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PCM2903C: USB Audio amplifier input

Jeff Wilhelm
Prodigy 30 points

Part Number: PCM2903C
Other Parts Discussed in Thread: PCM2903

I'm trying to get a USB audio input board working for an integrated amplifier. This design is based on the PCM2903EVM schematic. I did a previous version that worked flawlessly on a small 4 layer board and literally copied and pasted the schematic and layout into a larger board containing a couple other inputs. I've assembled three of these boards and am having the same issue with all three, USB device Not recognized when I plug it into the same two PCs that my initial layout worked perfectly on. All powers and grounds seem to measure correctly at all the appropriate pins on the PCM2903. The AND gate circuit is providing 3.3V to the D+ USB line when connected as should be. The only thing I haven't been able to get a valid measurement of is the crystal  XI pin. I've tried measuring the frequency of it with my scope with my probe set on 10x and AC coupled ground but I'm not getting a steady frequency reading. Any suggestions where to go from here?

  • 0
    Prodigy 30 points

    I think I've managed to make a more legible schematic. My software isn't great for printing.

  • 0 in reply to Jeff Wilhelm
    TI__Guru 64715 points

    Since you're having trouble seeing a stable frequency from the XTAL then our first guess is that the crystal load capacitance was perhaps not optimally selected and the device doesn't have it's main clock input.  

    Try 18pF is a little large for crystal shunt capacitances, perhaps the total capacitance was supposed to be 18pF and that means 9pF on each side.  Maybe moving from a 2-layer to 4-layer PCB added enough additional extra capacitance that it pushed it past the oscillatory limits.

    Otherwise our alternative guesses are that something didn't get connected between the internal GND/Power planes you've added and the device.  Have you verified all of the supply voltages and that all of the GND signals have continuity to the main GND?

  • 0 in reply to Collin Wells
    Prodigy 30 points

    I have verified all of the powers and grounds. I'll try changing the capacitors on the crystal and see if that does anything.