5

2487C–MICRO–03/05

AT89S51

4.8

Port 2

Port 2 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 2 output buffers can

sink/source four TTL inputs. When 1s are written to Port 2 pins, they are pulled high by the inter-

nal pull-ups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low

will source current (I

Port 2 emits the high-order address byte during fetches from external program memory and dur-

ing accesses to external data memory that use 16-bit addresses (MOVX @ DPTR). In this

application, Port 2 uses strong internal pull-ups when emitting 1s. During accesses to external

data memory that use 8-bit addresses (MOVX @ RI), Port 2 emits the contents of the P2 Special

Function Register.

Port 2 also receives the high-order address bits and some control signals during Flash program-

ming and verification.

4.9

Port 3

Port 3 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 3 output buffers can

sink/source four TTL inputs. When 1s are written to Port 3 pins, they are pulled high by the inter-

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

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.

4.10

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.

4.11

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.

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)