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2009 Microchip Technology Inc.

DS39689F-page 49

PIC18F2221/2321/4221/4321 FAMILY

5.2

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.

The MCLR pin is not driven low by any internal Resets,

including the WDT.

In PIC18F2221/2321/4221/4321 family devices, the

MCLR input can be disabled with the MCLRE Configu-

ration bit. When MCLR is disabled, the pin becomes a

digital input. See Section 11.5 “PORTE, TRISE and

LATE Registers” for more information.

5.3

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, tie the MCLR

pin through a resistor (1 k

Ω 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). For a slow rise

time, see Figure 5-2.

When the device starts normal operation (i.e., exits the

Reset

condition),

device

operating

parameters

(voltage, 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.

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; 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.

FIGURE 5-2:

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: R < 40 k

Ω is recommended to make sure that

the voltage drop across R does not violate

the device’s electrical specification.

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

Electrostatic Discharge (ESD) or Electrical

Overstress (EOS).

VDD

MCLR

PIC18FXXXX

VDD

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