Datasheet

4235K–8051–05/08

AT89C51RD2/ED2

ALE/PRO

33 27 68 56 O (I)

Address Latch Enable/Program Pulse: Output pulse for latching the

low byte of the address during an access to external memory. In normal

operation, ALE is emitted at a constant rate of 1/6 (1/3 in X2 mode) the

oscillator frequency, and can be used for external timing or clocking. Note

that one ALE pulse is skipped during each access to external data

memory. This pin is also the program pulse input (PROG) during Flash

programming. ALE can be disabled by setting SFR’s AUXR.0 bit. With

this bit set, ALE will be inactive during internal fetches.

PSEN 32 26 67 55 O Program Strobe ENable: The read strobe to external program memory.

When executing code from the external program memory, PSEN is

activated twice each machine cycle, except that two PSEN activations

are skipped during each access to external data memory. PSEN is not

activated during fetches from internal program memory.

EA 35 29 2 58 I External Access Enable: EA must be externally held low to enable the

device to fetch code from external program memory locations 0000H to

FFFFH. If security level 1 is programmed, EA will be internally latched on

Reset.

Table 4-1. Pin Description (Continued)

Mnemonic

Pin Number

Type

Name and FunctionPLCC44 VQFP44 PLCC68 VQFP64