Hi!
Check the attiny13 manual and find out that PWM freq can be set by the last 3 bits of TCCR0B register.
Use that code in setup section:
/*
Setting Divisor Frequency PWM on 9.6, 4.8, 1.2 MHz CPU
0x01 divisor is 1 37500, 18750, 4687 Hz
0x02 divisor is 8 4687, 2344, 586 Hz
0x03 divisor is 64 586, 293, 73 Hz
0x04 divisor is 256 146, 73, 18 Hz
0x05 divisor is 1024 36, 17, 5 Hz
*/
TCCR0B = TCCR0B & 0b11111000 | 0x02; // 0x02 divisor is 8 586 Hz
Measured real frequency is 555 Hz.
The question is - does this influence delay() function? Will test this in a near future 