AT89C55WD_08 datasheet(5/35 Pages) ATMEL

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1921D–MICRO–6/08

AT89C55WD

4.6

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

will source current (I

IL) because of the pull-ups.

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

Port 3 also serves the functions of various special features of the AT89C55WD, as shown in the

following 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 DISRTO

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 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 is the read strobe to external program memory.

When the AT89C55WD 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

external data memory.

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)

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