Hi,

> What happens if I run this code say at 4mhz 16F870 ? there will be no output, frequency wont match, or sound will be breaking up?

There's not enough time to actually execute the interrupt service routine before the timer fires another interrupt. The CPU will spend all of its time in the ISR, no time left for you to set the desired frequencies etc and the output frequenices will be wrong and...... Simply - It just won't work. You need 16MHz.

> on 16F876A, gives error during compile on this:
> IPR1.1 = 1 ' TMR2 high priority

The 16F series doesn't have interrupt priorities, remove the line.

> Gives same error on 12F1840.

The 12F series doesn't have interrupt priorities, remove the line.

/Henrik.