My memory is a little hazy on this - but when you read P2, you're reading the pin values, when you write P2 your write to the output register. So the way around your problem is to keep a copy of the value you write to P2 in a memory location or a register.


mov P2,my_p2
...
...
...
...
mov a,my_p2
push a
...
...
...
pop a
mov my_p2,a


DATA
my_p2: ds 1


I seem to recall some warning about what you're mentioning in some doc I read many moons ago. Checkout the 'bibles' on this site.

Also, a similar problem can happen when you read/write the ports. Say you want to flash a led, so you read the port, xor the bit in question and output back to the port. What happen if the output of the port is loaded so that a '1' values doesn't actually read as a '1' - the logic will not work. So when updating output bits, don't use a read/write method. Again, keep a copy of the bits you want to write, manipulate this copy, then write out to the port.

Hope this clears things up!