HCS08 datasheet(135/290 Pages) MOTOROLA

135 / 290 page

Data Sheet — MC9S08GB60/D V1.5

MOTOROLA

135

MC9S08GB/GT

8.4 Parallel I/O Controls

Provided no on-chip peripheral is controlling a port pin, the pins operate as general-purpose I/O pins that

are accessed and controlled by a data register (PTxD), a data direction register (PTxDD), a pullup enable

Reads of the data register return the pin value (if PTxDDn = 0) or the contents of the port data register (if

PTxDDn = 1). Writes to the port data register are latched into the port register whether the pin is controlled

by an on-chip peripheral or the pin is configured as an input. If the corresponding pin is not controlled by

a peripheral and is configured as an output, this level will be driven out the port pin.

8.4.1 Data Direction Control

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 control 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 control 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.

For the MC9S08GB/GT MCU, reads of PTG0/BKGD/MS will return the value on the output pin.

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.

8.4.2 Internal Pullup Control

An internal pullup device can be enabled for each port pin that is configured as an input (PTxDDn = 0).

The pullup device is available for a peripheral module to use, provided the peripheral is enabled and is an

input function as long as the PTxDDn = 0.

For the four configurable KBI module inputs on PTA7–PTA4, when a pin is configured to detect rising

edges, the port pullup enable associated with the pin (PTAPEn) selects a pulldown rather than a pullup

device.

8.4.3 Slew Rate Control

Slew rate control can be enabled for each port pin that is configured as an output (PTxDDn = 1) or if a

peripheral module is enabled and its function is an output. Not all peripheral modules outputs have slew

rate control, refer to the Pins and Connections section for more information about which pins have slew

rate control.

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