1. What are fuse bits?

   Fuse bits are a few bits stored in some special flash memory area. They are used to configure the how the chip will behave when powering on. (e.g. initilize system register to different value.) It is also said to be hardware set compared to software set by changing registers. For example, it can determine whether the chip has a bootloader, what speed (clock source) and voltage it is going to run at. Moreover, the status of the Fuse bits is not affected by Chip Erase.

   Terms

   • programmed: when saying a bit is programmed, it is equvilent to claim a bit is set. But it is written to 0 to be set.
   • unprogrammed: unset by written to 1.

2. Fuse bits in Atmega328/p

   The device has three Fuse bytes. Extended fuse bytes (contains 3 fuse bits), fuse high bytes (contains 8 fuse bits), and fuse low byte (contains 8 fuse bits).

   Extended Fuse byte

   Extended fuse bits are used to configure the BOD circuit (a internal circuit that guarantees a enough time-out delay for stable Vcc). Moreover, BOD would reset the MCU if the Vcc is below the trigger voltage. There are three reference voltage levels for triggering the device. The low three bits in the extended fuse byte are defined and used to specify the trigger voltage level. Atmega328p's trigger voltage efuse[2:0] = 111 (BOD disabled), 110 (1.8V), 101 (2.7V), 100 (4.3V), others (reserved)

   No.	Name	Meaning	Default value
   2	BODLEVEL2		1 (unprogrammed/unset)
   1	BODLEVEL1		1
   0	BODLEVEL0	Brown-out Detector trigger levels set the BOD reference voltage.	1

   High Fuse byte

   The high fuse byte enables/disables of the external reset, debugWIRE, serial program downloading, watchdog timer, and controls the behavior of EEPROM, the location of reset vector and the size of boot section.

   No.	Name	Meaning	Default value
   7	RSTDISBL	external reset disable	1 (unprogrammed/unset)
   6	DWEN	debugWIRE enable	1
   5	SPIEN	Enable serial program and data downloading	0 (programmed/set)
   4	WDTON	Watchdog timer is always on and works in reset mode	1
   0	BOOTRST	= 1 (unprogrammed) the reset vector is at the begining of the application code, otherwise at the beginning of boot section.	1 (arduino = 0)

   Low Fuse byte

   Low fuse byte contains 8 fuse bits. They are primarily used to configure the system clock, such as clock source, start-up time, and divided by 8.

   No.	Name	Meaning	Default value
   7	CKDIV8	Divide the clock source frequency by 8	0 (programmed/set)
   6	CKOUT	Output the clock signal in the CLKO (PB0 pin)	1 (unprogrammed/unset)
   5	SUT1		1
   4	SUT0	SUT1 and SUT0 set the minimum #cycles required for a stable clock (default is the max start-up time for default clock source)	0
   3	CKSEL3		0
   2	CKSEL2		0
   1	CKSEL1		1
   0	CKSEL0	CKSELn select the clock source (default 0010 internal 8MHz RC)	0

   The CKDIV8 bit's principle is to modify the Clock Prescaler Register (CLKPR) to divided the frequency by 8. In other words, if the CKDIV8 is programmed, then the initial value of CLKPR is 0x03.

3. How to set fuse bits?

   Attention

   1. Cannot directly modify the fuse bit via arduino bootloader. A ISP programmer has to be used.
   2. When using an Arduino as ISP, a 10uF cap is needed to connect GND and RST.

   Read arduino fuse bit

   Resource: two Arduino Nano, one 10 uF cap

   1. Upload the Arduino ISP to Arduino Nano A.
   2. 10uF cap connects Arduino Nano A's GND and RST.
   3. Connection:

      Master/Arduino Nano Programmer A	Slave/Arduino Nano B
      SCK (PB5, d13)	SCK (PB5, d13)
      MISO (PB4, d12)	MISO (PB4, d12)
      MOSI (PB3, d11)	MOSI (PB3, d11)
      ~SS (PB2, d10)	reset (RST)
      Vcc	Vcc
      GND	GND

   4. Arduino IDE burns bootloader to Arduino Nano B.
   5. Run

      avrdude -p m328p -c stk500v1 -b 19200 -P /dev/ttyUSB0 -v #v stands for verbose

   Result E: 0xFD, H:0xDA, L:0xFF

4. Resources

   1. Fuse bits calculator