Attiny85 forced to 8MHz

i need small attiny85 runs @ 8mhz because has make fft...

i bit lost... promise have read threads... have problems summarize info.

i know how use arduino uno programmer. know have modify boards.txt. there fuses cannot understand.

first step. original boards.txt, showing part corresponding uno programmer:

attiny85arduinoisp.name=attiny85 (w/ arduino isp)
attiny85arduinoisp.upload.using=arduino:arduinoisp
attiny85arduinoisp.upload.maximum_size=8192
attiny85arduinoisp.build.mcu=attiny85
attiny85arduinoisp.build.f_cpu=1000000l
attiny85arduinoisp.build.core=attiny45_85

so have modify frequency:
attiny85arduinoisp.build.f_cpu=8000000l

ok. now, think have add lines fuses... right?

what lines? read in 1 thread writing these additional lines, not work if not flash bootloader  ??

any help?

thanks in advance.

writing flags done part of write bootloader process in arduino ide.  if familiar 'avrdude' can use write fuses.

if attiny85 factory chips comes clock set 8mhz , pre-scaler flag set initialize pre-scaler divide 8, resulting in cycle rate of 1mhz.  can change pre-scaler in code: set clkpr = 0x80; (to enable changes) , clkpr = 0x00; (to set divisor 1).

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