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Keyboard Interrupt (KBI)

Keyboard Initialization

MC68HC908GR8 — Rev 4.0

Technical Data

MOTOROLA

Keyboard Interrupt (KBI)

179

NOTE:

Setting a keyboard interrupt enable bit (KBIEx) forces the corresponding

keyboard interrupt pin to be an input, overriding the data direction

software to read the pin.

13.5 Keyboard Initialization

When a keyboard interrupt pin is enabled, it takes time for the internal

pullup to reach a logic 1. Therefore, a false interrupt can occur as soon

as the pin is enabled.

To prevent a false interrupt on keyboard initialization:

1. Mask keyboard interrupts by setting the IMASKK bit in the

keyboard status and control register.

2. Enable the KBI pins by setting the appropriate KBIEx bits in the

keyboard interrupt enable register.

3. Write to the ACKK bit in the keyboard status and control register

to clear any false interrupts.

4. Clear the IMASKK bit.

An interrupt signal on an edge-triggered pin can be acknowledged

immediately after enabling the pin. An interrupt signal on an edge- and

level-triggered interrupt pin must be acknowledged after a delay that

depends on the external load.

Another way to avoid a false interrupt is:

1. Configure the keyboard pins as outputs by setting the appropriate

DDRA bits in data direction register A.

2. Write logic 1s to the appropriate port A data register bits.

3. Enable the KBI pins by setting the appropriate KBIEx bits in the

keyboard interrupt enable register.

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