Младший байт фьюзов Low Fuse (Low Byte) установить в значение 0xFF (все единицы).
Это соответствует внешнему кварцу на 8 МГц.

При программировании подключить кварц,
иначе программатор после программирования фьюзов перестанет видеть чип.
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ATmega8:

Старший байт фьюзов
Table 87. Fuse High Byte
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Fuse High	 Byte	Bit No.	Description			Default Value
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RSTDISBL	7	Select if PC6 is I/O pin or RESET pin	1 (unprogrammed, PC6 is RESET-pin)
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WDTON 		6	WDT always on			1 (unprogrammed, WDT enabled by WDTCR)
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SPIEN 		5	Enable Serial Program and Data	0 (programmed, SPI prog. enabled)
			Downloading
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CKOPT 		4 	Oscillator options 			1 (unprogrammed)
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EESAVE 		3	EEPROM memory is preserved	1 (unprogrammed, EEPROM not preserved)
			through the Chip Erase
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BOOTSZ1 	2	Select Boot Size			0 (programmed)
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BOOTSZ0 	1	Select Boot Size			0 (programmed) 
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BOOTRST 	0 	Select Reset Vector 			1 (unprogrammed)
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Младший байт фьюзов
Table 88. Fuse Low Byte
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Fuse High	 Byte	Bit No.	Description			Default Value
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BODLEVEL	7	Brown out detector trigger level		1 (unprogrammed)
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BODEN		6	Brown out detector enable		1 (unprogrammed, BOD disabled)
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SUT1  		5	Select start-up time			1 (unprogrammed)
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SUT0		4 	Select start-up time 			0 (programmed)
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CKSEL3  		3	Select Clock source			0 (programmed)
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CKSEL2 	 	2	Select Clock source			0 (programmed)
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CKSEL1 	 	1	Select Clock source			0 (programmed) 
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CKSEL0 		0 	Select Clock source 			1 (unprogrammed)
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