mc68hc908kx8 Freescale Semiconductor, Inc, mc68hc908kx8 Datasheet - Page 98

• Manufacturer Part Number: mc68hc908kx8
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Keyboard Interrupt Module (KBI)
The keyboard flag bit (KEYF) in the 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 register to configure the
pin as an input and read the data register.
9.4 Keyboard Initialization
When a keyboard interrupt pin is enabled, the pin may initially be low and cause a false interrupt to occur.
A false interrupt on an edge-triggered pin can be acknowledged immediately after enabling the pin. A false
interrupt on an edge- and level-triggered interrupt pin must be acknowledged after the pin has been pulled
high.
The internal pullup device, the pin capacitance, as well as the external load will factor into the actual
amount of time it takes for the pin to pull high. Considering only an internal pullup of 48 kΩ and pin
capacitance of 8 pF, the pullup time will be on the order of 1 µs.
To prevent a false interrupt on keyboard initialization:
Another way to avoid a false interrupt:
9.5 Low-Power Modes
The WAIT and STOP instructions put the MCU in low power-consumption standby modes.
9.5.1 Wait Mode
The keyboard module remains active in wait mode. Clearing the IMASKK bit in the keyboard status and
control register enables keyboard interrupt requests to bring the MCU out of wait mode.
9.5.2 Stop Mode
The keyboard module remains active in stop mode. Clearing the IMASKK bit in the keyboard status and
control register enables keyboard interrupt requests to bring the MCU out of stop mode.
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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.
1. Configure the keyboard pins as outputs by setting the appropriate DDRA bits in data direction
2. Write 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.
register A.
Setting a keyboard interrupt enable bit (KBIEx) forces the corresponding
keyboard interrupt pin to be an input, overriding the data direction register.
However, the data direction register bit must be a 0 for software to read the
pin.
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 Data Sheet, Rev. 2.1
NOTE
Freescale Semiconductor