Square wave 40kHz with ATtiny10

I'm using a 12 MHz ATtiny10 and I would like to generate a square wave on PB0 at 40 kHz. I think that I need CTC. What should be the values of TCCR0A, TCCR0B, OCR0A and OCR0B?

TCCR0A = COM0A(0x01); //Toggle OC0A on compare match