51 MCU reset status

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 operation does not change the content in the on-chip RAM area. The state of the 21 special function registers after reset is a fixed value, as shown in the table below.

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 program.

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 ​​(detailed explanation will be given when introducing special registers). The data in the internal RAM remains unchanged.