I once worked as the firmware developer for Sun's SPARCstation-1 computer. We were able to get most of the prototypes working reliably, but there was one unit that would intermittently reset itself and then display the firmware prompt. The resets didn't seem to correlate with any obvious workload or software. They seemed to happen randomly. Since firmware tends to get blamed for every problem, I was on the hook for solving it.
We looked at all of the usual suspects, but we came up with nothing. The problem continued to plague us for several weeks.
Finally, we caught a break in the case. This particular unit - instead of being powered by a normal, small computer power supply - was connected to a high-end and very expensive HP lab-grade supply. When we monitored the supply voltage to the unit over a long period of time, we found that the 5V supply to the computer was glitching for a few milliseconds just prior to the reset. Of course, that just prompted more questions. Why would a high-end lab power supply have a glitch?
We examined the setting carefully. The power supply and the computer were sitting on a lab bench. The bench was supported on either end by painted metal panels. Each panel had a round hole in it to support the ends of a cylindrical, chrome-plated footrest bar. The footrest bar was insulated from the support panels by the paint, and it could spin in the round holes.
When someone was working at the bench, it was common for them to rest a foot on the bar. Often they would idly spin the bar without thinking about it. A rubber-soled shoe spinning a metal bar that is insulated from the rest of the world is essentially a Van de Graaf generator. Static charges will build up on the bar until the potential is high enough to arc over to the metal bench supports. Of course. The charge would go to ground through the power supply's power cord, confusing the power supply.
Grounding the footrest stopped the resets.
This entry was submitted by Mitch Bradley and edited by Rob Spiegel.
Mitch Bradley is a firmware engineer who developed Open Boot/Open Firmware. He currently spearheads the firmware and diagnostics for the One Laptop per Child project.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Nice detective work. It always amazes me at how something seemingly so unimportant and unrelated could have such a huge impact on a product's performance.
This takes my thoughts immediately to ESD grounded workstations.Whenever working with developing prototypes for any electronic computing device, our labs were always monitored by our internal ESD cops – the QA safety team that insured that all benches had that light-blue anti static pad as the work-surface, and that any user at the station had the mandatory wrist-straps and ankle grounds.Even if a manager wandered down into the lab to check on the daily progress, they were in violation (with the ESD cops) if they reached into the prototype set-up without first grounding themselves with a strap.While I realize that ESD safety precautions are more prevalent today than they were when the SPARC station was introduced about 20+ years ago, I'm wondering if the author of the article had ESD cops at his facility-?
Boy does this bring back memories of grad school. We had an ultrafast-spectroscopy instrument that we assembled from $1.2 Million of lasers and fancy electronics equipment. In essence, our system was a very, very fast strobe light that would illuminate a repetitive chemical reaction at precisely the right time. We used a liquid nitrogen-cooled scientific CCD camera that would record the faint output signal viewed through a maze of optics and filters. We would darken the lab and use upwards of 20-min CCD exposures to collect the very faint signal. Every now and again our data would be extremely noisy and we would have to collect data again. After some major sleuthing, we discovered that one of the PCs on the far side of the lab running Windows3.0 had a screen saver consisting of a bouncing ball. All but the top 5% of the computer monitor was obscured by equipment and only when the image of the ball bounced near the top of the screen did the extra photons get entrained into our optics. I've disliked screen savers ever since...
Wow, Jim. Is that type of restriction common these days? I would imagine there are ways to ground equipment that doesn't require personnel to wear such elaborate gear.
Actually, the grounding precautions were not so much for the equipment, but for the safety of the developing prototypes and new product introductions.But the equipment is normally sitting right on the same bench as the product, which is the target of the precautionary measures. And yes, this was a recent practice used on manufacturing floors as late as last year when I was introducing a military computing device at a domestic OEM.
You're lucky you were able to isolate it. I've seen many intermittent failures in the lab, which were impossible to reliability repeat, and thus diagnose. I suspect this is something that's very widespread, and accounts for a lot of in-the-field glitches.
I agree, Rob. It's amazing that problems like this one ever get solved. This isn't just a glitch; it's a sub-system glitch, made even tougher to find because firmware was supposed to be the culprit.
So, you are saying the power supply wasn't properly grounded and the output was float in reference to ground. Sounds like things were not properly installed and grounded. I don't go along with the Vandergraf garbage,
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