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I have an ADC attached to my uC on a custom PCB. The traces are just directly connected between the uC and the ADC. Here is a photo of an oscilloscope trace of SCK and MISO. SCK is Blue, MISO is Red. I see my clock and my data, but they are only about 940mV of the 3.3V range. I have a bypass cap on the ADC (and the uC). Where did my voltage go? Its a hobby oscilloscope. I can try a better one tomorrow.

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It doesn't look to me like you see clock and data! Those look like the same signal which indicates something is really not right. Can you try disconnecting MISO and SCK from everything and see what the signal looks like? If it's identical again I would think there's something wrong in the uC configuration. – Kit Scuzz Oct 24 '12 at 6:51
MISO is master in slave out - it should be a response from ADC on this line, check MOSI line. Then check for flux residue on the board - it looks like signals are shorted to something. – miceuz Oct 24 '12 at 7:20
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...and add a link to datasheets of MCU and ADC chips to your question and your schematics. – miceuz Oct 24 '12 at 7:21
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Kit and miceuz you were right. There was a tiny solder ball between the 2 pins on the ADC (the package is MSOP-8). I managed to get rid of the short and it started working. Thanks for the answers. – BSEE Oct 24 '12 at 18:33

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These are not both data and clock, so I think you have a shorted circuit somewhere. Maybe between the ADC pin and some other pin

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