[alsa-devel] ASoC: at91sam9260-PCM1808/TAS5709-based board driver

[Pedro I. Sanchez]

Mark Brown wrote:
> On Fri, Aug 28, 2009 at 05:24:59PM -0400, Pedro I. Sanchez wrote:
> 
>> Well, it turns out that the TAS5709 documentation doesn't say the whole
>> story. It seems that after all the chip can only guess the incoming bit
>> rates in rare occasions and I can't really rely on this. What the chip
>> really needs is a MCLK signal which is a multiple of the sound file's
>> sample frequency. This is a third clock line to be provided to that chip in
>> addition to the regular TF (frame) and TK (bit) clocks from the SSC
>> interface.
> 
> That's fairly standard for CODECs - some have FLLs or similar which
> allow them to generate their master clock from the BCLK or LRCLK but
> most need a MCLK too.
> 
>> Now I know that I have to output this additional clock line via one of the
>> gpio pins of the SoC and therefore I have to somehow "attach" one of the
>> CPU internal timers to one of the SoC gpio pins (PC6, linux gpio102 pin to
>> be more exact). I have the following (borrowed) code now:
> 
> Be careful here.  Generally CODECs require some synchronisation between
> the master clock and the other audio clocks - normally the controller
> block in the CPU will have a clock it can provide with the intention
> that it be used as a master clock.
> 

Yes, but the manual says, if I still believe in it, that the MCLK needs 
no phase alignment with the bit clock and that in fact it can be 
generated by external circuitry. I'll have to wait and see.

>> 	/* PCK0 provides MCLK */
>> 	at91_set_A_periph(AT91_PIN_PC6, 0);
> 
>> How is this linking the internal pck0 timer to pin PC6? or is it?
> 
> You'd be better off asking the Atmel people on linux-arm-kernel (or some
> other Atmel-specific list if there is one) about this - it's more of a
> general question about the CPU port than an audio-specific question.
>

I was suspecting that, thank you anyway. I will keep my questions on 
this thread on the sound aspects only.

>>         ret = clk_set_parent(mclk, pllb);
>>         if (ret != 0) {
>>                 pr_err("%s: Failed to set MCLK parent\n", __func__);
>>                 goto err_parent;
>>         }
> 
>>         clk_set_rate(mclk, 11289600);
>>         clk_put(pllb);
>>         pr_info("%s: MCLK rate %luHz\n", __func__, clk_get_rate(mclk));
> 
>> ... but it prints "MCLK rate 6000000Hz". Where does the 600000 come from, I
>> don't know. And certainly there is no clock on the PC6 pin :-(
> 
> It'll be derived from the parent clock plus any dividers in the way.

Thanks again,

-- 
Pedro