AN1772 - APPLICATION NOTE

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Reset Thresholds

The Reset Circuits are available with multiple threshold voltages (refer to Table 1), so the designer can

choose the reset threshold level best suited to the system’s power supply.

The sensitivity of the Reset Circuit depends on:

■

the duration of the voltage undershoot,

■

the magnitude of the undershoot below the minimum threshold.

The generation of the reset signal depends on, the maximum voltage transient duration versus the reset

comparator overdrive, as is shown in Figure 4. Conditions falling into the area below the curve are filtered

and therefore do not generate a reset signal.

Figure 4. Reset Thresholds

Table 1. Reset Threshold Levels

Manual Reset

A manual reset is useful in many applications, from debugging new systems to waking-up from an idle or

sleep mode.

Usually an external reset is done with a switch, however switches are not ideal because of switching

bounce (voltage spikes that can occur during the switching phase). The problem can be overcome by de-

signing a specific circuit, but this requires additional discrete logic gates. The STM811 and STM812 Reset

Circuits solve the problem with a push-button reset input signal (MR). These devices have an internal de-

bounce circuit (see Figure 3), usually with an internal pull-up resistor, so there are no requirements for any

other discrete circuits connected to the device.

Device

Reset Threshold Voltage

Min

Max

Unit

STM8xxL

4.50

4.75

V

STM8xxM

4.25

4.50

V

STM8xxT

3.00

3.15

V

STM8xxS

2.85

3.00

V

STM8xxR

2.55

2.70

V

AI08628

µS

STM8xxL/ M

STM8xxR/S/T