Section 4.7 Fuse Bytes
Although the microcontroller contains flash memory, EEPROM, and SRAM, it also contains non-volatile fuse bytes to affect the functioning of the microcontroller external to the program operations. There are three fuse bytes used to control the microcontroller: the extended fuse byte, the low fuse byte, and the high fuse byte. Programmed fuses have a value of 0, while unprogrammed fuses have a value of 1.
Subsection 4.7.1 Extended Fuse Byte
The extended fuse byte configures the brown-out detection unit. If a power supply fluctuates, it is possible for the microcontroller to malfunction if the voltage supplied to the VCC pin dips below a certain value. The brown-out detection unit resets the microcontroller while the voltage is underneath a given threshold as programmed by the extended fuse byte.
Subsection 4.7.2 High Fuse Byte
The high fuse byte configures features that are relevant to programming and debugging the ATmega328P microcontroller. Among other things, the bits in the high fuse byte control the ability to use serial programming, to use the watchdog timer (introduced in Section 9.8), to preserve EEPROM memory when writing new program memory to the device, and to configure the amount of memory set aside to act as a bootloader.
Subsection 4.7.3 Low Fuse Byte
The low fuse byte configures features that are relevant to the clock source used to operate the microcontroller. There are two clock sources internal to the ATmega328P, but the device may also be clocked from an external source. The start-up time is also affected by changing the low fuse byte. Clock sources require a sufficiently high value of VCC before it starts oscillating, and it must oscillate a sufficient number of times before the clock can be considered stable. A time-out delay is used after device reset to ensure a sufficient value of VCC.
