PIC18F2450/4450
FIGURE 4-2:
EXTERNAL POWER-ON
RESET CIRCUIT (FOR
SLOW VDD POWER-UP)
4.2
Master Clear Reset (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.
VDD
VDD
The MCLR pin is not driven low by any internal Resets,
including the WDT.
D
R
R1
MCLR
In PIC18F2450/4450 devices, the MCLR input can be
disabled with the MCLRE Configuration bit. When
MCLR is disabled, the pin becomes a digital input. See
Section 9.5 “PORTE, TRISE and LATE Registers”
for more information.
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.
4.3
Power-on Reset (POR)
A
Power-on Reset pulse is generated on-chip
2: R < 40 kΩ is recommended to make sure that
the voltage drop across R does not violate
the device’s electrical specification.
whenever VDD rises above a certain threshold. This
allows the device to start in the initialized state when
VDD is adequate for operation.
3: R1 ≥ 1 kΩ will limit any current flowing into
MCLR from external capacitor C, in the event
of MCLR/VPP pin breakdown, due to Electro-
static Discharge (ESD) or Electrical
Overstress (EOS).
To take advantage of the POR circuitry, tie the MCLR pin
through a resistor (1 kΩ to 10 kΩ) 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 (parameter D004, Section267 “DC
Characteristics”). 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 operation. If these conditions are not met, the
device must be held in Reset until the operating
conditions are met.
POR events are captured by the POR bit (RCON<1>).
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 manually resets the bit to ‘1’
in software following any POR.
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 43
MICROCHIP [ MICROCHIP ]
