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SubjectRe: [PATCH v2] ASoC: atmel_ssc_dai: Allow more rates
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.
>>
>>> + case SND_SOC_DAIFMT_CBM_CFS:
>>> + case SND_SOC_DAIFMT_CBM_CFM:
>>> + 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?
>
> Cheers,
> Peter
>

Best Regards,
Bo Shen


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