Got a pcb with a teensy 3.2 trying to talk to a motor driver(DRV8434s). I know information is transferred to the chip because I'm able to run a stepper motor, the signal from the micro also looks crisp. The problem is the DRV8434s SDO line, naturally it's connected to the standard MISO input for the micro. The signal is strange looking, it does not quite reach 3.3v and has a very slow rise time. It rests at about 0, opposite to the MOSI signal, resting at 3.3v. I found this puzzling.
Among the things I've tried is lowering the frequency of the SPI bus, verifying values of supporting components, changing the IC, touching up solder joints and adding a pull-up resistor. Discovered I had connected the VSDO pin to the 3.3v rail though a resistor witch I swapped out for a wire.
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