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.
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