ADAU1701: external frequency change detector
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.
Audio Signal Processors
ADM823
Supervisory Circuits
Single-Supply Monitors
SigmaDSP Audio Processors
adau1701
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Hi Jon,
Could you use a Schmidt-Trigger XOR gate and a RC as shown? The IC is a 5-pin SOT, and the R and C could be 0402 size.
Best regards,
Bob
I am laughing at myself here, funny how I want to make some simple so complicated. I have a tendency to look for ICs first because *usually* they will take less space than discretes. Usually!
Well, at least I knew I needed a logic gate--and I still think that should be XNOR because RESET is active low?
Hi Jon.
Yep, should be XNOR to get the usual !Reset. Also, I didn't think of asking if your switch needs debouncing. Just in case, here's the simplest debouncer I know of (assuming you can use a SPDT mechanical switch):
Bob
The reset ICs have internal debounce, and a 240ms reset anyway. Shows why I've gotten lazy about discretes, because the folks at ADI make parts that are too clever!
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