MCP131X datasheet(29/44 Pages) MICROCHIP

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DS21985B-page 29

MCP131X/2X

4.3

Negative Going VDD Transients

The minimum pulse width (time) required to cause a

Reset may be an important criteria in the implementa-

tion of a Power-on Reset (POR) circuit. This time is

referred to as transient duration. The MCP131X/2X

devices are designed to reject a level of negative-going

transients (glitches) on the power supply line.

Transient duration is the amount of time needed for

these supervisory devices to respond to a drop in VDD.

The transient duration time (tTRAN) is dependant on the

magnitude of VTRIP – VDD (overdrive). Any combination

of duration and overdrive that lies under the duration/

overdrive curve will not generate a Reset signal. Gen-

erally speaking, the transient duration time decreases

with and increases in the VTRIP – VDD voltage. Combi-

nations of duration and overdrive that lies above the

duration/overdrive curve are detected as a brown-out

or power-down condition.

Figure 4-16 shows a typical transient duration vs.

Reset comparator overdrive, for which the MCP131X/

2X will not generate a Reset pulse. It shows that the far-

ther below the trip point the transient pulse goes, the

duration of the pulse required to cause a Reset gets

shorter. Figure 4-16 shows the transient response

characteristics for the MCP131X/2X.

Transient immunity can be improved by adding a

bypass capacitor (typically 0.1 µF) as close as possible

to the VDD pin of the MCP131X/2X device.

FIGURE 4-16:

Example of Typical

Transient Duration Waveform.

4.4

Manual Reset Input

The Manual Reset input pin (MR) allows the Reset pins

(RST/RST) to be manually forced to their active states.

The MR pin has circuitry to filter noise pulses that may

be present on the pin. Figure 4-17 shows a block dia-

gram for using the MCP131X/2X with a push button

switch. To minimize the required external components,

the MR input has an internal pull-up resistor.

A mechanical push button or active logic signal can

drive the MR input.

Once MR has been low for a time, tMRD (the Manual

Reset delay time), the Reset output pins are forced

active. The Reset output pins will remain in their active

states for the Reset delay timer time out period (t

ST)

Figure 4-18 shows a waveform for the Manual Reset

switch input and the Reset pins output.

FIGURE 4-17:

Push Button Reset and

Watchdog Timer.

FIGURE 4-18:

MR Input – Push Button.

4.4.1

NOISE FILTER

The noise filter filters out noise spikes (glitches) on the

Manual Reset pin (MR). Noise spikes less than 100 ns

(typical) are filtered.

Time (µs)

VTRIP(MIN) - VDD

tTRANS

VTRIP(MAX)

VTRIP(MIN)

(Overdrive)

(Duration)

VDD

VSS

RST

WDI

I/O

MCLR

+5V

MCP13XX

PIC® MCU

RST

VIL

tMR

RST

tMRD

VIH

tRST

The MR input typically ignores input pulses

of 100 ns.

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