Latchup? - Page 1
Forum section: electronics / projects

author: Poe
author_rank: Frequent Contributor
timestamp: August 05, 2014, 04:14:36 pm
content: I'm a bit stuck trying to troubleshoot a test fixture issue and could really use some help.The fixture is attempting to check a Microchip 23K256-I/ST on the DUT by writing/reading some data over the SPI buss. It does other things, but this is the issue area.The communication looked good and clean when we initially designed the fixture. 98 assemblies passed test. Two failed with an intermittent SRAM issue. Technicians replaced them concluded a bad IC.A month later we tested 1k assemblies. ~10% of assemblies constantly failed this test SRAM step. I could find no apparent build issue. I can't rule out a damaged SRAM IC since we have no new ones yet and the passing 90% was shipped out.Supply levels and dataintothe IC look good. Although when it tries tooutputdata on the SO line, it appears to latch-up.I'm guessing it's a latch-up condition because the SO line spiked up briefly then fell as the supply voltage oscillated. This would happen each time the SPI clock transitioned negative (That's when the IC initiates output data). I lifted and probed the SO pin. Everything looked great when connecting the scope directly to the pin. The output transitioned cleanly low to high and high to low. The power supply ringing was gone and the data was correct.Although when I soldered a small bit of wire to the pin (still not connected to anything else) and connected the other end to my scope, I got similar results to when this line was connected to the test fixture.The SO pin output would simply spike during each negative transition of the SLCK line and a noticeable spike would be seen on the supply voltage line.I thought it was an issue with poor grounding or decoupling, but they both look good.I thought it was some kind of reflection on the line causing a latch-up from over voltage. That kind of thing is a step beyond my knowledge level though.I connected a 13k ohm resistor from the SO pin to ground and another 13k ohm resistor to Vdd. Both connected at the end of the wire. Unfortunately that had zero effect. I did the same thing on the test fixture with no results either.We have some new SRAM ICs coming in, but regardless of if these work or don't.... I'm not sure what the next step would be.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 05, 2014, 08:24:58 pm
content: So, are you driving an implausible load with the SO pin? A long line, a capacitance you'd forgotten about, or another pin set to be an output (possibly transiently, for just a single cycle, as it thinks the bus is turning around?)(At a wild guess, it's more likely to be a second 'clock' edge turning up because of ground bounce than actual latchup, too).Also, time for some super-paranoid checking of power, ground and decoupling. Are your planes working right? Thermal reliefs not stopping copper being where you think it is?Does strapping extra grounds and decouplers onto a failing board, right at the chip, make any difference? Does bodging a 33R series launch resistor between the SO pin and the pad make any improvement?It does seem unlikely your RAMs are actually duff...

author: free_electron
author_rank: Super Contributor
timestamp: August 05, 2014, 08:35:17 pm
content: i smell a device that has a missing ground connection....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2014, 10:15:01 pm
content: How long is the connecting trace, and what all connects to it?What does the supply and ground look like? Any chance of a pic of layout?TIm

author: Poe
author_rank: Frequent Contributor
timestamp: August 06, 2014, 03:03:10 am
content: Thank you for the feedback. It is appreciated.I'm at home right now. I'll try to get some images/illustrations tomorrow.Precipice & T3sl4co1l:Although the SRAM operates correctly when not connected to anything and exhibits this 'latch-up' behavior when the SO line is connected to my test fixture, it also fails when the SO line is ONLY connected to a 3 inch wire-wrap wire. I lifted the pin and simply soldered a long wire to it. I cut off 1inch sections until it started to work. Anything less than ~2.5" and it operates correctly. The SRAM bypass cap is 25V 0.1uF (verified and even replaced several times).The board is simple and very small (2" x 1"). It has two SMPS (12V and 3.3V), four SPI SRAM ICs, and one SPI/Ethernet IC. In the field, it is connected to two other boards via headers. One board has a microcontroller/crystal/caps, the other board simply has terminal blocks. My test fixture attempts to simulate the microcontroller board. In the field, the SPI buss length is about three inches across the DUT through the header then into the microcontroller. In my test fixture, this is about four inches.24Vdc is supplied by the test fixture to the DUT through one of the headers. The DUT's 12Vdc SMPS feeds the 3.3Vdc SMPS. There are two solid internal power planes, no traces cutting them in half. One for ground and one for the 3.3Vdc. The SRAM ICs are about a half inch away from both SMPS on the bottom side. The decoupling caps are on the topside, directly above. They can't get any closer to the SRAM ICs. I wouldn't think them being on the opposite side of the board would cause an issue? Tomorrow I'll solder a cap directly to the leads.I have removed the Ethernet/SPI IC and all but one of the SRAMs from the DUT. The decoupling cap was replaced and I verified it and the SRAM were actually connected to the appropriate planes. I was worried that the DUT to tester connection was to blame, so I checked it several times. I verified the SO line was not shorted to an internal plane and even removed parts so I could visibly trace it's full length.I will recheck all of this again tomorrow with a fresh mind.Although the SRAM works when the SO line is only connected to the scope, could it still be damaged? If the tester to DUT connection was faulty and some of the chip select lines floated, causing multiple ICs to transmit on the SO line... I can see how that could cause damage. Although, I would think that, in that case, the SO line would be permanently damaged and wouldn't be able to operate at all.Free_electron - I'll check them all again tomorrow. That makes the most sense, but I'm 99% certain I checked all the ground lines..... ok, maybe I'm now like 90% certain......