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Can the crystal-less USB clock recovery system (CRS) support perfect USB Audio Iscochronous sample rate?

Question asked by crome.caleb on Jun 17, 2014
Latest reply on Jun 19, 2014 by knik
Hi there,
   I'd like to build a USB Audio Class device that can synchronize to the 1kHz SOF so that samples are never lost or added due to clock frequency differences between the host crystal and the sample rate oscillator.

In order to implement this, the clock recovery has to synchronize continuously, with the SOF so that exactly 1ms of audio data happens in one SOF period.  

I currently have a clock setup so that HCI48 is synchronized to the USB SOF, and the MCO is set to the HCI48 as 12 MHz.  The MCO then drives a PLL (in a codec) that does an exact frequency conversion of 12MHz -> 12.288MHz.  However, the problem is that apparently the CRS is close enough to 48 that no USB problems occur, the CRS is not pegged perfectly on to the USB SOF frequency, so drift between SOF and the sample clock still happens.

I guess we need software that tracks buffer levels and modifies the CRS via software to constantly tweak the CRS for perfect tracking, right?  

Has anybody been through this or know if it's possible?

Thanks,
  -Caleb

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