MC68HC11KW1 Datasheet, PDF (22/238 Pages) Motorola, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit

2.1
RESET
2
An active-low, bidirectional control signal, RESET acts as an input to initialize the MCU to a known
start-up state. It also acts as an open-drain output to indicate that an internal failure has been
detected in either the clock monitor or the COP watchdog circuit. The CPU distinguishes between
internal and external reset conditions by sensing whether the reset pin rises to a logic one in less
than four E clock cycles after an internal reset has been released. It is therefore not advisable to
connect an external resistor-capacitor (RC) power-up delay circuit to the reset pin of M68HC11
devices because the circuit charge time constant can cause the device to misinterpret the type of
reset that occurred. Refer to Section 5 for further information.
Figure 2-2 illustrates a typical reset circuit that includes an external switch together with a low
voltage inhibit circuit, to prevent power transitions, or RAM or EEPROM corruption.
V_DD
V_DD
2
IN
4.7 kΩ
1
RESET
MC34064
GND
V_DD
3
To M68HC11
RESET
Manual
reset
4.7 kΩ
4.7 kΩ
2
1µF
IN
RESET ¹
MC34164
GND
3
Figure 2-2 External reset circuitry
PIN DESCRIPTIONS
MC68HC11KW1
2-2