Principle of the single chip microcomputer minimum system reset circuit diagram

Reset Circuit

Figure 1. Reset circuit diagram

The setting and resetting of a microcontroller are both to initialize the circuit to a certain state. Generally speaking, the function of a microcontroller reset circuit is to initialize a state machine to an empty state, for example. Inside the microcontroller, when it is reset, the microcontroller loads some registers and storage devices with a value preset by the manufacturer.

The principle of the microcontroller reset circuit is to connect an external resistor and capacitor to the reset pin RST of the microcontroller to achieve power-on reset. The reset is valid when the reset level lasts for more than two machine cycles. The duration of the reset level must be greater than two machine cycles of the microcontroller. The specific value can be calculated by the time constant of the RC circuit.

The reset circuit consists of two parts: button reset and power-on reset.

(1) Power-on reset: The STC89 series single chip is reset at a high level. Usually, a capacitor is connected to VCC on the reset pin RST, and then a resistor is connected to GND. This forms an RC charge and discharge loop to ensure that the RST pin has a high level for a sufficient time to reset the microcontroller when it is powered on, and then returns to a low level to enter normal working state. The typical values ​​of this resistor and capacitor are 10K and 10uF.

(2) Push-button reset: Push-button reset is to connect a switch in parallel with the reset capacitor. When the switch is pressed, the capacitor is discharged and RST is pulled to a high level. Moreover, due to the charging of the capacitor, the high level will be maintained for a period of time to reset the microcontroller.