Analysis of internal reset circuit diagram of microcontroller

The principle of the reset circuit is to connect an external resistor and capacitor to the reset pin RST of the microcontroller to achieve power-on reset. 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 from the time constant by the RC circuit.

As shown in the figure below, the microcontroller reset circuit is composed of two parts: key reset and power-on reset. Among them, the STC89 series microcontroller has a high-level reset. Usually, a capacitor is connected to the reset pin RST to VCC, and then a resistor is connected to GND. This forms an RC charge and discharge circuit to ensure that the microcontroller is on the RST pin when it is powered on. There is enough time for the high level to reset, and then returns to the low level to enter the normal working state. The typical values ​​of this resistor and capacitor are 10K and 10uF. The button reset is to connect a switch in parallel with the reset capacitor. When the switch is pressed The capacitor is discharged, RST is also pulled to a high level, and due to the charging of the capacitor, it will remain high for a period of time to reset the microcontroller.