Datasheet

MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
Freescale Semiconductor 155
Chapter 13
Input/Output (I/O) Ports
13.1 Introduction
Bidirectional input-output (I/O) pins form five parallel ports. All I/O pins are programmable as inputs or
outputs. All individual bits within port A, port C, and port D are software configurable with pullup devices
if configured as input port bits. The pullup devices are automatically and dynamically disabled when a port
bit is switched to output mode.
13.2 Unused Pin Termination
Input pins and I/O port pins that are not used in the application must be terminated. This prevents excess
current caused by floating inputs, and enhances immunity during noise or transient events. Termination
methods include:
1. Configuring unused pins as outputs and driving high or low;
1. Configuring unused pins as inputs and enabling internal pull-ups;
1. Configuring unused pins as inputs and using external pull-up or pull-down resistors.
Never connect unused pins directly to V
DD
or V
SS
.
Since some general-purpose I/O pins are not available on all packages, these pins must be terminated
as well. Either method 1 or 2 above are appropriate.
Addr.Register Name Bit 7654321Bit 0
$0000
Port A Data Register
(PTA)
See page 158.
Read:
PTA7 PTA6 PTA5 PTA4 PTA3 PTA2 PTA1 PTA0
Write:
Reset: Unaffected by reset
$0001
Port B Data Register
(PTB)
See page 160.
Read:
PTB7 PTB6 PTB5 PTB4 PTB3 PTB2 PTB1 PTB0
Write:
Reset: Unaffected by reset
$0002
Port C Data Register
(PTC)
See page 162.
Read: 1
PTC6 PTC5 PTC4 PTC3 PTC2 PTC1 PTC0
Write:
Reset: Unaffected by reset
$0003
Port D Data Register
(PTD)
See page 164.
Read:
PTD7 PTD6 PTD5 PTD4 PTD3 PTD2 PTD1 PTD0
Write:
Reset: Unaffected by reset
= Unimplemented
Figure 13-1. I/O Port Register Summary