MC9S08JM16 Datasheet, PDF (78/386 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 6 Parallel Input/Output
6.2 Port Data and Data Direction
Reading and writing of parallel I/O is done through the port data registers. The direction, input or output,
is controlled through the port data direction registers. The parallel I/O port function for an individual pin
is illustrated in the block diagram below.
PTxDDn
DQ
Output Enable
PTxDn
DQ
Output Data
Port Read
Data
1
0
Synchronizer
Input Data
BUSCLK
Figure 6-1. Parallel I/O Block Diagram
The data direction control bits determine whether the pin output driver is enabled, and they control what
is read for port data register reads. Each port pin has a data direction register bit. When PTxDDn = 0, the
corresponding pin is an input and reads of PTxD return the pin value. When PTxDDn = 1, the
corresponding pin is an output and reads of PTxD return the last value written to the port data register.
When a peripheral module or system function is in control of a port pin, the data direction register bit still
controls what is returned for reads of the port data register, even though the peripheral system has
overriding control of the actual pin direction.
When a shared analog function is enabled for a pin, all digital pin functions are disabled. A read of the port
data register returns a value of 0 for any bits which have shared analog functions enabled. In general,
whenever a pin is shared with both an alternate digital function and an analog function, the analog function
has priority such that if both the digital and analog functions are enabled, the analog function controls the
pin.
Write to the port data register before changing the direction of a port pin to become an output. This ensures
that the pin will not be driven momentarily with an old data value that happened to be in the port data
register.
MC9S08JM16 Series Data Sheet, Rev. 2
78
Freescale Semiconductor