Thursday, April 4, 2013

PCM/PWM Clock Calculator

I was tired of calculating the same thing over and over again. Therefore, I build this little tool for calculating the Raspberry Pi PCM/PWM clock settings according to BCM2835 Audio & PWM clocks.
The tool is provided without warranty of any kind. Please check the settings manually before applying them to your hardware.

Hint: In most cases you want to set the external divider to 1, but it can be handy if you want for example adjust a derived clock, e.g. the I2S frame sync. You can get appropriate divider settings (i.e. all integer dividers of the divisor) by hovering the mouse over the (combined) divisor field.

Questions or suggestion are gladly welcome!

By the way: Can anyone explain why someone would prefer MASH-2 or MASH-3 over MASH-1? They have a higher variance, but I don't see any advantage.
Edit: I found a note in the datasheet, that this is because of noise-shaping. This means you increase the overall noise that is introduced by the jitter, but push it into higher frequencies. This has the advantage of reducing the perceived noise when using it for audio applications.

Oscillator PLLD