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I'm trying to control SPI-type display with PIC and I have this problem sending the needed 9 bits (data is in bit 0, clock in bit 1):

Works:


Doesn't work:


I just can't figure out why the shorter version doesn't work. Both look fine in the simulator.

I seem to recall some limitation with the PIC in that you have to wait a machine cycle between each port write - that's probably why the shorter code doesn't work.

This may be a read-modify-write issue. It's mentioned in the datasheet in the ports description section.

The BSF works that way : it reads the value, modifies it, and then writes it back. The problem is that depending on the impedance of the circuit connected to the output, the value read is not necessarily what was written before. So this instruction may modify more than one bit.