PIC18F2331/2431/4331/4431
FIGURE 5-2:
EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
5.2
Master Clear (MCLR)
The MCLR pin can trigger an external Reset of the
device by holding the pin low. These devices have a
noise filter in the MCLR Reset path that detects and
ignores small pulses.
VDD
VDD
D
The MCLR pin is not driven low by any internal Resets,
including the Watchdog Timer.
R
R1
MCLR
In PIC18F2331/2431/4331/4431 devices, the MCLR
input can be disabled with the MCLRE Configuration
bit. When MCLR is disabled, the pin becomes a digital
input. For more information, see Section 11.5
“PORTE, TRISE and LATE Registers”.
PIC18FXXXX
C
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.
5.3
Power-on Reset (POR)
A Power-on Reset pulse is generated on-chip when-
ever VDD rises above a certain threshold. This allows
the device to start in the initialized state when VDD is
adequate for operation.
2: R < 40 k is recommended to make
sure that the voltage drop across R does
not violate the device’s electrical
specification.
To take advantage of the POR circuitry, tie the MCLR
pin through a resistor (1 k to 10 k) to VDD. This will
eliminate external RC components usually needed to
create a Power-on Reset delay. The minimum rise rate
for VDD is specified (Parameter D004). For a slow rise
time, see Figure 5-2.
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).
When the device starts normal operation (i.e., exits the
Reset condition), device operating parameters (such
as voltage, frequency and temperature) must be met to
ensure operation. If these conditions are not met, the
device must be held in Reset until the operating
conditions are met.
5.4
Brown-out Reset (BOR)
A Configuration bit, BOREN, can disable (if clear/
programmed) or enable (if set) the Brown-out Reset
circuitry. If VDD falls below VBOR (Parameter D005A
through D005F) for greater than TBOR (Parameter 35),
the brown-out situation will reset the chip. A Reset may
not occur if VDD falls below VBOR for less than TBOR.
The chip will remain in Brown-out Reset until VDD rises
above VBOR. If the Power-up Timer is enabled, it will be
invoked after VDD rises above VBOR; it then will keep
the chip in Reset for an additional time delay TPWRT
(Parameter 33). If VDD drops below VBOR while the
Power-up Timer is running, the chip will go back into a
Brown-out Reset and the Power-up Timer will be
initialized. Once VDD rises above VBOR, the Power-up
Timer will execute the additional time delay. Enabling
the Brown-out Reset does not automatically enable the
PWRT.
Power-on Reset events are captured by the POR bit
(RCON<1>). The state of the bit is set to ‘0’ whenever
a POR occurs and does not change for any other Reset
event. POR is not reset to ‘1’ by any hardware event.
To capture multiple events, the user manually resets
the bit to ‘1’ in software following any Power-on Reset.
Note:
The following decoupling method is
recommended:
1. A 1 F capacitor should be connected
across AVDD and AVSS.
2. A similar capacitor should be
connected across VDD and VSS.
2010 Microchip Technology Inc.
DS39616D-page 49
MICROCHIP [ MICROCHIP ]
