Hello all,

I have two 8051s on an SPI bus; one is configured as master and one is configured as slave (obviously).

Whenever I try bi-directional communication, the master's output 'overrides' the slave's output. As an example, the master is outputting 0x33 and the slave is trying to output 0x55; however, looking at the two lines on a logic analyzer, both outputs are 0x33. This same problem occurs no matter what I change the master's or slave's output to--the slave's output always follows the master's output.

I have verified that the MOSI and MISO pins aren't shorted together (the obvious explanation). **Also, if I lift the slave's MISO (output) pin from the board (thereby excluding any possible external hardware problems on the board) and monitor that pin directly, it still follows the master's output (0x33 instead of 0x55 in the case above).** The only explanation this leaves (I think) is that something is goofed up *internally* in the slave 8051; however, I've swapped the part out and still have the same problem.

Does anyone have any ideas?

Thanks in advance.
Drew Rainwater