examples: update README

examples/README.md

@@ -1,31 +1,46 @@
-# 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`)
-- **DMA-based example** (`lpc55s28-evk-dma`)
+They can be programmed and debugged using the CMSIS-DAP that's on the board,
+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`)
 
-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
-structure of the class driver. It fills a `bbqueue` as data comes in from USB,
-and drains it into the I2S FIFO in the I2S interrupt. This requires a lot of
-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.
+structure of the class driver. The architecture is not recommended for a production
+device, but it does work reliably on the happy path.
 
-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`)
 
-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
 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
 keep emitting from the ring regardless of whether the data is valid, which
 sounds terrible.
-
-## Running the Examples
-
-You can flash and run either example using `cargo embed`:
-
-```sh
-cargo embed --release --example lpc55s28-evk --features usbfs