Resets and Interrupts
Resets
MC68HC11E Family
—
Rev. 4
Technical Data
MOTOROLA
Resets and Interrupts
109
5.3.1 Power-On Reset (POR)
A positive transition on V
DD
generates a power-on reset (POR), which is
used only for power-up conditions. POR cannot be used to detect drops
in power supply voltages. A 4064 t
cyc
(internal clock cycle) delay after
the oscillator becomes active allows the clock generator to stabilize. If
RESET is at logical 0 at the end of 4064 t
cyc
, the CPU remains in the
reset condition until RESET goes to logical 1.
The POR circuit only initializes internal circuitry during cold starts. Refer
to
Figure 2-6. External Reset Circuit
.
NOTE:
It is important to protect the MCU during power transitions. Most
M68HC11 systems need an external circuit that holds the RESET pin
low whenever V
DD
is below the minimum operating level. This external
voltage level detector, or other external reset circuits, are the usual
source of reset in a system.
5.3.2 External Reset (RESET)
The CPU distinguishes between internal and external reset conditions
by sensing whether the reset pin rises to a logic 1 in less than two
E-clock cycles after an internal device releases reset. When a reset
condition is sensed, the RESET pin is driven low by an internal device
for four E-clock cycles, then released. Two E-clock cycles later it is
sampled. If the pin is still held low, the CPU assumes that an external
reset has occurred. If the pin is high, it indicates that the reset was
initiated internally by either the COP system or the clock monitor.
CAUTION:
Do not 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.