MC68HC908JB8 Datasheet, PDF (230/286 Pages) Motorola, Inc – MICROCONTROLLERS

Keyboard Interrupt Module (KBI)
14.5 Functional Description
Writing to the KBIE7–KBIE0 bits in the keyboard interrupt enable register
independently enables or disables each port A pin as a keyboard
interrupt pin. Enabling a keyboard interrupt pin also enables its internal
pullup device. A logic 0 applied to an enabled keyboard interrupt pin
latches a keyboard interrupt request.
A keyboard interrupt is latched when one or more keyboard pins goes
low after all were high. The MODEK bit in the keyboard status and
control register controls the triggering mode of the keyboard interrupt.
• If the keyboard interrupt is edge-sensitive only, a falling edge on a
keyboard pin does not latch an interrupt request if another
keyboard pin is already low.
• If the keyboard interrupt is falling edge- and low level-sensitive, an
interrupt request is present as long as any keyboard pin is low.
NOTE: To prevent losing an interrupt request on one pin because another pin is
still low, software can disable the latter pin while it is low.
If the MODEK bit is set, the keyboard interrupt pins are both falling edge-
and low level-sensitive, and both of the following actions must occur to
clear a keyboard interrupt request:
• Vector fetch or software clear — A vector fetch generates an
interrupt acknowledge signal to clear the interrupt request.
Software may generate the interrupt acknowledge signal by
writing a logic 1 to the ACKK bit in the keyboard status and control
register (KBSCR). The ACKK bit is useful in applications that poll
the keyboard interrupt pins and require software to clear the
keyboard interrupt request. Writing to the ACKK bit prior to leaving
an interrupt service routine also can prevent spurious interrupts
due to noise. Setting ACKK does not affect subsequent transitions
on the keyboard interrupt pins. A falling edge that occurs after
writing to the ACKK bit latches another interrupt request. If the
keyboard interrupt mask bit, IMASKK, is clear, the CPU loads the
program counter with the vector address at locations $FFF0 and
$FFF1.
Technical Data
230
MC68HC908JB8•MC68HC08JB8•MC68HC08JT8 — Rev. 2.3
Keyboard Interrupt Module (KBI)
Freescale Semiconductor