6

2487C–MICRO–03/05

AT89S51

In normal operation, ALE is emitted at a constant rate of 1/6 the oscillator frequency and may be

used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped dur-

ing each access to external data memory.

If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set,

ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high.

Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode.

4.12

PSEN

Program Store Enable (PSEN) is the read strobe to external program memory.

When the AT89S51 is executing code from external program memory, PSEN is activated twice

each machine cycle, except that two PSEN activations are skipped during each access to exter-

nal data memory.

4.13

EA/VPP

External Access Enable. EA must be strapped to GND in order to enable the device to fetch

code from external program memory locations starting at 0000H up to FFFFH. Note, however,

that if lock bit 1 is programmed, EA will be internally latched on reset.

EA should be strapped to V

This pin also receives the 12-volt programming enable voltage (V

4.14

XTAL1

Input to the inverting oscillator amplifier and input to the internal clock operating circuit.

4.15

XTAL2

Output from the inverting oscillator amplifier

5.

Special Function Registers

A map of the on-chip memory area called the Special Function Register (SFR) space is shown in

Table 5-1.

Note that not all of the addresses are occupied, and unoccupied addresses may not be imple-

mented on the chip. Read accesses to these addresses will in general return random data, and

write accesses will have an indeterminate effect.