AT89C55 datasheet(3/24 Pages) ATMEL

3 / 24 page

AT89C55

4-171

The AT89C55 provides the following standard features:

20K bytes of Flash, 256-bytes of RAM, 32 I/O lines, three

16-bit timer/counters, a six-vector two-level interrupt archi-

tecture, a full duplex serial port, on-chip oscillator, and

clock circuitry. In addition, the AT89C55 is designed with

static logic for operation down to zero frequency and sup-

ports two software selectable power saving modes. The

I dle Mod e stops th e CPU wh ile allow ing the RAM ,

timer/counters, serial port, and interrupt system to continue

functioning. The Power Down Mode saves the RAM con-

tents but freezes the oscillator, disabling all other chip func-

tions until the next hardware reset. The low-voltage option

saves power and operates with a 2.7-volt power supply.

Pin Description

VCC

Supply voltage.

GND

Ground.

Port 0

Port 0 is an 8-bit open drain bidirectional I/O port. As an

output port, each pin can sink eight TTL inputs. When 1s

are written to port 0 pins, the pins can be used as high-

impedance inputs.

Port 0 can also be configured to be the multiplexed low-

order address/data bus during accesses to external pro-

gram and data memory. In this mode, P0 has internal pul-

lups.

Port 0 also receives the code bytes during Flash program-

ming and outputs the code bytes during program verifica-

tion. External pullups are required during program verifica-

tion.

Port 1

Port 1 is an 8-bit bidirectional I/O port with internal pullups.

The Port 1 output buffers can sink/source four TTL inputs.

When 1s are written to Port 1 pins, they are pulled high by

the internal pullups and can be used as inputs. As inputs,

Port 1 pins that are externally being pulled low will source

current (IIL) because of the internal pullups.

In addition, P1.0 and P1.1 can be configured to be the

timer/counter 2 external count input (P1.0/T2) and the

timer/counter 2 trigger input (P1.1/T2EX), respectively, as

shown in the following table.

Port 1 also receives the low-order address bytes during

Flash programming and verification.

Port 2

Port 2 is an 8-bit bidirectional I/O port with internal pullups.

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 internal pullups and can be used as inputs. As inputs,

Port 2 pins that are externally being pulled low will source

current (IIL) because of the internal pullups.

Port 2 emits the high-order address byte during fetches

from external program memory and during accesses to

external data memory that use 16-bit addresses (MOVX @

DPTR). In this application, Port 2 uses strong internal pul-

lups 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 programming and verification.

Port 3

Port 3 is an 8-bit bidirectional I/O port with internal pullups.

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 internal pullups and can be used as inputs. As inputs,

Port 3 pins that are externally being pulled low will source

current (IIL) because of the pullups.

Port 3 also serves the functions of various special features

of the AT89C55, as shown in the following table.

Port 3 also receives the highest-order address bit and

some control signals for Flash programming and verifica-

tion.

RST

Reset input. A high on this pin for two machine cycles while

the oscillator is running resets the device.

Port Pin

Alternate Functions

P1.0

T2 (external count input to Timer/Counter 2),

clock-out

P1.1

T2EX (Timer/Counter 2 capture/reload

trigger and direction control)

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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