Motorola MC68HC908AB32 - Keyboard Initialization; Keyboard Status and Control Register

Keyboard Interrupt Module (KBI)
Functional Description
MC68HC908AB32 — Rev. 1.0 Technical Data
MOTOROLA Keyboard Interrupt Module (KBI) 349
19.5.1 Keyboard Initialization
When a keyboard interrupt pin is enabled, it takes time for the internal
pull-up 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:
1. Configure the keyboard pins as outputs by setting the appropriate
DDR bits in the data direction register.
2. Write logic 1s to the appropriate port data register bits.
3. Enable the KBI pins by setting the appropriate KBIEx bits in the
keyboard interrupt enable register.
19.5.2 Keyboard Status and Control Register
• Flags keyboard interrupt requests.
• Acknowledges keyboard interrupt requests.
• Masks keyboard interrupt requests.
• Controls keyboard interrupt triggering sensitivity.