Hi all,

My background is mainly software on PCs but I've developed an interest in MCUs, electronics and PCB fabrication. I've been playing with the Atmel 89C2051 and have learned how to drive an LCD among other things.

Recently I started experimenting with the Atmel 89S53. I would like to use P0 to drive an LCD. My problem is that P0 on the 89S53 does not seem to work like the I/O ports on the 2051 and P1 to P3 on the '53. When I write 1s to all the P0 pins I can read 5 volts only on P0.0, the other 7 pins are low. The data sheet tells me P0 is open drain whereas the other ports have pullup resistors. So what do I do? Add pull up resistors? If so, how do I do that? What resistor value should I use?

Thanks for any help.