AVR-Chat

On Mon, 5 Dec 2005 10:55:06 -0800 you wrote:

>I'm just starting to play around with sine waves using PWM and a lookup table. 
>
>I'm concerned as to the maximum frequency of a sine that can be generated. Walking through an example, based on Fast PWM.
>
>Xtal=9830400Hz
>8bit Fast PWM
>This gives PWM frequency of (9830400/256) 19200Hz.
>
>So, if I wanted to generate a sine wave at 2200Hz, I'd only be changing the pulse width ~9 times per cycle. This seems completely inadequate.
>
>As this is my first attempt at sine wave generation, I have a couple of questions
>
>1) Does the math above seem correct, or am I mistaken in my limited understanding of using PWM

The division looks incorrect. 9830400/256 = 38400.

>2) Assuming my math is correct, then there must be one rule-of-thumb for how how a frequency one can generate using fast-pwm for a given Xtal frequency. What is the rule?

>3) Anything else you'd care to add/offer?

You could sacrifice resolution to get a faster pwm by setting the top to a lower value
than 255. Also the ATtiny26 has a 64MHz PLL clock for PWM giving a PWM frequency of 250KHz
at 8 bits. 

An alternative is a dedicated synth chip. The AD9833 is not too expensive and isn't
difficult to program. 
--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
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