This is a modification of the original CXADC ClockGen at https://gitlab.com/wolfre/cxadc-clock-generator-audio-adc
- Cost reduction: The main board PCB and the Si5351A is not necessary for this mod.
- Alternative use cases: Nearly any combination of the Domesday Duplicator, CXADC cards, and this build works.
- Simplicity: Some niche use cases were cut.
There are several changes that make this possible:
- The PCM1802 is configured in 512 division mode. This allows us to directly use a 40 MHz clock with it, while producing 78125 Hz sample rate recordings.
- USB configuration was edited to allow enough buffer for the higher sample rate recording, as well as remove alternative clocks.
- The Pi Pico is underclocked to 120 MHz. This allows a simple divisor for getting 40 MHz, without fractionals which introduce jitter.
- The RP2040 has a feature where you can output divided clocks on a certain set of GPIO pins. Using this, we output 40 MHz on GPIO 21.
- PCM1802 board (€4) AliExpress
- Raspberry Pi Pico (€2) AliExpress
- Wires
- Soldering setup (iron, solder, flux)
- Enable 512 fs master mode by bridging MODE0
Unfortunately the most common PCM1802 board has a bug where the + side of the bridgable connections is not actually connected to 3V3. To fix this, you will need to add a cable.
- Connect the PCM1802 board to the Pi Pico
- 5V to VBUS
- GND to GND (any)
- PDW to GPIO17
- DOUT to GPIO10
- BCK to GPIO11
- LRCK to GPIO12
- Flash the firmware on the Pi Pico
Build and flash the contents of the firmware folder. Alternatively, you can use the prebuilt version to skip the building step.
- Connect PCM1802's SCK to GPIO21
- Build the necessary amount of clock generator adapters as in the original
- Connect their GND to GND and their clock input to GPIO21
How you make the connection is up to you, but cutting an SMA cable may be a very easy solution if using the SMA version of the adapter boards.
- Connect GNDs, connect PCM1802's SCK to the Domesday Duplicator's pin 40
Use as an externally clocked audio ADC with a MISRC
1. Connect the SMA clock output of the MISRC to SCK and GND of the PCM1802
Use the AUX pins and pcm_extract instead.