Hi Mattew,

Think about it: during external code fetch PSEN is the output enable for the prom. During external data fetch RD is the output enable for the ram. If RD was active during a code fetch it would trash the data bus.

Data sheet writers seem to be pretty good at hiding stuff; rather than simply stating when RD is active my Atmel data sheet refers to the timing diagrams. In the Philips' hardware documentation, look for External Program Memory Fetches and External Data Memory Read Cycle (figures 28 and 29, page 23).

Good luck,

Dennis