MC68HC908GR4MP datasheet(123/286 Pages) FREESCALE

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Keyboard Initialization

MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

Freescale Semiconductor

123

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.

NOTE

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.

13.4 Keyboard Initialization

When a keyboard interrupt pin is enabled, it takes time for the internal pullup to reach a 1. Therefore, a

false interrupt can occur as soon as the pin is enabled.

To prevent a false interrupt on keyboard initialization:

Mask keyboard interrupts by setting the IMASKK bit in the keyboard status and control register.

Enable the KBI pins by setting the appropriate KBIEx bits in the keyboard interrupt enable register.

Write to the ACKK bit in the keyboard status and control register to clear any false interrupts.

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:

Configure the keyboard pins as outputs by setting the appropriate DDRA bits in data direction

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

Enable the KBI pins by setting the appropriate KBIEx bits in the keyboard interrupt enable register.

13.5 Low-Power Modes

The WAIT and STOP instructions put the MCU in low power-consumption standby modes.

13.5.1 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.

13.5.2 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.

13.6 Keyboard Module During Break Interrupts

The system integration module (SIM) controls whether the keyboard interrupt latch can be cleared during

the break state. The BCFE bit in the SIM break flag control register (SBFCR) enables software to clear

status bits during the break state.

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