PWM and sound

Development Boards MicroMod
1

Hi, I know that this has been covered a lot, but I just can’t seem to understand.

I am trying to generate phone-quality(8bit 8khz sampling rate) sound using an atmega32 running at 16mhz.

I was tinking about using Timer0 (8bit) for that.

Technically, All I would need to do is setup the timer correctly and write to OCR0 on every TIMER0_OVF interupt right?

I am using the Fast PWM mode and If I am using a prescaller of 8, my frequency will be 16MHZ/8/TOP=7812hz. Which is not 8KHZ

I have seen somewhere that I could use a prescaler of 8 and set TOP to 250. But how do I set TOP=250?

Another place suggested to use the OCR0 to limit TOP, but that is not possible when in fast PWM mode since OCR0 is used to drive the OC0 pin.

2

dumais:
8bit 8khz sampling rate

The question not in how to get 8khz, question how you are going to get 8bit (256 different voltage levels on output pin).

3

The ATmega fast PWM mode will run at a fixed frequency (your example calculation is quite correct). There’s no way to fine-tune this frequency. CTC mode lets you generate other frequencies (that, I think, is what the “other place” meant about using OCR0 to limit TOP), but it doesn’t let you PWM the results.

You could upsample your data to 62.5k samples/second, digitally low-pass filter it (to remove alias bands), and then PWM at 62.5ks/s in fast PWM mode, if you have enough processor bandwidth to do that. Or you could buy a cheap external DAC and run it at 8ks/s. Or you could really really cheat and PWM at 7.8ks/s, and drop a sample every now and then: 8ks/s and 8 bits is pretty atrocious sound quality so you may well decide that Cheap is Better and live with the dropped samples (the cheater’s down-sample filter).