General Purpose I/O Ports
• Port 0 whose lines can serve as either general purpose inputs or alternatively as input to the analog-to-digital converter family.
• Port 1 is a quasi-bidirectional I/O port.
• Port 2 includes four input lines, two output lines, and two quasi-bidirectional I/O lines.
• Port 3 and 4 when used as ports, they have open drain outputs.
• By writing anything but a 1 to a line, it can serve as an input even as other lines serve as outputs.
• Each output line needs the addition of a pullup resistor having a value of 15kΩ.
• In the expanded mode the bus lines gain the ability to drive both high and low, forming the expansion bus without the need of pullup resistors.
