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examples: update README

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Keenan Tims
2026-05-16 16:24:57 -07:00
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# usbd-uac2 Examples Two example backends are provided, both based on the LPCXpresso55S28 demo board, using the onboard WM8904 DAC.
This repository contains example implementations of a USB Audio Class 2 (UAC2) device. ## LPC55S28 (LPCXpresso55S28)
Two example backends are provided, both based on the LPCXpresso55S28 demo board, using the onboard WM8904 DAC: These examples work on the
(LPCXpresso55S28)[https://www.nxp.com/design/design-center/software/development-software/mcuxpresso-software-and-tools-/lpcxpresso-boards/lpcxpresso55s28-development-board:LPC55S28-EVK]
development board. They use the onboard WM8904 codec, so don't require any
additional hardware for audio I/O.
- **Interrupt-driven example** (`lpc55s28-evk`) They can be programmed and debugged using the CMSIS-DAP that's on the board,
- **DMA-based example** (`lpc55s28-evk-dma`) though due to the security features you may need to reset into the DFU
bootloader first.
```
cargo embed --release
```
## Examples Overview Enable logging with `DEFMT_LOG` when building, but beware that enabling defmt
logs can cause timing failures, and _must_ be drained by the host, as it is
blocking.
They work on either USBFS or USBHS, but not both at the same time. The default
is USBHS. If you would like to run the example on USBFS, disable default
features and select USBFS:
```
cargo embed --release --no-default-features --features usbfs
```
### Interrupt-driven (`lpc55s28-evk`) ### Interrupt-driven (`lpc55s28-evk`)
Running at 32bit/48khz. It can't keep up at 96khz. Works on USBFS and USBHS. Running at 32bit/48khz. Simultaneous input and output. Works on USBFS and USBHS.
This is a minimal implementation intended to demonstrate the fundamental This is a minimal implementation intended to demonstrate the fundamental
structure of the class driver. It fills a `bbqueue` as data comes in from USB, structure of the class driver. The architecture is not recommended for a production
and drains it into the I2S FIFO in the I2S interrupt. This requires a lot of device, but it does work reliably on the happy path.
time-critical CPU work managing buffers. Particularly, the USB peripheral driver
uses a lot of interrupt-free critical sections which can cause late interrupts
and underruns.
This is intended primarily as a learning/reference implementation This is intended primarily as a reference implementation to aid understanding of
the class driver.
### DMA-based (`lpc55s28-evk-dma`) ### DMA-based (`lpc55s28-evk-dma`)
Running at 32bit/96khz. Works on USBFS and USBHS. Running at 32bit/96khz. Output only. Works on USBFS and USBHS.
A more realistic and robust implementation using DMA. It fills a static ring A more realistic and robust implementation using DMA. It fills a static ring
buffer as data comes in from USB, while the DMA chases it around the ring, buffer as data comes in from USB, while the DMA chases it around the ring,
@@ -39,10 +54,3 @@ edge cases, error conditions and so on you would want in a fully fleshed out
implementation. Particularly, it behaves poorly in underrun, since the DMA will implementation. Particularly, it behaves poorly in underrun, since the DMA will
keep emitting from the ring regardless of whether the data is valid, which keep emitting from the ring regardless of whether the data is valid, which
sounds terrible. sounds terrible.
## Running the Examples
You can flash and run either example using `cargo embed`:
```sh
cargo embed --release --example lpc55s28-evk --features usbfs