AT89C51ED2-RDTIM datasheet(13/137 Pages) ATMEL

13 / 137 page

AT89C51RD2/ED2

4235E–8051–04/04

ALE/PRO

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 mem-

ory. This pin is also the program pulse input (PROG) during Flash pro-

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

bit set, ALE will be inactive during internal fetches.

PSEN

Program Strobe ENable: The read strobe to external program memory.

When executing code from the external program memory, PSEN is acti-

vated twice each machine cycle, except that two PSEN activations are

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

vated during fetches from internal program memory.

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 13. Pin Description (Continued)

Mnemonic

Pin Number

Type

Name and Function

PLCC44 VQFP44

PLCC68

VQFP64

PDIL40

AT89C51ED2-RDTIM 数据表(PDF) 13 Page - ATMEL Corporation