Datasheet

2005-2012 Microchip Technology Inc. DS21985D-page 33
MCP131X/2X
4.3 Negative Going V
DD
Transients
The minimum pulse width (time) required to cause a
Reset may be an important criteria in the
implementation 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 V
DD
.
The transient duration time (t
TRAN
) is dependant on the
magnitude of V
TRIP
– V
DD
(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, and increases in, the V
TRIP
–V
DD
voltage. Combinations of duration and overdrive that lie
above the duration/overdrive curve are detected as a
brown-out or power-down condition.
Figure 4-16 shows a typical transient duration versus
Reset comparator overdrive, for which the
MCP131X/2X will not generate a Reset pulse. 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. 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 V
DD
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
diagram 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, t
MRD
(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
RST
).
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)
0V
Supply Voltage
5V
V
TRIP(MIN)
- V
DD
t
TRANS
V
TRIP(MAX)
V
TRIP(MIN)
(Overdrive)
(Duration)
V
DD
MR
V
SS
RST
WDI
I/O
MCLR
+5V
MCP13XX
PIC
®
MCU
RST
V
IL
t
MR
RST
t
MRD
V
IH
t
RST
MR
The MR input typically ignores input pulses
of 100 ns.