Thursday, January 26, 2012

Another Tragic Victim of Electrostatic Discharge

If Chip spends more than a day trying to solve a software problem, it's a hardware problem. -- Mrs. Overclock's Law

This isn't always true, but it's been true often enough, going all the way back to 1976 when a recalcitrant disk controller on a PDP-11/05 caused my device driver to work in the morning but not in the afternoon after the lab had warmed up. I typically work with hardware engineers of such caliber that I am extremely reluctant to invoke this law. However, when it's my hands on the hardware, all bets are off.

After writing the article "Remote Sensing with Arduino" I anxiously awaited for Mrs. Overclock (a.k.a. Dr. Overclock, Medicine Woman) to return home from a hard day courageously saving lives so that I could demonstrate the latest part of my Amigo project on her own laptop. Here is what she saw.

Analog Devices TMP36: Another Victim of ESD

Let me assure you that it is seldom if ever 38.77° Fahrenheit in my home office where the Arduino board was connected to the network. Convinced that I had somehow botched some recent software changes, or maybe knocked something loose on the breadboard, I spent several hours fiddling with the code, swapping jumper wires, peering at the Arduino source code that supports the analog pins, reading documentation about the analog-to-digital convertor in the Atmel ATmega328 microcontroller, and just generally wasting my time making all of this a lot harder than it needed to be. Everything pointed to the Analog Devices TMP36 temperature sensor as the problem.

Finally, first thing this morning, I invoked Mrs. Overclock's Law and found this excellent description of how to test the TMP36 by (as I was to find out only just now) none other than Limor Fried herself. Here's a close-up of the tiny TMP36 surrounded by probes to my multimeter and wires providing power right from the Arduino board.

Analog Devices TMP36: Surrounded by Probes

Initially the device reported 140 millivolts; that's around -33°F. A few minutes later it was 350mV or 5°F. Right now it reports 624mV or 54°F. Since I have my sleeves rolled up, this seems unlikely. This is a semiconductor device that had been working just fine.

Analog Devices TMP36: Testing

I have no one to blame but myself. While everything you see here is placed carefully on an anti-static mat, I am not always careful to ground myself using the anti-static wrist strap, to my woe. Especially in the dry winter months, it is not unusual for me to experience a spark here and there. I suspect I fried the part while putting my finger on it to warm it up during testing.

It's not really the semiconductor part that's the issue here, although if I had fried an OMAP microprocessor or some other expensive or hard to get component, it might have been. The TMP36 costs two bucks U.S. or less. It's the fact that my own carelessness cost me hours of my time. When you are self-employed and you bill by the hour, you are constantly aware of the value of your time in real dollars. Using a grounded wrist strap to protect against electrostatic discharge (ESD) would have saved me a lot of time and money.

I have a grounded ESD mat and wrist strap permanently wired up on my work table in my office.

Board Setup

I even carry a portable model in my tool bag.

Field Service ESD Mat

Back in my Bell Labs days we were required to take an annual refresher course in ESD practices. Or, I can just learn the hard way. If you lay hands on bare hardware, get yourself an ESD mat and wrist strap. And have the discipline to use them.

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