At a previous job, we had a bit-mapped display that was driven by serial data. Eight bits were transmitted at a time, which corresponded to eight pixels on the display’s field with a particular address. The two-byte address followed by the byte of data was transmitted serially to the display by a transmitter board.
After EMC testing, we made a design change at the main processor board. Then we occasionally noticed ghost images on the display. At the time, we didn’t connect these two events because display image problems were usually the result of a bad connection between the display and the serial transmitter.
However, we soon noticed the ghost images were always at the exact same location on the display. This wasn’t the same symptom as a bad connection, which would sprinkle dots everywhere on the display. So it was obvious the problem was address related.
After reviewing the address map of the display’s pixels and realizing that one bit position in the address would vector the data bits to the ghost location, I began to look at how that one bit could occasionally change while all the other bits were OK. I finally calculated the time in the message to that bit using the baud rate, and calculated the clock frequency shift required to change that one bit.
Then it dawned on me -- our processor board change had been to replace the clock oscillator with a new spread spectrum version. Its spread frequency range just matched the frequency shift to change that one pesky address bit. Once we slaved the serial transmitter board’s clock to the new spread spectrum clock, the display worked fine.
This entry was submitted by Steve Goss and edited by Rob Spiegel.
Steve Goss received his electronics training in the US Navy then went to college on the G.I. Bill and graduated with a BS in EE from the University of Maryland. His 35-year career in electronics has primarily been with stabilization controls.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
I think the one-change-at-a-time rule should be written into law,or at least a physical punishment given for ignoring it. The little or much time given to its implementation pays off more than we know!
Battar: I think you need to add a corollary, only change one thing at a time. It drives me crazy when someone makes wholesale changes that make the problem worse and then they cannot recall everything that was done so we can reverse the procedure to get back where we were and start the trouble shoot process anew.
I agree Battar. Hitting the product can be a real solution. I had a TV for a few years that needed a smack on the side every time it was turned on in order to bring up the picture. It was quite amusing to watch my kids each morning as they turned on the TV, and then smacked it on the side.
Rob, thats 97% for one of the 4 options. "hit it" is good for 30% - it cures bad connections and cold solder joints fast (but temporarily). As one of my colleagues was fond of saying, a fault isn't a rabbit - it doesn't run away, it always returns (that rhymes in the local lingo here).
Battar, did you mean that 97% of the time one of the four attempts works? Or did you mean that 97% of the time it's the number four try (looking at what you touched last) that delivers is the solution?
You're right, Batter. And I think the 97% statistic for step #4 is pretty close. In this case the problem was on a board 5 slots away on the backplane and some time had elapsed since the previous design change. Regarding the pesky spread spectrum clock oscillator, we were probably thinking if a little is good, a lot will be better.
Steve, actually you just had to follow the 4 rules of fault-finding for technicians. It's under rule 4. The rules are - 1)Turn it off, turn it on again 2)hit it 3) read the data sheet 4) look for the problem at the last place you touched before it stopped working. That usually solves 97% of all problems.
You're exctly right. The thing was, the first spread spectrum clock oscillator (I'm not a fan of those) had a narrower "spread" and didn't cause the problem. It was when we widened the spread, to get the emitted noise spikes lower, that the ghost images started showing up.
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