MC68HC908JW32_09 Datasheet, PDF (192/232 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Keyboard Interrupt Module (KBI)
15.4 Functional Description
INTERNAL BUS
KBA0
.
KBIE0
.
TO PULLUP ENABLE
.
KBA7
V_REG
D ^CLR Q
CK
ACKK
RESET
KEYBOARD
INTERRUPT FF
VECTOR FETCH
DECODER
KEYF
SYNCHRONIZER
Keyboard
Interrupt
Request
IMASKK
KBIE7
TO PULLUP ENABLE
MODEK
Figure 15-2. Keyboard Module Block Diagram
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 $FFE0 and $FFE1.
• Return of all enabled keyboard interrupt pins to logic 1 — As long as any enabled keyboard
interrupt pin is at logic 0, the keyboard interrupt remains set.
MC68HC908JW32 Data Sheet, Rev. 6
192
Freescale Semiconductor