AVR microcontroller working status

　　When the Vcc of the AVR chip is connected to the system power supply, the microcontroller will enter

　　different states according to the different levels of the RESET pin: reset state, normal working state, and programming state.

　　1. The RESET pin level is high

　　Normally, the RESET pin is connected to the system power supply through a pull-up resistor and is high level "1", see Figure 2-13. Under

　　this condition, once the power is turned on, the AVR will enter the power-on reset state. After a short internal reset operation, the chip

　　enters the normal working state (the reset caused by BOD and WDT is similar).

　　When the AVR is in the normal working state, there are two working modes: normal program execution working mode and sleep power saving working mode

　　.

　　z Normal program execution working mode

　　The normal program execution working mode is the basic working mode of the microcontroller. Since the hardware reset operation sets the program counter

　　to zero (PC=$0000), the execution of the program always starts from the Flash address $0000 (referring to non-BOOT LOAD mode

　　startup).

　　Sleep power saving mode

　　The sleep power saving mode is a mode that puts the microcontroller in low power consumption and power saving. When the microcontroller needs to

　　wait for an external trigger signal for a long time, and only performs corresponding processing after an external trigger, or only needs to perform processing at regular intervals

　　, the sleep power saving mode can be used to reduce power consumption. When the CPU is waiting (standby state)

　　, it can enter the sleep power saving mode. At this time, the CPU suspends work and does not execute any instructions. In the sleep power saving mode,

　　only part of the microcontroller's circuits are in working state, while other circuits stop working, which can save the microcontroller's power

　　consumption and form a power-saving standby state for the system. Once there is an external trigger signal or the waiting time is up, the CPU

　　is awakened from the sleep state and re-enters the normal program execution mode.

　　2 RESET pin level is low

　　After the AVR is powered on, if the level of the RESET pin is pulled low by the outside to "0", the chip will enter and be in the reset state

　　, see Figure 2-16 and Figure 2-17. Normally, this reset state continues until the low level of the RESET pin is cancelled.

　　Once RESET returns to a high level, the AVR will restart and enter the normal working state. This feature can be used to implement

　　manual reset or external forced reset operation of the AVR system.

　　In particular, it should be noted that once the level of the RESET pin is pulled low by the outside, when certain special conditions are met, the chip will

　　enter the programming state. For example, if the chip has an SPI interface and supports SPI serial programming, the following methods will make the chip

　　enter the SPI programming state: z Pull down the SCK pin of the SPI port externally, and then apply 　　a low-level pulse

　　of at least 2 system cycles to the RESET pin externally ; 　　z After a delay of 20ms, the external SPI programming instruction is sent to the chip through the SPI port of the AVR; 　　In the Serial 　　Downloading section of the Memory Programming chapter of the AVR device manual, the hardware connection of ISP programming using the SPI interface of the AVR, the 　　process of entering the programming mode state, and the commands of serial programming are specifically introduced. 　　Once the chip enters the programming state, the running code can be written into the program memory of the AVR through the SPI port, 　　the Flash and EEPROM in the chip can be erased, data can be written (including running code), and data can be read out, and the setting of the AVR 　　configuration fuse bit, the reading of the chip model, and the locking of the encryption bit can be realized.