M68HC08 datasheet(99/188 Pages) FREESCALE

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Keyboard/External Interrupt Module (KBI)

Functional Description

MC68HC908RF2 — Rev. 4.0

Data Sheet

MOTOROLA

Keyboard/External Interrupt Module (KBI)

An IRQ1/keyboard interrupt is latched when one or more keyboard pins goes low

after all were high. The MODEK bit in the keyboard status and control register

controls the triggering mode of the keyboard interrupt.

•

If the keyboard interrupt is edge-sensitive only, a falling edge on a keyboard

pin does not latch an interrupt request if another keyboard pin is already low.

To prevent losing an interrupt request on one pin because another pin is still

low, software can disable the latter pin while it is low.

•

If the keyboard interrupt is falling edge- and low level-sensitive, an interrupt

request is present as long as any keyboard pin is low.

If the MODEK bit is set, the keyboard interrupt pins are both falling edge- and low

level-sensitive, and both of these actions must occur to clear a keyboard interrupt

request:

•

Vector fetch or software clear — A vector fetch generates an interrupt

acknowledge signal to clear the interrupt request. Software may generate

the interrupt acknowledge signal by writing a 1 to the ACKK bit in the

keyboard status and control register (INTKBSCR). The ACKK bit is useful in

applications that poll the keyboard interrupt pins and require software to

clear the keyboard interrupt request. Writing to the ACKK bit prior to leaving

an interrupt service routine also can prevent spurious interrupts due to

noise. Setting ACKK does not affect subsequent transitions on the keyboard

interrupt pins. A falling edge that occurs after writing to the ACKK bit latches

another interrupt request. If the keyboard interrupt mask bit, IMASKK, is

clear, the CPU loads the program counter with the vector address at

locations $FFFA and $FFFB.

•

Return of all enabled keyboard interrupt pins to logic 1. As long as any

enabled keyboard interrupt pin is at logic 0, the keyboard interrupt remains

set.

The vector fetch or software clear and the return of all enabled keyboard interrupt

pins to logic 1 may occur in any order.

If the MODEK bit is clear, the keyboard interrupt pin is falling edge-sensitive only.

With MODEK clear, a vector fetch or software clear immediately clears the

keyboard interrupt request.

Reset clears the keyboard interrupt request and the MODEK bit, clearing the

interrupt request even if a keyboard interrupt pin stays at logic 0.

The keyboard flag bit (KEYF) in the IRQ and keyboard status and control register

can be used to see if a pending interrupt exists. The KEYF bit is not affected by the

keyboard interrupt mask bit (IMASKK) which makes it useful in applications where

polling is preferred.

To determine the logic level on a keyboard interrupt pin, use the data direction

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