Damage on uC from continuity test ?? - Page 1
Forum section: electronics / projects

author: Mat
author_rank: Contributor
timestamp: May 12, 2013, 11:11:14 am
content: Hi,I just had to replace the uC (dsPIC30f6010A) from this prototype board I'm working on because of a fried GPIO output buffer...Is it possible that the damage was done by probing the pins for continuity against ground or other adjacent pins with a Fluke 23 III multimeter (unpowered circuit of course)?And while I'm at it : what's the difference between the 23 III and the 70-series multimeters ?Thanks in advance guys,M.

author: Psi
author_rank: Super Contributor
timestamp: May 12, 2013, 01:47:27 pm
content: Good question, I'm interested to know what the official answer isMany years back i had a plasma display driver IC stop working immediately after I probed pins with DMM on continuity range.I still don't know for sure that's what caused it to fail but i've always suspected it was damage from the DMM.

author: kizzap
author_rank: Unknown
timestamp: May 12, 2013, 02:24:30 pm
content: Not 100% sure on this, but depending on the uC and the DMM, is it possible that the continuity setting put out a voltage that was higher then the allowable voltage on the pin?-kizzap

author: Dave
author_rank: Super Contributor
timestamp: May 12, 2013, 02:34:35 pm
content: Try measuring the voltage on the open contacts of the multimeter on continuity test. Post the result.

author: mikes
author_rank: Regular Contributor
timestamp: May 12, 2013, 02:58:22 pm
content: Manual for 23-III shows open circuit continuity test voltage <3.1 V and a short circuit current ~400 uA. That's unlikely to have caused a problem.

author: KJDS
author_rank: Super Contributor
timestamp: May 12, 2013, 03:02:37 pm
content: I've fried stiff before when probing just by clumsily bridging a port to ground or supply line.

author: Neilm
author_rank: Super Contributor
timestamp: May 12, 2013, 03:45:07 pm
content: I have seen this happen as well. Damage can happen as you are effectively taking the GPIO port above the power rail (as it is turned off at the time) 400uA isn't much but it can be enough to damage the protection circuit in this case. Generally, what is in chips is designed to stop very small ESD events and not much more.Neil

author: FrankBuss
author_rank: Unknown
timestamp: May 12, 2013, 04:05:38 pm
content: I don't think that the low current of the multimeter can destroy it. But the datasheet of the dsPIC30f6010A says: "The analog inputs have diodes to VDD and VSS as ESD protection.". Does this mean, that the digital inputs don't have ESD protection? Then this could be the reason, when touching a contact with a finger.I've not used dsPICs so far, but the PIC 16F and 18F series and never had such a problem, even when I bridged incidentally a GPIO to VDD or VSS.

author: Neilm
author_rank: Super Contributor
timestamp: May 12, 2013, 06:20:05 pm
content: ESD damage doesn't always result in immediate failure. It can just reduce the life of a component so it will fail later.Quote from: FrankBuss on May 12, 2013, 04:05:38 pmBut did you take them above VDD? That is when damage can happen.Neil

author: FrankBuss
author_rank: Unknown
timestamp: May 12, 2013, 07:17:50 pm
content: Quote from: Neilm on May 12, 2013, 06:20:05 pmI've just tried it the hard way: connecting GND to one of my circuits with a PIC18F252, then applying 3.1V with a 1k series resistor to each pin for some seconds each. The circuit is still working. There were other chips connected as well, but the voltage at the pins was still between 1V and 2V, so I guess not much ESD protection, otherwise one of the ESD protection diode would have reduced the voltage to 0.7V or below (but I don't know much about the analog side of chips). So I would say it is no problem. But to be sure that it doesn't reduce the life time of the chip, someone could ask the Microchip technical support, usually they are very helpful.

author: SeanB
author_rank: Super Contributor
timestamp: May 12, 2013, 07:22:02 pm
content: Many IC's only have limited ESD protection, small feature sizes limit the amount of energy they can handle. You might have just had enough static charge on you dischargeds through the junction to kill it.

author: jimmc
author_rank: Frequent Contributor
timestamp: May 13, 2013, 03:41:59 pm
content: Looking at p177 of the dsPIC30F6010A data sheet (http://ww1.microchip.com/downloads/en/DeviceDoc/70150E.pdf)under Absolute Maximum Ratings it givesInput clamp current, IIK(VI< 0 or VI> VDD) .........±20 mAOutput clamp current, IOK(VO< 0 or VO> VDD) ...±20 mASo the 400uA from the meter was well within permissible limits; as has been said ESD is the most likely culprit.Jim

author: Mat
author_rank: Contributor
timestamp: May 13, 2013, 09:03:56 pm
content: Thanks a lot everyone!Yeah, I'm leaning towards ESD too now that I think about it. I'm 99% sure I was wearing the ESD bracelet thing but could have forgotten....M.

author: Baliszoft
author_rank: Frequent Contributor
timestamp: May 13, 2013, 09:36:09 pm
content: dsPIC30F withstands a lot more abuse than the specified 20mA input clamp. I have tested up to 800mA without damaging the pin itself.