Ed1960:
Have been using the attiny13 core for some time with much pleasure.
However, the core already uses the TIM0_OVF_vect, therefore making it impossible to use it for own applications
There is only one timer in the t13. I don't know what else can be done.
I added 8, 16 and 20 MHz for arduino ide 1.6.5 they are not very well tested (tested on attiny13a-pu 8MHz, by chip number it should be 20 MHz but delay(1000) says it is ~8 MHz), so be aware that it could not work on your attiny13a or worse... And 20 MHz micros will not work as expected, you can fix it by editing wiring.c line 102 for 20000000 Hz. Instalation is thesame, put folder "tiny13" from zip to Documents\Arduino\hardware folder.
Sorry. I don't understand. Are you using an external clock gen?
The Attiny13 do not support 8 or 20MHz on the internal clock.