81C55 View Datasheet(PDF) - Oki Electric Industry

Part Name

Description

Manufacturer

81C55

2048-Bit CMOS STATIC RAM WITH I/O PORTS AND TIMER

Oki Electric Industry 

¡ Semiconductor

MSM81C55-5RS/GS/JS

(2) Method using RESET

5 V VCC

4.5 V

2.2 V

VCCH

tSU

tR

Standby Mode

RESET

GND

Note: In this case, the C/S register is reset, the port is set into the input mode, and the timer stops.

PIN FUNCTION

Symbol

RESET

ALE

AD0 - 7

CE

IO/M

RD

WR

PA0 - 7

(PB0 - 7)

PC0 - 5

TIMER IN

TIMER OUT

VCC

GND

Function

A high level input to this pin resets the chip, places all three I/O ports in the input mode, resets all

output latches and stops timer.

Negative going edge of the ALE (Address Latch Enable) input latches AD0 - 7, IO/M, and CE signals into

the respective latches.

Three-state, bi-directional address/data bus. Eight-bit address information on this bus is read into

the internal address latch at the negative going edge of the ALE. Eight bits of data can be read from

or written to the chip using this bus depending on the state of the WRITE or READ input.

When the CE input is high, both read and write operations to the chip are disabled.

A high level input to this pin selects the internal I/O functions, and a low level selects the memory.

If this pin is low, data from either the memory or ports is read onto the AD0 - 7 lines depending on

the state of the IO/M line.

If this pin is low, data on lines AD0 - 7 is written into either the memory or into the selected port

depending on the state of the line IO/M line.

General-purpose I/O pins. Input/output directions can be determined by programming the command/

status (C/S) register.

Three pins are usable either as general-purpose I/O pins or control pins for the PA and PB ports.

When used as control pins, they are assigned to the following functions:

PC0: A INTR (port A interrupt)

PC1: A BF (port A full)

PC2: A STB (port A strobe)

PC3: B INTR (port B interrupt)

PC4: B BF (port B buffer full)

PC5: B STB (port B strobe)

Input to the counter/timer

Timer output. When the present count is reached during timer operation, this pin provides

a square-wave or pulse output depending on the programmed control status.

3–6V power supply

GND

9/19

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