The fuse bit is a bit that can be read at a specific address to determine the state of the fuse. A value of 0 indicates that it has been blown, while a value of 1 indicates that it has not been blown. When a high level is applied to specific pins of a microcontroller (MCU) or CPU chip, the current generated can blow the fuse inside, making the program in the chip non-readable and non-writable, thus only executable. This is the effect of the fuse functionality. Many earlier microcontrollers or secure encryption chips had this fuse feature. After the product debugging is finalized, the fuse bits are processed to be blown during mass production. Although it does not directly manipulate the program and data inside the chip, it becomes unreadable, preventing pirates from conducting data analysis and indirectly achieving intellectual property protection, which is to prevent piracy. Some models of Atmel microcontrollers have this capability.
The reason why the fuse mechanism has gradually faded from history in recent years is mainly due to the upgrading of testing and analysis equipment, which has significantly reduced the cost of forcefully cracking chips. The fuse mechanism can only ensure that software development cannot achieve read and write capabilities for the program, but brute force cracking can restore the blown fuse through decapsulation, magnifying the internal circuit, and then read out the program again; or locate the program storage area and use probes to directly read the internal program and data of the chip, thus analyzing and cracking the chip.
