The SPI code, and therefore the SCI-to-SPI bridge will be dependent on whether the SPI facility is a master or a slave. If SCI receive traffic exceeds the transmit traffic, I would tend to choose a SPI master. If the SCI transmit traffic is greater, the SPI would operate as a slave. It will also depend on whether the other end of the SPI link is master or slave. If the other end is existing equipment, you will also need to match the SPI parameters CPHA and CPOL.
The slave SPI would generally be interrupt driven, whereas the master operation might be polled.
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