TC1270AMAVRCTR datasheet(20/42 Pages) MICROCHIP

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TC1270A/70AN/71A

DS20002035F-page 20

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4.3

Negative-Going VDD Transients

The minimum pulse width (time) required to cause a

Reset

may

important

criteria

the

implementation of a Power-on Reset (POR) circuit.

This time is referred to as transient duration. The

TC127XA 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 dependent 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.

Generally speaking, the transient duration time

decreases with an increase in the VTRIP – VDD voltage.

Figure 4-9 shows an example transient duration vs.

Reset comparator overdrive. It shows that the farther

below the trip point the transient pulse goes, the shorter

the duration of the pulse required to cause a Reset

gets. So, any combination of duration and overdrive

that lays under the curve will not generate a Reset

signal. Combinations above the curve are detected as

a brown-out or power-down.

Transient immunity can be improved by adding a

bypass capacitor (typically 0.1 µF) as close as possible

to the VDD pin of the TC127XA device.

FIGURE 4-9:

Example of Typical

Transient Duration Waveform.

4.4

Manual Reset with Glitch Filter

Circuit

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 4-10 shows a block

diagram for using the TC127XA 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, tMD (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 (tRST).

Figure 4-11 shows a waveform for the manual Reset

switch input and the Reset pins output.

FIGURE 4-10:

Push Button Reset.

FIGURE 4-11:

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

tTRAN (Duration)

VTRIP(MAX)

VTRIP(MIN)

(Overdrive)

Transient Overdrive Voltage (mV)

Area below curve will

not generate a Reset signal

Area above curve will

generate a Reset signal

VDD

VSS

RST

MCLR

+5V

TC127XA

PIC® MCU

RST

VIL

tMR

RST

tMD

VIH

tRST

The MR input typically ignores input pulses

of 100 ns.

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