Port 0 is an 8-bit open drain bi-directional 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 highimpedance
inputs.
Port 0 can also be configured to be the multiplexed loworder
address/data bus during accesses to external program
and data memory. In this mode, P0 has internal
pullups.
Port 0 also receives the code bytes during Flash programming
and outputs the code bytes during program
verification. External pullups are required during program
verification.