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

Peter Ujfalusi peter.ujfalusi at nokia.com
Wed May 13 12:41:57 CEST 2009

On Wednesday 13 May 2009 02:28:58 ext Ernesto Torres wrote:
> Hi,
> I would like to know the actual configuration for the output of the Beagle
> Board I2S, from the OMAP35x Reference Manual, on page 2887:

Is you external codec supports master mode (provides the needed clocks on the 
I2S bus)?
I would suggest to use it in that way, since configuring OMAP to provide close 
enough frequency can be tricky.
I have did this for experimenting on our custom board, just to see how it 
This is the code is taken from the soc board file's hw_params function:

div = clk_id = freq = 0;
switch (params_rate(params)) {
case 44100: /* 44.117 */
	div = 68;
	freq = 96000000;
case 48000: /* 48.032 */
	div = 54;
	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);
if (ret < 0) {
	printk(KERN_ERR "can't set cpu system clock\n");
	return ret;

ret = cpu_dai->dai_ops.set_clkdiv(cpu_dai, OMAP_MCBSP_CLKGDV, div);
if (ret < 0) {
	printk(KERN_ERR "can't set cpu clock divider\n");

As you can see I needed to select different clock source for 44.1 and 48 KHz 
sample rate for OMAP to use as a base for the dividing...

So: try to use the OMAP as slave, if it is possible.


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