8031... can P0 and/or P2 actually be used as ports?

[rxraine]

Hi,
I'm having some holiday fun teaching myself MCU history with an 8031. It brings me to a question that Google can't answer for me though, and perhaps someone here would be kind enough to help me.
As the 8031 has no ROM, it appears it must always use P0 and P2 for data and address buses. I've seen it written that this is true for P2, but not explicitly for P0. So...
- can !ALE control a latch on P0 (or P2) to use it for output?
- if 256 bytes is enough ROM, can P2 be used as a normal port?

[yy2013]

Thank you for an interesting question.

Please see Figure 16 on page 25 of MCS-51 Users' Manual:
http://web.mit.edu/6.115/www/document/8051.pdf

Here, the timing of external ROM/RAM access is displayed.
During instruction fetch, P2 outputs the higher byte of the program counter throughout the cycle. So, it cannot be used as an I/O port.
However, during external data memory access (that is, when !PSEN is high), the content of P2 is either "DPH out" or "P2 out". So, it should be possible to use it as an I/O port.

Also, Figure 5 (page 14) displays the case where part of P2 is used as an address bus for external RAM, while the remaining part is used as I/O ports.

[Guest]

Thank you for your answer, and the link to the very useful user manual.