Oct 132012

I was beginning to look at the blinky pov firmware and how it interprets the SCLK and SDAT signals but got sidetracked looking at the scope traces and phasing between the clock and data signals.

I scoped both the clock (yellow) and data (blue) and captured these traces.

scope hookup clock and data

It is pretty clear where the clock and data are supposed to be “1” and “0”.

According to the W&L Blinky POV design page

"The above diagram shows how we prepare the binary data values for transmission. First, we look at the binary value of each byte. For example, the hex value 0x6D is binary 01101101. We are using a “data and clock” transmission scheme, where the data value is sampled when the clock changes."

This says the data will be sampled each time the clock transitions, from low to high or from high to low.

If we zoom in on a particular transition, say from high to low, we see something interesting.

The clock is the yellow trace and data the blue trace. The first question I have is, where does the clock decide it is high or low? Looking at the nearly full scale swing in both clock and data it is hard to see where the “rising” and “falling” edges of the clock are and what the corresponding value of the data is when the clock rises or falls. Looking at the last scope trace in this series it is not clear what the value of the data will be read as when the clock is finally determined to be low. Clearly the data, the blue trace, is supposed to be high before and after the negative going transition of the clock.

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