Dictionary of Electronics
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Power-On Reset Circuits

The classical approach to resetting a circuit is to momentarily pulse a circuit which than causes an IC to reset. The require reset pulse could be a high or low depending on the circuit receiving the reset, and or how the pulse is generated. This circuit combines a reset switch with a Debounce Circuit, and the IC also functions as a buffer. The Time Constant of the RC circuit is chosen to filter out any switch bounce.

Switched RC passive reset circuit
Reset Switch

The momentary push button applies a reset to the circuit, the resistor capacitor filters any switch bounce, and the schmitt trigger adds hysteresis and buffers the signal. This arrangement is also called an integral reset circuit because of the location of the capacitor, or because the output is taken across the capacitor.

RC active reset circuit
Active Reset

The potential problem with this circuit arrangement occurs when sudden rapid drop-outs develop on the power supply line [Vcc]. Momentary voltage drops may be so short in duration that the capacitor voltage does no have time to react. So voltage glitches could be occurring on the voltage plane with out the capacitor having time to discharge to a level that would cause a reset.

Another reset approach is to reverse the position of the resistor and capacitor in the RC circuit. This circuit arrangement is called the differential reset circuit, again, because of the location of the output tap across the resistor. This circuit suffers form malfunction when used with power supplies that have a slow rise time, or take a long time to reach operational voltage.

RC passive reset circuit
RC Reset

As the capacitor begins to charge up during power on, the voltage rise could be so slow that the current through the resistor never reaches the threshold point required to reset the circuit. Once the voltage of the capacitor reaches Vcc, current through the resistor capacitor network drops to zero and the voltage across the resistor drops to zero. So this circuit design should only be used in a system that also uses a power supply with a rapid turn-on. Otherwise the current through the circuit just slowly flows through the RC network and the voltage across the resistor is never able to rise to the required voltage level.

Power-Up Sequence

Microcontrollers and some complex ICs have different internal functions that power up at different rates. Memory function might power up before the logic that controls it. When this is the case, the memory cells are in an unknown state as the rest of the chip power up. Some kind of power-up reset circuitry is required to insure that the IC comes up in a known state. Some processor have their own power-on reset circuit internal to the chip. The processors or circuits that use more than one voltage may required some kind of power-up sequencing circuit.

PC motherboard

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