Cristiano,

> I have got the feeling it is not good spending most of the CPU time
> handling interrupt routines

Where the PIC is spending its time is largely irrelevant. An interrupt is
just another routine

> Signals change from 0V to 5V, and frequency from 0Hz to 3KHz

> If anybody has a good idea, I would be very grateful.

In this project of mine for a sensor exciter, the PIC generates low
frequency 6-bit PWM using a timer. Frequency and duty are settable
with pots. There is plenty of processing time left, which is going to be
used for other controls and outputs

http://www.piclist.com/techref/microchip/HV_PWM_Proto.htm

At your highest frequency, 3kHz, that means at least 6,000 interrupts
per second, which is not a lot for a fast PIC. That would be 1666.66
instruction cycles at 40MHz

What you need to decide is the resolution and accuracy you need for
these waves. If you want a 3000Hz and 2999Hz wave at the same
time then obviously the timer will have to interrupt more frequently

How are you handling the USB ? I'm sure it could be processed under
interrupt too, without affecting the waves

wbr
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