RP2040 Software Design

Hardware to Interface

• TI TAA3040 - 4 channel audio ADC
  • SPI or I2C control interface
  • TDM audio sample output
• some stereo DAC - probably another TI part with TDM or I2S input
• preamp (optional, for other applications) https://www.analog.com/en/products/ssm6322.html?doc=SSM6322.pdf
• USB interface to present device as input and output channels to a host computer
• some user interface controls
  • knobs
  • buttons
  • a display would be cool

4-Channel ADC samples in via TDM Interface - WIP

First pass of the PIO program was written, intending to test with another pico providing the TDM output signal

• PIO program to receive samples on serial pin as a slave to the TAA3040 ADC providing bit clock and sync pulse
• one 32-bit sample will be pushed to the RX_FIFO at a time, one sample from each channel per TDM frame

3 digital pins

ADC control and configuration via SPI or I2C - TODO

There are a lot of features on the TAA3040 that are configurable by software. For a first pass, just set the chip up in a static configuration and let it go. For future improvement, there are a ton of ways it could be set up and integrated with the USB audio specification.

2-Channel stereo output to DAC via I2S or TDM - WIP

This will take samples returned from the USB host and output to the DAC. This could be I2S as that supports 2 channels no problem. Probably a slightly simpler PIO program than TDM. Select between line output and headphone output.

TDM output of 4 channels has been implemented with PIO. This will allow me to test the 4 channel input side of the code. The program should easily adapt to 2 channels.

3 digital pins

DAC control interface via SPI or I2C - TODO

select a chip that's also controlled via SPI or I2C.

USB Audio interface - TODO

RP2040 can implement a Full-speed 12Mbps device

transmit 4 channels of samples to the host, and receive 2 channels back from the host. Also handle configuration - there are a ton of things that could be exposed to the USB host as configurable options and it would be cool to support a lot.

Asynchronous interface since the ADC generates its own clock

USB Audio will transmit a packet of samples every 1ms

Moving sample data with DMA

• samples can move one at a time from ADC PIO RX FIFO to USB transmit buffer
  • seems like the TX buffer can't directly interface with DMA
  • instead handle interrupt from TinyUSB when tx buffer is empty to set up DMA descriptor to transfer from elsewhere in ram to the USB buffer
  • two steps
    • from pio to generic buffer in memory
    • from buffer to TX buffer when data is required
• samples can move one at a time from USB receive buffer to DAC output PIO TX FIFO
  • may need to do the same thing handling an interrupt when the rx buffer is filled
    • RX buffer to RAM buffer
    • RAM buffer to PIO TX FIFO
• bypass mode could move samples from primary input channel to output without going through USB
  • a little latency but definitely less than the 2ms at minimum round trip to/from USB host
• could have some self-contained mixing capability that would go from ADC channels -> pico software -> DAC channels
• probably don't need to also implement a total analog bypass, but could provide a physical switch