[alsa-devel] SoC Atmel SSC stereo problem

[Patrick Ziegler]

Hi Alex,

>> The SSC is connected to a FPGA that assigns the bus to different devices
>> depending on the application. And for all applications the FPGA
>> generates the clocks. Maybe this is not the best solution but I will try
>> to deal with this limitation first before I try to persuade other people
>> to change it.
>>
>>      
> Hi Patrick,
>
> It looks like you are running the SSC TX in slave mode, with both the SCLK (the
> i2S clock) and LRCK provided by your FPGA.  It is trickier to prevent channel
> inversion in this mode.
>
> One possible  is to:
>
> (1)  test for the LRCK level with a gpio pin connected to the LRCK.  This should
> normally be the same pin assigned to TX_FRAME_SYNC.  After any USB set interface
> to the alternate setting for playback of your active device, or after any
> sampling rate change etc., you re-sync the transfer to the correct LRCK edge:
>
>          pdca_disable(PDCA_CHANNEL_SSC_TX);
>
>          // reset the audio buffer pointers to the start of the LEFT channel etc
>          // if required
>
>      	// re-sync SSC to LRCK
>          // Wait for the next frame synchronization event
>          // to avoid channel inversion.  Start with left channel - FS goes low
>
> 	while (!gpio_get_pin_value(_LRCK));
> 	while (gpio_get_pin_value(LRCK));	
>          // exit when FS goes low
>
>          // Enable now the transfer.
>          pdca_enable(PDCA_CHANNEL_SSC_TX);
>
> (2)  start clocking data out at any LRCK edge after a suitable delay (of 1
> SCLK).  You may need to use LRCK level change rather than any edge depending on
> the hardware timing.  The delay may also need to be adjusted depending on
> hardware timing.
>
> (3)  clock out one sample (either left or right) per LRCK edge (or level).
>
> Of course, if you are running the SSC in master mode, there are easier ways to
> ensure channel synchronization :-)
>
> The above suggestion is extrapolated from my project using the AT32UC3A3 which
> has a similar (more capable) SSC as the AT91.  So it may or may not work for
> you.
>
>    

Thank you for your suggestion, I will try to adept this approach for our 
needs.

Best regards

Patrick

-- 

Dipl.-Inf. (FH) Patrick Ziegler

University Of Applied Sciences
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