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1701 PLL filter ... How to verify it's working?

Question asked by dzingoni on Dec 15, 2015
Latest reply on Dec 23, 2015 by DaveThib

Hi everybody, I've a problem with a set of boards using the 1701.

Due to design constraints we use a 24.576Mhz clock (an XO module), connected to the master clock input.Of course the PLL mode pins are both set to 1 so that we should get a 2X multiplier for the internal clock..

 

The board is running fine, but only runs if the 1701 "Program Length" is set at 2X (512 instructions) even if I'm running at 48 Khz.

Moreover an oscillator source set at 1Khz actually produces 500Hz at the outputs.

 

After much investigation I realized that probably the PLL is not working, and the chip instead of multiplying the external clock x 2 to generate the core clock it just uses the external one (so it runs at 24.576 Mhz instead of 49...). This is the only possible explaination for this behaviour.

 

If I set the program lenght to 1X everything stops, although the DSP is alive (I can read / write registers). If I set to 2X or 4X it works.

 

Now I checked everything several times. Clocks, dividers, GPIOs, power supplies ... everything is OK.

 

The only doubt that I have is that I cannout understand if the PLL is working. Checking pin PLL_LF with a scope I see it's at 3.3V. I don't know if I should see something on this pin, or if the scope is loading the circuit and actually stops the pll from working (but I don't hear any changes when connecting the probe even when the dsp is running at 2X).

 

Is there a way to understand if the PLL is working? What waveform should be present on that pin?

I'm sure that the PLL components are OK, and the board is working fine.

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