Chapter 11
Input/Output Ports (PORTS)
11.1 Introduction
The MC68HC908QC16 has up to 24 bidirectional input-output (I/O) pins and two input only pins
depending on the package selection. All I/O pins are programmable as inputs and outputs.
11.2 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;
2. Configuring unused pins as inputs and enabling internal pull-ups;
3. 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.
11.3 Port A
Port A is an 6-bit special function port that shares all six of its pins with the keyboard interrupt (KBI)
module (see
Each port A pin also has a software
configurable pullup device if the corresponding port pin is configured as an input port.
NOTE
PTA2 is input only. PTA2 has a high voltage detector to enable entry into
special modes. Do not exceed the V
DD
level on this pin in normal operation.
When the IRQ function is enabled in the configuration register 2
(CONFIG2), bit 2 of the port A data register (PTA) will always read a 0. In
this case, the BIH and BIL instructions can be used to read the logic level
on the PTA2 pin. When the IRQ function is disabled, these instructions will
behave as if the PTA2 pin is a logic 1. However, reading bit 2 of PTA will
read the actual logic level on the pin.
MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 5
Freescale Semiconductor
107
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
