MC68HC908LJ12 Datasheet, PDF (366/413 Pages) Freescale Semiconductor, Inc – 8-bit microcontroller units

Keyboard Interrupt Module (KBI)
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 data direction register.
2. Write logic 1s to the appropriate data register bits.
3. Enable the KBI pins by setting the appropriate KBIEx bits in the
keyboard interrupt enable register.
19.6 Keyboard Interrupt Registers
Two registers control the operation of the keyboard interrupt module:
• Keyboard status and control register
• Keyboard interrupt enable register
MC68HC908LJ12 — Rev. 2.1
Freescale Semiconductor
Keyboard Interrupt Module (KBI)
Technical Data
367