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ADAU1701: external frequency change detector

Question asked by jon71 on Jan 9, 2015
Latest reply on Jan 15, 2015 by jon71

In my design (ADAU1701 self-booting) I have a switch that selects sample rate by changing an external oscillator's frequency.  I need to generate a reset when that switch changes so that -1701 can load a new program for the new sample rate, but I am completely tapped out on GPIOs, and I'm not sure how -1701 will respond when its sample rate suddenly changes anyway.  In DSP I would code a sample/hold & compare, but not having that ability I thought of comparing the output of a D flip-flop to its input (from the switch), and triggering a reset if different.  The trouble is the flip-flop would have to be clocked by the -1701 (thinking of using LRCK), since the MCLK frequency is too fast to trigger a reset chip like ADM823.  But when the -1701 is reset, the LRCK will go away.  So that would mean an external frequency divider on the oscillator as well, and now I've exceeded my available PCB space.

 

Can anyone think of a better method?  Other than adding a MCU, which has the same problem of space?  I suppose I could add a low-frequency oscillator, but I'm not keen on another clock if there is a better solution.

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