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A SPI test which read/writes and expects MOSI to be loopbacked to MISO. It checks that what it writes is what it reads. The values put in the buffer are a circular ring of 8-bit values, starting with an initial value and incrementing by 1 on each write. So if the first write is [33, 34, … , 32], the next write will be [34, 35, …, 34]. You can set the speed of the operation to check that configurations are being set correctly: running two tests in parallel with different bit rates should see different clock frequencies.