AN1772 - APPLICATION NOTE

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POWER-UP/ RESET

Power-up is the first thing that must be considered in a new design, as every system requires a time in-

terval for internal initialization after the power supply has stabilized.

During the power-up sequence, microprocessors and other complex digital devices, wait for the clock sig-

nal to stabilize and load the internal registers and boot-code required for the microprocessor to function

properly. For this reason, they always require a predefined time for power-up/reset. If the reset time is not

long enough, the system may not function properly.

Resistor-Capacitor Filter Circuit

One possible approach to delay the reset signal after power-up, is to use a Resistor-Capacitor (RC) low-

pass filter at the microprocessor reset (RST) input. This solution is inexpensive, but cannot be guaranteed

to work in all circumstances. It uses an exponential RC rise time for the reset delay after power-up (see

Figure 1). The delay can be changed easily by selecting the appropriate RC circuit.

Unfortunately, this approach has two drawbacks.

■

The reset delay depends on the power-up voltage slew rate - if the slew rate is too slow then the RC

curve tracks this rising voltage and the reset delay may be inadequate (See the dashed line in top

diagram in Figure 1).

■

The slew rate of reset signal at the microprocessor’s input depends on the reset time (typically 110-200

ms) - if longer pulses are required, the RC circuit values must be increased, which will slow the slew

rate of the reset signal and may cause the microprocessor to malfunction, due to insufficient overdrive

at the Reset input.

Some manufacturers implement power-up/reset functions in the microprocessor to avoid these problems,

however in most cases the monitoring is not very precise.

Another solution is to use an external Reset Circuit device. ST Reset Circuits have precise, predefined

voltage thresholds and reset delays and so do not encounter any of the drawbacks of the RC Filter Circuit

(see Integrated Reset Circuits section).