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Chapter 6 Parallel Input/Output Control

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

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.

Figure 6-1. Parallel I/O Block Diagram

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.

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.

Slew rate control can be enabled for each port pin by setting the corresponding bit in the slew rate control

reduce EMC emissions. Slew rate control has no effect on pins that are configured as inputs.

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

sinking greater current. Even though every I/O pin can be selected as high drive, the user must ensure that

the total current source and sink limits for the MCU are not exceeded. Drive strength selection is intended

to affect the DC behavior of I/O pins. However, the AC behavior is also affected. High drive allows a pin

to drive a greater load with the same switching speed as a low drive enabled pin into a smaller load.

Because of this, the EMC emissions may be affected by enabling pins as high drive.

Port Read

PTxDDn

PTxDn

Output Enable

Output Data

Input Data

Synchronizer

Data

BUSCLK

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