Reset of 51 MCU

Reset is the initialization operation of the microcontroller. When the microcontroller starts running, it needs to be reset first. Its function is to put the CPU and other components in the system in a certain initial state and start working from this state. Therefore, reset is a very important operation mode. However, the microcontroller itself cannot reset automatically, and must be implemented with the corresponding external circuit.

Reset circuit:

When the reset pin RST (full name RESET) of the MCS-5l series microcontroller is at a high level for more than 2 machine cycles, the microcontroller performs a reset operation. If RST continues to be at a high level, the microcontroller is in a cyclic reset state.

Depending on the application requirements, reset operations usually have two basic forms: power-on reset and power-on or switch reset.

Power-on reset requires that the reset operation is automatically realized after the power is turned on. The commonly used power-on reset circuit is shown in the left figure of Figure A below. In the figure, capacitor C1 and resistor R1 form a differential circuit for power supply + 5V. After power-on, RST is kept at a high level for a period of time. Due to the effect of the equivalent resistance in the microcontroller, the power-on reset operation function can be achieved without the resistor R1 in the figure, as shown in the right figure of Figure (A) below.

Power-on or switch reset requires that the microcontroller automatically resets after the power is turned on, and the microcontroller can also be reset by using a switch during operation. The commonly used power-on or switch reset circuit is shown in the figure (B) above. After power-on, due to the charging of capacitor C3 and the action of the inverting gate, RST remains at a high level for a period of time. When the microcontroller is already running, pressing the reset button K and then releasing it can also make RST a high level for a period of time, thereby realizing the power-on or switch reset operation.

Based on actual operating experience, the reference values ​​of capacitance and resistance for these two reset circuits are given below.

In the figure above (A): Cl = 10-30uF, R1 = 1kO

In Figure 1.27(B) above: C: = 1uF, Rl = 1kO, R2 = 10kO

The state of the microcontroller after reset:

The reset operation of the MCU puts the MCU into the initialization state, which includes setting the program counter PC = 0000H, which means that the program starts to execute from the address unit 0000H. After the MCU is cold started, the on-chip RAM is a random value. The reset operation in progress does not change the contents of the on-chip RAM area. The states of the 21 special function registers after reset are fixed values, as shown in the table below. [page]

It is worth pointing out that remembering the main states of some special function registers after reset is necessary to understand the initial state of the microcontroller and reduce the initialization part in the application.

Note: The symbol * in the table indicates a random state;

A=00H, indicating that the accumulator has been cleared;

PSW=00H, indicating that register group 0 is selected as the working register group;

SP = 07H, indicating that the stack pointer points to the 07H byte unit of the on-chip RAM. According to the stack operation rule of first adding and then pushing, the first content pushed is written to the 08H unit;

Po-P3 = FFH, indicating that 1 has been written to each port line. At this time, each port can be used for both input and output;

IP=×××00000B, indicating that each interrupt source is at a low priority;

IE=0××00000B, indicating that all interrupts are turned off;

System reset is the first step of any microcomputer system, which makes the whole control chip return to the default hardware state. The reset of 51 single chip microcomputer is controlled by RESET pin. After this pin is connected to high level for more than 24 oscillation cycles, 51 single chip microcomputer enters the internal reset state of the chip and waits in this state until RESET pin turns to low level, then checks whether EA pin is high or low level. If it is high level, the program code inside the chip is executed, if it is low level, the external program is executed.

When the 51 microcontroller is reset, some important registers inside it are set to specific values, while the data in the internal RAM remains unchanged.