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ok. I found I need a pull-up resistor after several night debug...

my environment: attiny45, default circuits which use internal RC oscillator. So PB5, PB4, PB3 and PB0 are left for own using. It works well at most test case. And now I want to use it to monitor common 4pin PC Fan speed. I use Timer 0 overflow interrupt: TCCR0B = 0x02;// 1:8 presc. TCNT0 =0; TIMSK= ...