AVR MCU Status and Programming

 (1) In the AVR device manual, the terms Programmed and Unprogrammed are used to define the fuse status. Unprogrammed means the fuse status is "1" (disabled); Programmed means the fuse status is "0" (allowed).    
   (2) AVR fuses can be programmed multiple times and are not one-time OPT fuses. (3) Fuse configuration can be achieved through parallel, ISP serial, and JTAG serial modes. 
   (4) After the AVR chip is encrypted and locked (LB2/LB1=1/0,0/0), the FLASH and EEPROM data inside the chip cannot be read in any way, but the fuse status can still be read, but the configuration cannot be modified. 
   (5) The chip erase command clears the data in the FLASH and EEPROM, and at the same time configures the two lock bit statuses to the unlocked state (LB2/LB1=1/1), but the chip erase command does not change the configuration of its fuses. 
   (6) The correct operation procedure for downloading programming is: when the chip is unlocked, download the running code and data, configure the relevant fuse bits, and finally configure the chip's encryption lock bits. (7) If the chip is locked after encryption, and it is found that the fuse bits are configured incorrectly, you must use the erase command to clear the chip data, unlock the encryption, and then re-download the running code and data, modify the configuration of the relevant fuse bits, and finally configure the chip's encryption lock bits again.