[alsa-devel] [PATCH 2/2] ASoc: wm8731: add normal mode with 12MHz XTAL
Rob Calhoun
rcalhoun at gmail.com
Sun Jun 19 23:46:02 CEST 2016
Hi Matt,
Based on my experience with the WM8737, the bitclock will always run
at 12 MHz on these devices in USB mode. While the carrying capacity of
the channel is always 12 Mbps, the data put on the wire by the codec
depend on the sample rate, number of channels, and bit depth. This
leaves some unused capacity in the channel. It is the responsibility
of the receiving digital audio interface to pull off the appropriate
number of bits every time the LRCLK/frameclock fires and ignore the
rest of it. For example in I2S mode, 2-channel, 32-bit, the DAI should
pull off the first 32 bits following each LRCLK transition.
Whether you can use USB mode or not depends on the capabilities of the
receiving digital audio interface. For example, we tried to use WM8737
USB mode on a TI AM335x-based board because the AM335x has a 24 MHz
master clock. This will not work because the MCASP unit on AM335x has
a hard limit of 32 bits per "slot" (thus 64 bits per frame with
two-channel acquisition) and there are 128 bits per frame at 96 kHz,
which is as fast as the WM8737 will go. (Oddly the MCASP would be fine
192 kHz x 2 or 96 KHz x 4 ; the limit is based on the size of the
XRBUF register MCASP uses to swizzle data, not the speed of the
interface.)
12.288 MHz (or 24.576 MHz) are much more commonly used a master clock
frequencies in the audio world than 12 MHz, so unless you are trying
to squeeze the last dollar out of your board BOM cost you will
probably find it easier to get everything working if you switch to a
12.288 MHz master clock.
Best regards,
Rob Calhoun
On Sat, Jun 11, 2016 at 11:22 PM, Matt Flax <flatmax at flatmax.org> wrote:
> 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;
>>
>>
>
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