Thanks - I downloaded the rpi image and had a play with spitest-D
Has anyone else seen their chips go into a mode where the in_miso pin starts oscillating very slowly - e.g. about 1Hz, even when the SPI clock isn't running?
My guess is that the MISO pin keeps showing the last bit that you read (or the one after that). In the case of normal miner operation, that would be the register that indicates the job it is working on, which toggles between 1 and 0 every 2^22 clock cycles.