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2010-2012 Microchip Technology Inc.
DS41412F-page 61
PIC18(L)F2X/4XK22
4.3
Master Clear (MCLR)
The MCLR pin provides a method for triggering an
external Reset of the device. A Reset is generated by
holding the pin low. These devices have a noise filter in
the MCLR Reset path which detects and ignores small
pulses. An internal weak pull-up is enabled when the
pin is configured as the MCLR input.
The MCLR pin is not driven low by any internal Resets,
including the WDT.
In PIC18(L)F2X/4XK22 devices, the MCLR input can
be disabled with the MCLRE Configuration bit. When
MCLR is disabled, the pin becomes a digital input. See
Section 10.6
“PORTE
Registers”
for
more
information.
4.4
Power-on Reset (POR)
A Power-on
Reset
pulse
is
generated on-chip
whenever VDD rises above a certain threshold. This
allows the device to start in the initialized state when
VDD is adequate for operation.
To take advantage of the POR circuitry either leave the
pin floating, or tie the MCLR pin through a resistor to
VDD. This will eliminate external RC components
usually needed to create a Power-on Reset delay. A
minimum rise rate for VDD is specified. For a slow rise
time, see Figure 4-2.
When the device starts normal operation (i.e., exits the
Reset condition), device operating parameters (volt-
age, frequency, temperature, etc.) must be met to
ensure proper operation. If these conditions are not
met, the device must be held in Reset until the operat-
ing conditions are met.
POR events are captured by the POR bit of the RCON
register. The state of the bit is set to ‘0’ whenever a
POR occurs; it does not change for any other Reset
event. POR is not reset to ‘1’ by any hardware event.
To capture multiple events, the user must manually set
the bit to ‘1’ by software following any POR.
FIGURE 4-2:
EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
Note 1:
External Power-on Reset circuit is required
only if the VDD power-up slope is too slow.
The diode D helps discharge the capacitor
quickly when VDD powers down.
2:
15 k
< R < 40 k is recommended to make
sure that the voltage drop across R does not
violate the device’s electrical specification.
3:
R1
1 k will limit any current flowing into
MCLR from external capacitor C, in the event
of
MCLR/VPP
pin
breakdown,
due
to
Electrostatic Discharge (ESD) or Electrical
Overstress (EOS).
C
R1
R
D
VDD
MCLR
VDD
PIC MCU
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