W78LE51C/W78L051C

- 4 -

4.

PIN DESCRIPTION

SYMBOL

DESCRIPTIONS

EA

EXTERNAL ACCESS ENABLE: This pin forces the processor to execute out of

external ROM. It should be kept high to access internal ROM. The ROM address and

data will not be presented on the bus if EA pin is high and the program counter is

within on-chip ROM area.

PSEN

PROGRAM STORE ENABLE: PSEN enables the external ROM data onto the Port 0

address/ data bus during fetch and MOVC operations. When internal ROM access is

performed, no PSEN strobe signal outputs from this pin.

ALE

ADDRESS LATCH ENABLE: ALE is used to enable the address latch that separates

the address from the data on Port 0.

RST

RESET: A high on this pin for two machine cycles while the oscillator is running resets

the device.

XTAL1

CRYSTAL1: This is the crystal oscillator input. This pin may be driven by an external

clock.

XTAL2

CRYSTAL2: This is the crystal oscillator output. It is the inversion of XTAL1.

GROUND: Ground potential

POWER SUPPLY: Supply voltage for operation.

P0.0

−P0.7

PORT 0: Port 0 is a bi-directional I/O port which also provides a multiplexed low order

address/data bus during accesses to external memory. The Port 0 is also an open-

drain port and external pull-ups need to be connected while in programming.

P1.0

−P1.7

PORT 1: Port 1 is a bi-directional I/O port with internal pull-ups. The bits have alternate

functions which are described below:

T2(P1.0): Timer/Counter 2 external count input

T2EX(P1.1): Timer/Counter 2 Reload/Capture control

P2.0

−P2.7

PORT 2: Port 2 is a bi-directional I/O port with internal pull-ups. This port also provides

the upper address bits for accesses to external memory.

PORT 3: Port 3 is a bi-directional I/O port with internal pull-ups. All bits have alternate

functions, which are described below:

RXD(P3.0) : Serial Port receiver input

TXD(P3.1) : Serial Port transmitter output

INT0

(P3.2) : External Interrupt 0

P3.0

−P3.7

INT1(P3.3) : External Interrupt 1

T0(P3.4)

: Timer 0 External Input

T1(P3.5)

: Timer 1 External Input

WR (P3.6) :External Data Memory Write Strobe

RD (P3.7) : External Data Memory Read Strobe

P4.0

−P4.3

PORT 4: Another bit-addressable bidirectional I/O port P4. P4.3 and P4.2 are alternative

function pins. It can be used as general I/O port or external interrupt input sources