
M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-17
Figure 2-12 Reset Circuit Example
2.2.6 Interrupt Pins (XIRQ, IRQ)
The XIRQ pin provides a means for requesting non-maskable interrupts after reset ini-
tialization. During reset, the X bit in the condition code register (CCR) is set, and any
interrupts are masked until MCU software enables them. Since the XIRQ input is level
sensitive, it can be connected to a multiple-source wired-OR network with an external
pull-up resistor. XIRQ is often used as a power loss detect interrupt.
The IRQ input provides a means for requesting asynchronous interrupts to the
MC68HC11A8. IRQ is program selectable (OPTION register), having a choice of ei-
ther level-sensitive or falling-edge-sensitive triggering. After reset, IRQ is configured
for level-sensitive operation by default.
Whenever XIRQ or IRQ are used with multiple interrupt sources (IRQ must be config-
ured for level-sensitive operation if there is more than one source of IRQ interrupt),
each source must drive the interrupt input with an open-drain-type driver to avoid con-
tention between outputs. There should be a single pull-up resistor near the MCU inter-
rupt input pin (typically 4.7 k
). There must also be an interlock mechanism at each
interrupt source so that the source holds the interrupt line low until the MCU recogniz-
es and acknowledges the interrupt request. If one or more other interrupt sources are
still pending after the MCU services a request, the interrupt line will still be low; thus,
the MCU will be interrupted again as soon as the interrupt mask bit in the MCU be-
comes clear (normally upon return from an interrupt).
The IRQ pin is used during factory testing as a bulk V
PP
programming voltage source,
which allows for parallel programming of as many as half of the bytes in the EEPROM
in a single programming operation. Since the on-chip charge pump does not have suf-
ficient drive capability to simultaneously program this many EEPROM locations, the
external 20-V power supply is needed to supplement the on-chip charge pump. The
4.7 k
3
1
2
3
1
2
[3]
TO RESET
OF M68HC11
VDD
MC34064
GND
RESET
IN
RESET
GND
IN
MANUAL
RESET SWITCH
4.7 k
1.0
μ
F
MC34164
4.7 k
VDD
VDD