Pull-Up, Slew Rate, And Drive Strength; Port Internal Pull-Up Enable; Port Slew Rate Enable; Port Drive Strength Select - ROHS MC9S08QE128 Reference Manual

Chapter 6 Parallel Input/Output Control
It is a good programming practice to 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.
Port Read
Data
BUSCLK
6.2

Pull-up, Slew Rate, and Drive Strength

Associated with the parallel I/O ports is a set of registers located in the high page register space that operate
independently of the parallel I/O registers. These registers are used to control pull-ups, slew rate, and drive
strength for the pins and may be used in conjunction with the peripheral functions on these pins.
6.2.1

Port Internal Pull-Up Enable

An internal pull-up device can be enabled for each port pin by setting the corresponding bit in the pull-up
enable register (PTxPEn). The pull-up device is disabled if the pin is configured as an output by the parallel
I/O control logic or any shared peripheral function regardless of the state of the corresponding pull-up
enable register bit. The pull-up device is also disabled if the pin is controlled by an analog function.
6.2.2

Port Slew Rate Enable

Slew rate control can be enabled for each port pin by setting the corresponding bit in the slew rate control
register (PTxSEn). When enabled, slew control limits the rate at which an output can transition in order to
reduce EMC emissions. Slew rate control has no effect on pins that are configured as inputs.
6.2.3

Port Drive Strength Select

An output pin can be selected to have high output drive strength by setting the corresponding bit in the
drive strength select register (PTxDSn). When high drive is selected, a pin is capable of sourcing and
112
PTxDDn
D Q
PTxDn
D Q
Figure 6-1. Parallel I/O Block Diagram
MC9S08QE128 MCU Series Reference Manual, Rev. 2
1
Synchronizer
0
Output Enable
Output Data
Buffer???
Input Data
Freescale Semiconductor