MCP809M3-2.63 NSC [National Semiconductor], MCP809M3-2.63 Datasheet - Page 6

• Manufacturer Part Number: MCP809M3-2.63
• Description: 3-Pin Microprocessor Reset Circuits
• Manufacturer: NSC [National Semiconductor]

www.national.com
Applications Information
Benefits of Precision Reset Thresholds
A microprocessor supply supervisor must provide a reset
output within a predictable range of the supply voltage. A
common threshold range is between 5% and 10% below the
nominal supply voltage. The 4.63V and 3.08V options of the
MCP809/810 use highly accurate circuitry to ensure that the
reset threshold occurs only within this range (for 5V and 3.3V
supplies). The other voltage options have the same tight
tolerance to ensure a reset signal for other narrow monitor
ranges. See Table 1 for examples of how the standard reset
thresholds apply to 3V, 3.3V, and 5V nominal supply volt-
ages.
Ensuring a Valid Reset Output Down to V
When V
longer sinks current. A high-impedance CMOS logic input
connected to RESET can therefore drift to undetermined
voltages. To prevent this situation, a 100k
be connected from the RESET output to ground, as shown in
Figure 1 .
A 100k pull-up resistor to V
MCP810, if RESET is required to remain valid for V
Negative-Going V
The
negative-going transients or glitches on V
the maximum pulse width a negative-going V
TABLE 1. Reset Thresholds Related to Common
FIGURE 1. RESET Valid to V
Threshold
4.63
4.38
4.00
3.08
2.93
2.63
MCP809/810
Reset
CC
±
±
±
±
±
±
falls below 1V, the MCP809 RESET output no
3%
3%
3%
3%
3%
3%
CC
85 - 90%
Supply Voltages
Transients
are
3.0V
relatively
CC
is also recommended for the
90 - 95%
86 - 90%
77 - 81%
CC
3.3V
20022310
= Ground Circuit
immune
CC
. Figure 2 shows
CC
resistor should
90 - 95%
85 - 90%
78 - 82%
CC
= 0V
5.0V
to
CC
transient
<
short
1V.
6
can have without causing a reset pulse. In general, as the
magnitude of the transient increases, going further below the
threshold, the maximum allowable pulse width decreases.
Typically, for the 4.63V and 4.38V version of the
MCP809/810, a V
reset threshold and lasts 20µs or less will not cause a reset
pulse. A 0.1 µF bypass capacitor mounted as close as
possible to the V
jection.
Interfacing to µPs with Bidirectional Reset Pins
Microprocessors with bidirectional reset pins, such as the
Motorola 68HC11 series, can be connected to the MCP809
RESET output. To ensure a correct output on the MCP809
even when the microprocessor reset pin is in the opposite
state, connect a 4.7k
SET output and the µP reset pin, as shown in Figure 3 .
Buffer the MCP809 RESET output to other system compo-
nents.
FIGURE 2. Maximum Transient Duration without
FIGURE 3. Interfacing to Microprocessors with
Causing a Reset Pulse vs. Reset Comparator
CC
Bidirectional Reset I/O
CC
pin will provide additional transient re-
transient that goes 100mV below the
resistor between the MCP809 RE-
Overdrive
20022309
20022311