What would cause some random pins of an MCU to fry? (circuit protection) - Page 1
Forum section: electronics / projects

author: gndloop
author_rank: Newbie
timestamp: December 05, 2015, 02:25:53 pm
content: We have an ARM based ASIC chip that is powered by a step-down SMPS (24VAC to 3.3DC) and from time to time, some IO pins of the MCU stop working.For example, the JTAG pins still work and you can put new firmware on the chip, but the SPI pins can't drive the LCD or in another case I2C reads all zeros from all sensors. but the MCU core processing has always remained intact.My best guess is that transients make it through the rail and are just high enough voltage to blow some internal clamping diode, etc.The question is how do you protect against such failure modes? A TVS on the 24VAC side? A 3V3 zener on the 3.3 side? How is this type of circuit protection done properly?Thanks!

author: DanielS
author_rank: Frequent Contributor
timestamp: December 05, 2015, 02:36:25 pm
content: Your buck regulator, combined with its input and output filter caps, should already be enough to handle typical input transients unless your source is susceptible of producing exceptionally nasty ones, as may be the case with "load dumps" in automotive applications.If the microcontroller is used in an environment where high ground/neutral currents may exist, another possibility to look at is ground voltage differences between the MCU and the external stuff it is connected to.

author: Jeroen3
author_rank: Super Contributor
timestamp: December 05, 2015, 02:45:39 pm
content: What is the startup/shutdown bahavior of the smps like?I'm not a big fan of running the mcu directly from the primary supply. When I can, I add a small 5 to 3.3 LDO, it will take away most of the ripple, and commit suicide if the primary supply fails.

author: RobK_NL
author_rank: Frequent Contributor
timestamp: December 08, 2015, 01:35:38 pm
content: Quote from: Jeroen3 on December 05, 2015, 02:45:39 pmI've had suicidal LDO's that failed with shorted in- and output pins ...

author: Fungus
author_rank: Super Contributor
timestamp: December 08, 2015, 01:43:56 pm
content: I'd have a very close look at grounding between the MCU and whatever it's connected to.Edit: Also measure current through the ground connectors/wires, see if any of there's a noticeable current flow anywhere (ground loop).

author: DanielS
author_rank: Frequent Contributor
timestamp: December 08, 2015, 06:26:31 pm
content: Quote from: RobK_NL on December 08, 2015, 01:35:38 pmThat's how most linear regulators I have seen fail failed too, in-to-out short/low-Z.

author: station240
author_rank: Unknown
timestamp: December 08, 2015, 07:06:01 pm
content: My guess is it's common ground from the negative side of the rectified 24VAC to the +3.3V output. Hence surges can get into the ARM via clamping diodes, pull down resistors or external devices (eg your LCD).Have you checked what the power on state of those ports is ?Also consider adding a "power good" circuit, so the micro doesn't run when the power is glitchy/low/high.

author: poorchava
author_rank: Super Contributor
timestamp: December 08, 2015, 07:43:24 pm
content: You could try adding resistors in series with MCU pins that have been failing and if that doesn't help, also low Vf Schottky diodes to supply rails.Sent from my HTC One M8s using Tapatalk.

author: Nauris
author_rank: Regular Contributor
timestamp: December 09, 2015, 09:59:29 am
content: QuoteCustom ASIC? My gut feeling says that is the prime suspect. Maybe you have a wirebond problem or something else screwed up. Have you measured non-working pins? Are they shorted to rails or open or what?

author: krivx
author_rank: Frequent Contributor
timestamp: December 09, 2015, 10:52:53 am
content: Quote from: Nauris on December 09, 2015, 09:59:29 amYeah this is a red flag for me too. Do you get the same values under "perfect" conditions, like running from a linear supply? Can you induce one of these failures?

author: tszaboo
author_rank: Super Contributor
timestamp: December 09, 2015, 11:07:56 am
content: I've had a legacy Infineon MCU, which got 24V on some of the pins (maybe use different connector next time DOH!). I clearly saw a bit of the MCU explode into air, a small part of the package shot some half a meter away.After that, the MCU was still looking functional, it was executing code. It was running very hot, but minutes after it seemed operational. The pins and parts of it was fried of course.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 09, 2015, 11:57:35 am
content: Energy limit the +3.3V by putting a small resistor in series from the switcher output to the actual processor +3.3V. Clamp that side with Zeners. Look into nice ones like ON Semi 5W Surmetic. Run the switcher feedback from the +3.3V side of the current-limiting resistor.Are any of these pins connected to cables that lead off-board?Are any of these pins connected to parts that then connect to cables that lead off-board?

author: Jeroen3
author_rank: Super Contributor
timestamp: December 09, 2015, 03:02:30 pm
content: Please notice how "gndloop" did not reply to any of the above comments yet.