Reset circuit principle

Crystal oscillator

Quartz Oscillator 3225 20M OSC

In order to ensure stable and reliable operation of the circuit in the microcomputer system, the reset circuit is an indispensable part. The first function of the reset circuit is the power-on reset. Generally, the normal operation of the microcomputer circuit requires 5V ± 5% of the power supply, which is 4.75 ~ 5.25V. Since the microcomputer circuit is a sequential digital circuit, it needs a stable clock signal. Therefore, when the power supply is powered up, the reset signal will be removed only when VCC exceeds 4.75V below 5.25V and the crystal oscillator is stable. The microcomputer circuit starts normal. jobs.

The concept of reset circuit principle

The reset circuit uses it to restore the circuit to its starting state. Just like the calculator's clear button, in order to return to the original state, recalculate. The difference with the calculator clear button is that the means of resetting the circuit is different. First, the reset operation is performed immediately when the circuit is energized; second, it can be manually operated when necessary; and third, it is automatically performed according to the needs of the program or the operation of the circuit. The reset circuit is relatively simple, and most of them can be done only with a combination of resistors and capacitors. In addition, there are triodes and the like in conjunction with the program.

Reset circuit principle

The working principle of the reset circuit is as shown in the figure below. When VCC is powered on, C charges, and a voltage appears on the 10K resistor, which causes the microcontroller to reset. After a few milliseconds, C is full, the current on the 10K resistor is reduced to 0, and the voltage is also 0. The microcontroller enters the working state. During operation, press S, C discharge, voltage appears on the 10K resistor, so that the microcontroller reset. Release S, C is charged again, after a few milliseconds, the microcontroller enters the working state.

Reset circuit type reset circuit type

So far, there are four main types of microcontroller reset circuits: (1) differential reset circuit; (2) integral reset circuit; (3) comparator reset circuit; (4) watchdog reset circuit.

Differential reset circuit

The differential reset circuit is shown above. Ucc is connected to the single-chip power supply Us, and Urst is connected to the reset pin of the MCU. (In the case of the case, if the MCU is reset, only the high level of the MCU reset pin must be maintained for a certain duration.

Integral reset circuit of reset circuit principle

After power-on, due to the charging of capacitor C3 and the action of the inverting gate, RST is held high for a period of time. When the MCU is already running, pressing the reset button K and releasing it can also make RST a high level for a period of time, thus realizing the operation of power-on or switch reset.

Comparator type reset circuit of reset circuit principle

At power-on reset, since an RC low-pass network is formed, the voltage at the non-inverting input of the comparator is delayed by a certain time from the input voltage of the negative-phase terminal. The time constant of the comparator's negative phase network is much smaller than the time constant of the positive phase RC network. Therefore, when the positive terminal voltage has not exceeded the negative terminal voltage, the comparator outputs a low level, which is generated after the inverter. Level. The width of the reset pulse is mainly determined by the speed at which the normal voltage rises.

Watchdog type reset circuit

The watchdog type reset circuit mainly uses the CPU to reset the counter periodically, so that the counter value does not exceed a certain value; when the CPU cannot work normally, since the counter cannot be reset, its count will exceed a certain value, thereby A reset pulse is generated to return the CPU to normal operation.

Circuit