Tsuneo,

Thank you for your informative reply.

Yes - it is the same SPI on the same board (Master - C8051F310, Slave - C8051F410). The master was done by someone else, and I am coding the slave to interface with the master.

8 bytes are sent by the master to my board about 20 times a second, and I also have to send 8 bytes back. However, as you have mentioned, the timing is very tight. After I read from SPI0DAT and buffer up the byte, then try to write the byte back to the master - there will be a RX buffer overrun.

Your idea of dummy bytes is great. However, since I have no control over the master board, it is not possible to implement your idea. Fortunately, the author of the master firmware may be able to slow down the clock so there will be more time in the interrupt handling. That may help.