4

3053B–MICRO–3/05

AT89LS51

4.

Pin Description

4.1

VCC

Supply voltage.

4.2

GND

Ground.

4.3

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 program and data memory. In this mode, P0 has internal pull-ups.

Port 0 also receives the code bytes during Flash programming and outputs the code bytes dur-

ing program verification. External pull-ups are required during program verification.

4.4

Port 1

Port 1 is an 8-bit bidirectional I/O port with internal pull-ups. 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 inter-

nal pull-ups and can be used as inputs. As inputs, Port 1 pins that are externally being pulled low

will source current (I

Port 1 also receives the low-order address bytes during Flash programming and verification.

4.5

Port 2

Port 2 is an 8-bit bidirectional 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.

Port Pin

Alternate Functions

P1.5

MOSI (used for In-System Programming)

P1.6

MISO (used for In-System Programming)

P1.7

SCK (used for In-System Programming)