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 pre vent low-pow er conditions tha t can
cause many different system problems. The most
common causes are brown-out conditions, where the
system supply drop s belo w the operatin g level mome n-
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: Typic al App li ca tio n Circui t.
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 volt age re fere nce is us ed to s et the v olt age tri p
point (VTRIP). Additionally, there is a hysteresis on this
trip point.
When the falling edge of VDD crosses this voltage
threshold, the reset power-down timer (TRPD) starts.
When this delay timer times out, the RST pin is f orc ed
low.
When the rising-edge of VDD crosses this voltage
threshold, the reset power-up timer (TRPU) starts.
When this delay timer times out, the RST pin is f orc ed
high, TRPU is active and there is additional system
current.
The actual voltage trip point (VTRIPAC) will be be twe en
the minimum trip point (VTRIPMIN) and the maximum
trip point (VTRIPMAX). The hysteresis on this trip point
and the delay timer (TRPU) are to remove any “jitter”
that would occur on the RST pi n w h en th e de vi c e VDD
is at the trip point.
Figu re 4-2 shows t he wavef orm of the RST pin as deter-
mined by the VDD voltage, while Table 4-1 shows the
state of the RST pin. The VTRIP specification is for falling
VDD voltages. When the VDD voltage is risi ng, the RS T
will not be driven high until VDD is at VTRIP + VHYS. Once
VDD has crossed the voltage trip point, there is also a
minimal delay time (TRPD) before the R S T pin is driven
low.
TABLE 4-1: RST PIN STATES
FIGURE 4-2: RST Operation as Determined by the VTRIP and VHYS.
VDD
VDD
MCLR
(Reset input)
(Active-low)
VSS
PIC®
Microcontroller
RPU
Note 1: Resistor RPU may be required with the
MCP121 due to the open-drain output.
Resistor RPU 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
VDD
RST
VSS
State of RST Pin when:
Device VDD < VTRIP V
DD >
VTRIP + 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 res is tor
VDD VTRIPMAX
VTRIPMIN VTRIPAC
VTRIPAC
VTRIPAC + VHYSAC
RST
1V
< 1V is ou tside the
device spec ifications
tRPD
tRPU
tRPD tRPU