Why would this set bit 5?
TCCR1B = (1<<WGM12) + (1<<CS10);
OCR1A = nnnn;
TIMSK = (1<<OCIE1A);
(1<<6) really does (1<<5), as well as (1<<5) really does (1<<6)
0x20 really does 0x40, as well as 0x40 really does 0x20
In the TIMSK, i.e. setting bit 5 (OCIE1B) sets bit 6 (OCIE1A) and vice versa, with an ATtiny2313!
The photo is of setting OCIE1A
#define OCIE1A 6
#define OCIE1B 5
Please someone try this and let me know if I am doing something wrong.