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MOTOROLA
2-16
PINS AND CONNECTIONS
M68HC11
REFERENCE MANUAL
capacitors as if the external inverter input were the EXTAL pin and the inverter output
were the XTAL pin. Use a 74HCU04 for this inverter. This device is an unbuffered HC-
MOS hex inverter. Avoid Schmitt-trigger devices because the oscillator may fail to
start. Buffer the output of the external Pierce oscillator to drive additional logic.
2.2.4.4 AT-strip vs AT-cut Crystals
The AT-strip is a new-technology low-power crystal. Connecting one of these crystals
to the M68HC11 may cause problems due to the NAND gate in the MCU overdriving
the crystal. Use an AT-cut crystal with the M68HC11 to avoid this problem.
2.2.5 Reset Pin (RESET)
This active-low, bidirectional control signal is used as an input to initialize the
MC68HC11A8 to a known startup state and as an open-drain output to indicate that
an internal failure has been detected in either the clock monitor or computer operating
properly (COP) watchdog circuit. This RESET signal is significantly different from the
RESET signal used on earlier MCUs. More detailed information about this pin can be
found in
SECTION 5 RESETS AND INTERRUPTS
.
The reset circuitry is specifically designed to work with lower levels of V
DD
than other
MCU circuitry. Thus, RESET can be used to prevent undesirable performance as V
DD
power is applied or decays, which is important for applications in which the contents
of on-chip RAM must be maintained in the absence of V
DD
. In this situation, the RAM
and reset input logic in the MCU would be powered from a standby power source con-
nected to the MODB/V
STBY
pin whenever V
DD
is too low to support proper MCU oper-
ation. Secondly, RESET must be controlled when V
DD
is below legal operating limits
to prevent unintentional corruption of EEPROM data. Even if an application is not us-
ing the 512-byte EEPROM, the CONFIG register is still an EEPROM byte and must be
protected from corruption.
Virtually all MC68HC11A8 systems should include automatic control of RESET to
drive it low whenever V
DD
is below legal limits. A simple, inexpensive, low voltage in-
hibit (LVI) device such as the MC34064 or MC34164 can be used. The MC34064 is
available in TO-92 or SOT-8 plastic packages and provides an open-drain output to
directly drive the RESET pin of the MC68HC11A8. This device is connected to V
DD
,
V
SS
, and the RESET pin of the MCU. A pull-up resistor from RESET to V
DD
is the only
other component required for the reset circuit in most applications.
Figure 2-12
shows
a typical reset circuit.