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 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
duration of the pulse required to cause a Reset gets
shorter. 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.
VTRIP(MIN) - VDD
Area above curve will
generate a reset signal
Area below curve will
not generate a reset signal
Transient Overdrive Voltage (mV)
Example of Typical
Transient Duration Waveform.
4.4 Manual Reset with Glitch Filter
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.
Push Button Reset.
The MR input typically ignores input pulses
of 100 ns.
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.
© 2007 Microchip Technology Inc.