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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

information.

4.4

Power-on Reset (POR)

A Power-on

Reset

pulse

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

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.

15 k

< R < 40 k is recommended to make

sure that the voltage drop across R does not

violate the device’s electrical specification.

1 k will limit any current flowing into

MCLR from external capacitor C, in the event

MCLR/VPP

breakdown,

due

Electrostatic Discharge (ESD) or Electrical

Overstress (EOS).

VDD

MCLR

VDD

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