About

Embedded engineers working with AVR microcontrollers must configure hardware behavior using non-volatile fuse bits. These settings control critical parameters like the clock source (internal RC vs. external crystal), brown-out detection levels, and bootloader memory size. Incorrectly setting these bits can render a chip unresponsive or "bricked" (e.g., by disabling the Reset pin or selecting a clock source that doesn't exist). This tool visualizes the hexadecimal fuse values required for programming. It translates cryptic bit flags into human-readable options for popular chips like the ATmega328P. The calculated hex values are directly compatible with avrdude command-line arguments.

Formulas

Fuses are active LOW (0 = Programmed/Enabled, 1 = Unprogrammed/Disabled). The calculator performs bitwise logic to generate the final Byte:

ByteValue = ∑ (BitState × 2^position)

Where BitState is 1 if the feature is disabled (unchecked) and 0 if enabled (checked), adhering to AVR convention.

Reference Data

Chip	Default Low	Default High	Default Ext	Common Usage
ATmega328P	0xE2	0xD9	0xFF	Factory Settings (Internal 8MHz / 8)
Arduino Uno	0xFF	0xDE	0xFD	External 16MHz Crystal, BOD 2.7V
ATtiny85	0x62	0xDF	0xFF	Factory Settings (Internal 8MHz / 8)
ATmega16	0xE1	0x99	N/A	Factory Settings (Internal 1MHz)

Frequently Asked Questions

The SPIEN (Serial Peripheral Interface Enable) bit controls the ability to program the chip via standard ISP programmers. If you disable this (set to 1), you cannot reprogram the chip using ISP. You would need a High Voltage Parallel Programmer (HVPP) to reset the fuses. This tool warns you against disabling it.

The Clock Select (CKSEL) bits determine the source of the clock (e.g., Crystal, Internal RC). The CKDIV8 bit is a prescaler that divides that source frequency by 8. For example, an 8MHz internal clock with CKDIV8 enabled runs the CPU at 1MHz.

In AVR architecture, a fuse bit is considered "programmed" when it is logic 0 and "unprogrammed" when it is logic 1. This is a physical property of the EEPROM technology used. Therefore, checking a feature usually results in a 0 bit.