5

2487D–MICRO–6/08

AT89S51

nal pull-ups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low

will source current (I

Port 3 receives some control signals for Flash programming and verification.

Port 3 also serves the functions of various special features of the AT89S51, as shown in the fol-

lowing table.

4.7

RST

Reset input. A high on this pin for two machine cycles while the oscillator is running resets

the device. This pin drives High for 98 oscillator periods after the Watchdog times out. The DIS-

RTO bit in SFR AUXR (address 8EH) can be used to disable this feature. In the default state of

bit DISRTO, the RESET HIGH out feature is enabled.

4.8

ALE/PROG

Address Latch Enable (ALE) is an output pulse for latching the low byte of the address during

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

programming.

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.

4.9

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.

4.10

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.

Port Pin

Alternate Functions

P3.0

RXD (serial input port)

P3.1

TXD (serial output port)

P3.2

INT0 (external interrupt 0)

P3.3

INT1 (external interrupt 1)

P3.4

T0 (timer 0 external input)

P3.5

T1 (timer 1 external input)

P3.6

WR (external data memory write strobe)

P3.7

RD (external data memory read strobe)