 volts, then it sounds like you're measuring a logic low (ie ~620 mv). So, I would check for shorts at the AtoD input pin at the micro. It could possibly be a small string of solder shorting the pin to ground. This sort of thing has happened to me in the past. Another thing it could be is if the PC board is multilayer and the board house didn't do a 'bed of nails' test on it, it could be filaments on one of the inner layers causing problems (ie shorts to ground). Anyway, just a thought. Let us know what the problem was when you find it. Regards, Jim {Quote hidden}> I dont recall the exact impedance, but I know its under the 10K > (which....if you go over, affects the timing of the A/D more than > anything). > > Robert Young <spamBeGonerwybeakerspamBeGonehotmail.com> wrote:You mention a resitive > divider. What is the total impedance seen by the PIC due to the > divider? Can you booger in a buffer op-amp just for testing the "bad" > client board? > > > > Rob > > > --

Sounds like the next options are board fabrication fault or manufacturing fault. Clean (leakage current) and inspect (solder bridges). Rob {Quote hidden}>From: alan smith <TakeThisOuTmicro_eng2EraseMEspam_OUTyahoo.com> >Reply-To: "Microcontroller discussion list - Public." <RemoveMEpiclistTakeThisOuTmit.edu> >To: "Microcontroller discussion list - Public." <piclistEraseME.....mit.edu> >Subject: RE: [PIC] Strange analog behavior >Date: Wed, 19 Oct 2005 13:49:22 -0700 (PDT) > >I dont recall the exact impedance, but I know its under the 10K >(which....if you go over, affects the timing of the A/D more than >anything). > >Robert Young <EraseMErwybeakerhotmail.com> wrote:You mention a resitive divider. >What is the total impedance seen by the PIC >due to the divider? Can you booger in a buffer op-amp just for testing the >"bad" client board? > > > >Rob > > >

Alan, My 2 cents worth. If you're using a 10 bit AtoD converter and you constantly get 0x7F back every time, and if the reference voltage is 5 volts, then it sounds like you're measuring a logic low (ie ~620 mv). So, I would check for shorts at the AtoD input pin at the micro. It could possibly be a small string of solder shorting the pin to ground. This sort of thing has happened to me in the past. Another thing it could be is if the PC board is multilayer and the board house didn't do a 'bed of nails' test on it, it could be filaments on one of the inner layers causing problems (ie shorts to ground). You could use an ohmmeter to compare between the working and non-working boards between the pins on the micro and ground. This might ferret out the problem. Anyway, just a thought. Let us know what the problem was when you find it. Regards, Jim {Quote hidden}> I dont recall the exact impedance, but I know its under the 10K > (which....if you go over, affects the timing of the A/D more than > anything). > > Robert Young <RemoveMErwybeakerEraseMEEraseMEhotmail.com> wrote:You mention a resitive > divider. What is the total impedance seen by the PIC due to the > divider? Can you booger in a buffer op-amp just for testing the "bad" > client board? > > > > Rob > > > --

I didn't read all of the discussion so perhaps I'm completely wrong here. Could it be latch-up by one supply comming up before another , do the analog sources have a powersupply that is up and running before the logic supply. Just a thought Peter van Hoof --- alan smith <RemoveMEmicro_eng2spam_OUTKILLspamyahoo.com> wrote: {Quote hidden}> I'm not discounting firmware yet, other than I have > two boards that have NO issues at all, while two > others do. And I can take a chip from one board, > drop into the other and it works just fine. Thats > why I am looking for some sort of external stimuli > that would be present on the bad boards and not the > good, but the question is....what to look for that > might cause such a condition. > > Olin Lathrop <RemoveMEolin_piclistTakeThisOuTspamembedinc.com> wrote:alan > smith wrote: > > So, making a long story short....anyone ever seen > where the PIC A/D > > gets "stuck" on a certain value and doesn't want > to change? > > I have never seen this. It sounds like a firmware > bug. > > ***************************************************************** > Embed Inc, embedded system specialists in Littleton > Massachusetts > (978) 742-9014, http://www.embedinc.com > --

>I'm not discounting firmware yet, other than I have >two boards that have NO issues at all, while two others >do. And I can take a chip from one board, drop into >the other and it works just fine. Thats why I am looking >for some sort of external stimuli that would be present >on the bad boards and not the good, but the question >is....what to look for that might cause such a condition. ISTR that this is the sort of funny thing that can happen when ANY pin that has an A/D input on it goes outside the VCC/GND limits. i.e. is any of the other A/D inputs floating/tied to some strange voltage on the other boards?

Perhaps....I might have one floating, but I believe it has a voltage divider on it as well. I don't have the schematic in front of me. Now, if I setup to make that pin an output...should eliminate that from being an issue. Now, one thing I did late last night (after taking care of the other days fires) was set up both boards, ran them both independently and looked at the register data from the A/D. Again, good board good...bad board...bad. So I took power from the good board and jumpered over to the bad board and it APPEARS with at least with the voltage divider input, it was no longer getting wierd 7F data but the sensor still was not reading (yes, same exact firmware load). Thus the problem...gotta be power! No wait....thats why you don't do stuff at midnight when you have been up since 4am. The voltage input that was being measured was the INPUT to the DC/DC so by jumpering over power, I eliminated the source. Makes sense that the reading was not consistant, musta been leakage I was measuring. But of course, didnt think about that till this morning...... "Alan B. Pearce" <EraseMEA.B.PearcespamspamBeGonerl.ac.uk> wrote: >I'm not discounting firmware yet, other than I have >two boards that have NO issues at all, while two others >do. And I can take a chip from one board, drop into >the other and it works just fine. Thats why I am looking >for some sort of external stimuli that would be present >on the bad boards and not the good, but the question >is....what to look for that might cause such a condition. ISTR that this is the sort of funny thing that can happen when ANY pin that has an A/D input on it goes outside the VCC/GND limits. i.e. is any of the other A/D inputs floating/tied to some strange voltage on the other boards?