MCP809 datasheet(6/7 Pages) NSC

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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.

TABLE 1. Reset Thresholds Related to Common

Supply Voltages

Reset

Threshold

3.0V

3.3V

5.0V

4.63 ± 3%

90 - 95%

4.38 ± 3%

85 - 90%

4.00 ± 3%

78 - 82%

3.08 ± 3%

90 - 95%

2.93 ± 3%

86 - 90%

2.63 ± 3%

85 - 90%

77 - 81%

Ensuring a Valid Reset Output Down to V

CC =0V

When V

CC falls below 1V, the MCP809 RESET output no

longer sinks current. A high-impedance CMOS logic input

connected to RESET can therefore drift to undetermined

voltages. To prevent this situation, a 100k

Ω resistor should

be connected from the RESET output to ground, as shown in

Figure 1.

A 100k

Ω pull-up resistor to V

CC is also recommended for the

MCP810, if RESET is required to remain valid for V

CC < 1V.

Negative-Going V

CC Transients

The

MCP809/810

are

relatively

immune

short

negative-going transients or glitches on V

CC. Figure 2 shows

the maximum pulse width a negative-going V

CC transient

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

the

MCP809/810, a V

CC transient that goes 100mV below the

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

CC pin will provide additional transient re-

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

Ω resistor between the MCP809 RE-

SET output and the µP reset pin, as shown in

Figure 3.

Buffer the MCP809 RESET output to other system compo-

nents.

20022310

FIGURE 1. RESET Valid to V

CC = Ground Circuit

20022309

FIGURE 2. Maximum Transient Duration without

Causing a Reset Pulse vs. Reset Comparator

Overdrive

20022311

FIGURE 3. Interfacing to Microprocessors with

Bidirectional Reset I/O

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