Oleg posted:
In case you indicated (port pins are '0's), writing '1's into the latch causes exactly OUTPUT behaviour of a port: all pins activate strong pull-up for 2 OSC cycles and emit '1's with possible very high current for this time.
By other words, if external peripheral provides low level to these pins at this time then short-circuit happens. Indeed, this is only for two OSC cycles but remember that 8051 may work at low frequences so this period may be long enough to destroy hardware.

a valid, sometimes overlooked point. In most cases this is not a problem (the ports reset to '1') but when you use the ports for truly bidirectional communication (e.g. uC to uC) it important to take it into consideration.


Erik