Datasheet

MCP102/103/121/131
DS21906C-page 12  2004-2013 Microchip Technology Inc.
4.0 APPLICATION INFORMATION
For many of today’s microcontroller applications, care
must be taken to prevent low-power conditions that can
cause many different system problems. The most
common causes are brown-out conditions, where the
system supply drops below the operating level momen-
tarily. The second most common cause is when a
slowly decaying power supply causes the
microcontroller to begin executing instructions without
sufficient voltage to sustain volitile memory (RAM), thus
producing indeterminate results. Figure 4-1 shows a
typical application circuit.
The MCP102/103/121/131 are voltage supervisor
devices designed to keep a microcontroller in reset
until the system voltage has reached and stabilized at
the proper level for reliable system operation. These
devices also operate as protection from brown-out
conditions.
FIGURE 4-1: Typical Application Circuit.
4.1 RST Operation
The RST output pin operation determines how the
device can be used and indicates when the system
should be forced into reset. To accomplish this, an
internal voltage reference is used to set the voltage trip
point (V
TRIP
). Additionally, there is a hysteresis on this
trip point.
When the falling edge of V
DD
crosses this voltage
threshold, the reset power-down timer (T
RPD
) starts.
When this delay timer times out, the RST
pin is forced
low.
When the rising-edge of V
DD
crosses this voltage
threshold, the reset power-up timer (T
RPU
) starts.
When this delay timer times out, the RST
pin is forced
high, T
RPU
is active and there is additional system
current.
The actual voltage trip point (V
TRIPAC
) will be between
the minimum trip point (V
TRIPMIN
) and the maximum
trip point (V
TRIPMAX
). The hysteresis on this trip point
and the delay timer (T
RPU
) are to remove any “jitter”
that would occur on the RST
pin when the device V
DD
is at the trip point.
Figure 4-2 shows the waveform of the RST
pin as deter-
mined by the V
DD
voltage, while Table 4-1 shows the
state of the RST
pin. The V
TRIP
specification is for falling
V
DD
voltages. When the V
DD
voltage is rising, the RST
will not be driven high until V
DD
is at V
TRIP
+ V
HYS
. Once
V
DD
has crossed the voltage trip point, there is also a
minimal delay time (T
RPD
) before the RST pin is driven
low.
TABLE 4-1: RST PIN STATES
FIGURE 4-2: RST Operation as Determined by the V
TRIP
and V
HYS
.
V
DD
V
DD
MCLR
(Reset input)
(Active-low)
V
SS
PIC
®
Microcontroller
R
PU
Note 1: Resistor R
PU
may be required with the
MCP121 due to the open-drain output.
Resistor R
PU
may not be required with
the MCP131 due to the internal pull-up
resistor. The MCP102 and MCP103 do
not require the external pull-up resistor.
0.1
µF
MCP1XX
V
DD
RST
V
SS
State of RST Pin when:
Device V
DD
< V
TRIP
V
DD
>
V
TRIP
+ V
HYS
Ouput Driver
MCP102 LHPush-pull
MCP103 LHPush-pull
MCP121 LH
(1)
Open-drain
(1)
MCP131 LH
(2)
Open-drain
(2)
Note 1: Requires External Pull-up resistor
2: Has Internal Pull-up resistor
V
DD
V
TRIPMAX
V
TRIPMIN
V
TRIPAC
V
TRIPAC
V
TRIPAC
+ V
HYSAC
RST
1V
< 1V is outside the
device specifications
t
RPD
t
RPU
t
RPD
t
RPU