USB Audio Class demo

For the icebreaker, the USB hardware connections are :

P1B4: USB DP
P1B3: USB DN
P1B2: Pull up. Resistor of 1.5 kOhm to USB DP

and the Audio PMOD on P1A.

For the bitsy, refer to the PCF (or edit it) to know what pins are used for audio out.

To run :

Build and flash the bitstream
- This will build fw/boot.hex and include it as the BRAM initial data
- make prog or make dfuprog
Flash the main application code in SPI at offset 1M
- make -C fw prog or make -C fw dfuprog