I've integrated Microwatt uart and spi flash test cases into caravel, both appear good. @User one thing I did notice is the SPI flash simulation model in the caravel tree requires a power up command before it accepts commands. Microwatt doesn't issue one, and we don't think it should be required.

Yes, probably. The SPI flash module used in the caravel simulations was taken directly from Claire Wolf's picorv32 repository and only lightly edited.