[alsa-devel] Output frequency configuration for I2S in beagleboard? (underrun problem)

Peter Ujfalusi peter.ujfalusi at nokia.com
Thu May 14 08:16:22 CEST 2009

On Wednesday 13 May 2009 21:08:20 ext Ernesto Torres wrote:
> Hi,
> thanks for your replies, unfortunately, the codec has to be slave, it
> doesn't support yet to send clock via i2s.

Too bad.

> I have another question, it
> might be seem simple but it will clarify some of my thoughts.
> From omap3beagle.c hw_params:
>     /* Set the codec system clock for DAC and ADC */
>     ret = snd_soc_dai_set_sysclk(codec_dai, 0, 26000000,
>                      SND_SOC_CLOCK_IN);
> The sysclk is set to 26Mhz and it is coming from the TWL which acts as
> master.

26MHz is the MCLK, which is used by the TWL4030 codec's APLL as a master 

> I had it configured to 12288000(info from datasheet) which was suppose to
> work until I knew the codec I'm using can't be master. (It is still is on
> 12288000)

So you modified the omap3beagle.c file to use your codec, which is not the 
TWL4030, and it is not on the McBSP2 bus, right? Or have you written a new 
board file for your codec and removed the twl4030 codec from the kernel?

> How should it be configured to make the OMAP Master? Should I leave it as
> 26Mhz and do manually all the divisions? I'm using SND_SOC_DAIFMT_CBS_CFS
> for the Beagle.

You seams to be confusing things here...

In order to use the OMAP as master:
In the hw_param function (on my custom board with twl4030 codec as slave):

/* codec is slave */
ret = snd_soc_dai_set_fmt(codec_dai,
/* OMAP is master */
ret = snd_soc_dai_set_fmt(cpu_dai,
/* codec MCLK is 19.2MHz in my board, 26 MHz on beagle */
ret = snd_soc_dai_set_sysclk(codec_dai, 0, 19200000,
/* McBSP clock settings for my custom board.
 * Note that the freq is there for reference, it is not
 * used by the McBSP code. Only the divider is used.
 * You need to figure out the correct frequencies and dividers
 * for beagle. You can try these also values, but I would not
 * count on that these are correct for beagle...
 * The important things in here:
 * div: divider
 * clk_id: Master clock used for the divider */
div = clk_id = freq = 0;
switch (params_rate(params)) {
case 44100: /* 44.117 */
        div = 68;
        freq = 96000000;
case 48000: /* 48.032 */
        div = 54;
        clk_id = OMAP_MCBSP_SYSCLK_CLK;
        freq = 83000000;
        printk(KERN_ERR "hw params: unknown rate %d\n",
        return -EINVAL;

ret = snd_soc_dai_set_sysclk(cpu_dai, clk_id, freq, SND_SOC_CLOCK_IN);
ret = cpu_dai->dai_ops.set_clkdiv(cpu_dai, OMAP_MCBSP_CLKGDV, div);

Since you want OMAP as master you need to configure the cpu_dai correctly to 
provide the clocks for the codec.

Does this clear things for you?

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