[alsa-devel] [PATCH 2/2] ASoc: wm8731: add normal mode with 12MHz XTAL

[Matt Flax]

It appears that my logic here is flawed.
 From what I can tell, it isn't possible to get a suitable bit clock out 
of the codec when using a 12 MHz crystal. For that reason, in my 
hardware I will change crystals from 12 MHz.

I just want to check ... are there any versions of this codec which 
output the bit clock when in USB mode on the BCLK pin ?

thanks
Matt

On 11/06/16 19:16, Matt Flax wrote:
> I have realised that this second patch may not be necessary. It is 
> also likely that it will skew the actual sample rate.
>
> Charles, can you please advise me on the following...
>
> I notice that I can probably get the correct sample rates by spoofing 
> the crystal frequencies in my machine driver. For example ...
> The ratio between the USB and Normal mode fs multipliers with BOSR=0 is :
> 250/256 = 0.97656
> Similarly, I can match this with the following crystal frequency ratio 
> (crystal f in MHz) :
> 12/12.288 = 0.97656
> By induction, I assume that I can spoof a 12.288 clock with BOSR=0 for 
> sample rate = 48kHz, 96 kHz when using a 12 MHz crystal.
>
> Again, I can match ratios between the following crystal frequecies (in 
> MHz) :
> 12/16.9344 = 0.70862
> With BOSR = 1, I assume that can get 44.1 kHz and 88.2 kHz because the 
> fs ratio between normal and usb mode is :
> 272/384 = 0.70833
>
> Now these two ratios are skew by 0.41% which is equvalent to the skew 
> in 12 MHz USB mode clocks of 44.1kHz to 44.118kHz as reported in the  
> wm8731 datasheet.
>
> Can you advise me whether my idea of spoofing clocks in the machine 
> driver is perhaps a better way of acquiring the correct sample rates, 
> rather then this patch ?
>
> thanks
> Matt
>
> On 10/06/16 10:11, Matt Flax wrote:
>> This patch allows a machine to set normal mode when using a
>> 12 MHz crystal. In USB_MODE the 12 MHz crystal signal is output on
>> the BCLK pin. In NORMAL_MODE the bit clock is output on the BCLK pin.
>> The previously ignored direction variable input to the
>> wm8731_set_dai_sysclk function is used to indicate whether crystal
>> or bit clock is output on the BCLK codec pin.
>> I have ensured that this does not effect the db1200.c machine driver
>> (the only other driver to use a 12 MHz crystal in USB_MODE). It also
>> does not effect the other machine drivers which use the wm8731 codec
>> as they don't use USB_MODE nor 12 MHz crystals.
>>
>> When driving the wm8731 in normal mode with a 12 MHz crystal, the
>> DSP filters are expected to be effected. If the filters are activated
>> then it is expected that the filter cut offs are shifted by 2.3% at
>> 250 fs and 29.1% at 272 fs. The assumption is that the filters are
>> linear, which implies a linear shift in rate scaling effects the
>> filter cut off frequencies linearly. If the filters are not active,
>> there will be no effects.
>>
>> Signed-off-by: Matt Flax <flatmax at flatmax.org>
>> ---
>>   sound/soc/codecs/wm8731.c | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/sound/soc/codecs/wm8731.c b/sound/soc/codecs/wm8731.c
>> index d18261a..ec98d5a 100644
>> --- a/sound/soc/codecs/wm8731.c
>> +++ b/sound/soc/codecs/wm8731.c
>> @@ -5,6 +5,7 @@
>>    * Copyright 2006-12 Wolfson Microelectronics, plc
>>    *
>>    * Author: Richard Purdie <richard at openedhand.com>
>> + * Author: Matt Flax <flatmax at flatmax.com> 12Mhz XTAL normal mode 
>> and 32bit mode.
>>    *
>>    * Based on wm8753.c by Liam Girdwood
>>    *
>> @@ -53,6 +54,7 @@ struct wm8731_priv {
>>       int sysclk_type;
>>       int playback_fs;
>>       bool deemph;
>> +    bool bclk_12_mhz;
>>         struct mutex lock;
>>   };
>> @@ -341,8 +343,11 @@ static int wm8731_hw_params(struct 
>> snd_pcm_substream *substream,
>>       struct wm8731_priv *wm8731 = snd_soc_codec_get_drvdata(codec);
>>       u16 iface = snd_soc_read(codec, WM8731_IFACE) & 0xfff3;
>>       int i = get_coeff(wm8731->sysclk, params_rate(params));
>> -    u16 srate = (coeff_div[i].sr << 2) |
>> -        (coeff_div[i].bosr << 1) | coeff_div[i].usb;
>> +    u16 srate = (coeff_div[i].sr << 2) | (coeff_div[i].bosr << 1);
>> +
>> +    /* this determines whether to output bit or crystal clk */
>> +    if (wm8731->bclk_12_mhz)
>> +        srate |= coeff_div[i].usb;
>>         wm8731->playback_fs = params_rate(params);
>>   @@ -420,6 +425,11 @@ static int wm8731_set_dai_sysclk(struct 
>> snd_soc_dai *codec_dai,
>>         wm8731->sysclk = freq;
>>   +    if (dir == SND_SOC_CLOCK_IN) /* output the 12 MHz clock */
>> +        wm8731->bclk_12_mhz = 1;
>> +    else /* SND_SOC_CLOCK_OUT, output the bit clock */
>> +        wm8731->bclk_12_mhz = 0;
>> +
>>       snd_soc_dapm_sync(dapm);
>>         return 0;
>