Re: [PATCH v2] ASoC: atmel_ssc_dai: Allow more rates

From: Bo Shen
Date: Sun Feb 08 2015 - 22:07:24 EST

Hi Peter,

On 02/07/2015 06:51 PM, Peter Rosin wrote:
Mark Brown wrote:
On Wed, Feb 04, 2015 at 12:52:25PM +0100, Peter Rosin wrote:

One thing remains a bit unclear, and that is the 500ppm deduction. Is
that really warranted? The number was just pulled out of my hat...

I don't really get what this is supposed to be protecting against.

+ t.min = 8000;
+ t.max = ssc_p->mck_rate / mck_div / frame_size;
+ /* Take away 500ppm, just to be on the safe side. */
+ t.max -= t.max / 2000;
+ t.openmin = t.openmax = 0;
+ t.integer = 0;
+ ret = snd_interval_refine(i, &t);

As I understand it this is a straight divider rather than something that's doing
dithering or anything else more fancy. Given that it seems as well just to
trust the clock rate we've got - we don't do any error tracking with the clock
API (perhaps we should) and for many applications some degree of
divergence from the nominal rate is not
*too* bad for audio systems (for application specific values of "some"
and "too" of course). If it is just dividers I'm not sure the situation is really
improved materially by knocking off the top frequency.

If we are doing something more fancy than divididing my analysis is off base
of course.

I'm thinking that the SSC samples the selected BCK pin using the (possibly
divided) peripheral clock. Getting too near the theoretical rate limit would
be bad, if these two independent clocks drift the wrong way. At least that
is my take on it, but I don't know the internal workings of the SSC, so...

I was hoping that someone from Atmel could chime in? Maybe I'm totally

Sorry for late response.

off base, and the SSC is doing this completely differently?

What you mean here? I am not sure I fully understand.

In our application, we're not near the limit. Therefore, it really doesn't
matter much to us.

Should I resend w/o the 500ppm deduction?


Best Regards,
Bo Shen
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