Hardware manual

Common Features Description
Atmel 8051 Microcontrollers Hardware Manual 2-29
4316A–8051–01/04
When the CPU is executing out of external Program Memory, all 8 bits of Port 2 are ded-
icated to an output function and may not be used for general purpose I/O. During
external program fetches they output the high byte of the PC. During this time the Port 2
drivers use the strong pull-ups to emit PC bits that are 1’s.
Figure 2-8. External Program Memory Execution
2.7 PSEN The read strobe for external fetches is PSEN. PSEN is not activated for internal fetches.
When the CPU is accessing external Program Memory, PSEN
is activated twice every cycle
(except during a MOVX instruction) whether or not the byte fetched is actually needed for the
current instruction. When PSEN
is activated its timing is not the same as RD. A complete RD
cycle, including activation and deactivation of ALE and RD, takes 12 oscillator periods. A com-
plete PSEN
cycle, including activation and deactivation of ALE and PSEN, takes 6 oscillator
periods. The execution sequence for these two types of read cycles are shown in Figure 2-8 for
comparison.
2.8 ALE The main function of ALE is to provide a properly timed signal to latch the low byte of an
address from P0 to an external latch during fetches from external Program Memory. For