SM72240

SNIS154C – JANUARY 2011 – REVISED APRIL 2013

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RESET THRESHOLD

The SM72240 is available with a reset voltage of 4.63V or 3.08V which are suitable for monitoring 5.0V or 3.3V

supplies respectively.

MANUAL RESET INPUT (MR)

Many µP-based products require a manual reset capability, allowing the operator to initiate a reset. The MR input

is fully debounced and provides an internal 22 k

more than 100 ns, reset is asserted after a typical delay of 2 µs. Reset remains active as long as MR is held low,

reset or to daisy chain supervisory circuits.

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 SM72240 uses

highly accurate circuitry to ensure that the reset threshold occurs only within this range (for 5.0V and 3.3V

supplies). Table 1 shows how the standard reset threshold applies to 5.0V and 3.3V nominal supply voltages.

Table 1. Monitored Tolerance Table

Supply Voltage

Reset Threshold

3.3V

5.0V

4.63 ± %

90.8-94.4%

3.08 ± %

91.8–95.2%

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 11.

OPEN DRAIN OUTPUT

An open drain output allows easy paralleling of multiple microprocessor reset circuits without requiring additional

logic gates. Open drain outputs also allow interfacing devices of differing logic levels or families, since the output

The pull up resistor is calculated so that maximum current flow into RESET is less than 10 mA when activated.

The resistor must be small enough so that the leakage current of all connected devices does not create an

excessive voltage drop when the output is not activated. A resistor value of 100 k

Ω will generally suffice.

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