57 lines
2.3 KiB
Markdown
57 lines
2.3 KiB
Markdown
Two example backends are provided, both based on the LPCXpresso55S28 demo board, using the onboard WM8904 DAC.
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## LPC55S28 (LPCXpresso55S28)
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These examples work on the
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[LPCXpresso55S28](https://www.nxp.com/design/design-center/software/development-software/mcuxpresso-software-and-tools-/lpcxpresso-boards/lpcxpresso55s28-development-board:LPC55S28-EVK)
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development board. They use the onboard WM8904 codec, so don't require any
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additional hardware for audio I/O.
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They can be programmed and debugged using the CMSIS-DAP that's on the board,
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though due to the security features you may need to reset into the DFU
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bootloader first.
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```
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cargo embed --release
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```
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Enable logging with `DEFMT_LOG` when building, but beware that enabling defmt
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logs can cause timing failures, and _must_ be drained by the host, as it is
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blocking.
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They work on either USBFS or USBHS, but not both at the same time. The default
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is USBHS. If you would like to run the example on USBFS, disable default
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features and select USBFS:
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```
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cargo embed --release --no-default-features --features usbfs
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```
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### Interrupt-driven (`lpc55s28-evk`)
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Running at 32bit/48khz. Simultaneous input and output. Works on USBFS and USBHS.
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This is a minimal implementation intended to demonstrate the fundamental
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structure of the class driver. The architecture is not recommended for a production
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device, but it does work reliably on the happy path.
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This is intended primarily as a reference implementation to aid understanding of
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the class driver.
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### DMA-based (`lpc55s28-evk-dma`)
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Running at 32bit/96khz. Output only. Works on USBFS and USBHS.
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A more realistic and robust implementation using DMA. It fills a static ring
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buffer as data comes in from USB, while the DMA chases it around the ring,
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draining into the TX FIFO. This is efficient and decouples interrupt latency
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from data delivery. It uses the DMA interrupt to track consumed slots, but as
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long as the USB doesn't catch up to the read slot before this happens, there is
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a lot of slack for other things to be happening.
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This is a more useful demonstration, but is still lacking correct handling of
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edge cases, error conditions and so on you would want in a fully fleshed out
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implementation. Particularly, it behaves poorly in underrun, since the DMA will
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keep emitting from the ring regardless of whether the data is valid, which
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sounds terrible.
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