
MOTOROLA
2-18
PINS AND CONNECTIONS
M68HC11
REFERENCE MANUAL
switchover mechanism, which decides whether EEPROM is powered by the internal
charge pump or the external voltage source, is similar to the V
STBY
logic at the MODB/
V
STBY
pin. When the external voltage is more than the charge-pump voltage, the
switch connects the external high-voltage source to the internal V
PP
line. The added
circuitry at this pin has no effect on normal IRQ functions, but it does have some effect
on the way the pin reacts to illegal levels.
In addition to XIRQ and IRQ, five other pins on the MC68HC11A8 can also be used to
generate interrupt requests to the MCU. These pins are associated with other on-chip
peripherals such as the timer or handshake I/O systems. The pins are PA0/IC3, PA1/
IC2, PA2/lC1, PA7/PAI/OC1, and AS/STRA. The input-capture pins can be configured
to detect rising edges, falling edges, or any edge. The PAI and STRA inputs can be
configured to detect rising edges or falling edges. The STRA input is only available if
the MCU is operating in a single-chip mode because the pin is used as the address
strobe (AS) output when the MCU is in expanded modes. These five pins have advan-
tages over the IRQ and XIRQ pins in that each of these five interrupts is independently
maskable with a local control bit as well as the global I bit in the CCR. Each of these
five interrupts also has a readable status indication, and a pending request can be
cleared without being serviced.
2.2.7 A/D Reference and Port E Pins (V
REFL
, V
REFH
, PE[7:0])
The V
REFH
and V
REFL
pins provide the reference voltages for the A/D converter cir-
cuitry. Since the A/D converter is an all-capacitive charge-redistribution converter,
there is essentially no dc current associated with these pins. Very small dynamic cur-
rents are caused by charge-redistribution switching during conversions (see
SEC-
TION 12 ANALOG-TO-DIGITAL CONVERTER SYSTEM
). These pins are normally
connected to V
DD
and V
SS
through a low-pass filter network (see
Figure 2-13
) to iso-
late noise on the logic power supply from the relatively sensitive analog measure-
ments. A low-noise precision reference supply can alternatively be used. There should
be at least 2.5 V between V
REFL
and V
REFH
for full A/D accuracy. Lower values will
result in more inaccuracy, but the converter will continue to operate. The A/D system
is tested at 4.5 V and 5.5 V across the reference supply pins.
There is an inherent diode from V
REFL
to V
SS
. If V
REFL
goes below V
SS
by more than
this diode drop, any conversion in progress may be corrupted, but no permanent phys-
ical damage will result until significant current is drawn. The only documented cases
of damage have been caused by blatant misapplication, such as connecting –12 V di-
rectly to the V
REFL
pin. Since no P-channel devices are associated with the V
REFH
pin,
there is no diode clamping to V
DD
. The gates of analog switches associated with the
A/D reference and input pins are controlled by signals that switch between V
SS
and
about 7 V. This higher-than-V
DD
supply is the output of a charge pump (separate from
the charge pump used for programming on-chip EEPROM). There is no special re-
quirement to keep V
REFH
below V
DD
. In fact, the converter will continue to produce
good results up to approximately 6 V on V
REFH
.