[sdiy] Driving SSI2144 freq control with PWM from a microcontroller

[rburnett at richieburnett.co.uk]

>> The easiest way to get PDM is just to reverse the bits from the PWM
>> counter to the PWM comparator, but not all MCUs support this mode.
> 
> Oooh, that's clever! I'd never seen that before. That should
> *absolutely* be a hardware feature on every chip with PWM...

I haven't noticed this as a hardware feature in the PWM peripherals of 
any microcontrollers that I've worked with in the past, but it would 
definitely be a worthwhile addition.  Also, easy enough to implement in 
an FPGA where you're designing your own hardware.

Tom, I suspect that one of the reasons why it's not more common is that 
most users of microcontrollers with PWM are doing "power electronics" 
type stuff...  Chopping up large currents to drive motors, dim LED 
lighting or otherwise convert power of one kind into another.  In 
conventional power-electronics applications every switching transition 
represents energy loss, decreased efficiency and heat that must be got 
rid of, so you certainly wouldn't want more than the minimum (2) 
transitions in every cycle.

So, it's a nice technique for shaping the switching noise and pushing it 
up the frequency spectrum in audio applications, but probably not high 
on the wish-list of the typical customer using built-in PWM peripherals 
for general power applications.

-Richie,