feat: implement USB serial with COBS-framed postcard protocol

Wire up UsbSerialJtag on ESP32-S3 boards for host communication:
- Read COBS-framed postcard commands from host via serial
- Route DisplayOn/Off to display task, everything else to radio
- Serialize responses back to host as COBS frames
- Use postcard::CobsAccumulator for chunked deserialization
- Concurrent read/write via embassy select

RAK nRF52840 USB (embassy-usb CDC-ACM) is still a stub — different
init path, will be implemented separately.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -347,6 +347,7 @@ dependencies = [
  "embassy-time",
  "embassy-usb",
  "embedded-graphics",
+ "embedded-io-async",
  "esp-backtrace",
  "esp-hal",
  "esp-hal-embassy",

Cargo.toml

@@ -23,7 +23,8 @@ static_cell = "2.1"
 embedded-graphics = "0.8"
 ssd1306 = { version = "0.10", features = ["async"] }
 
-# Async combinators (select)
+# Async IO traits and combinators
+embedded-io-async = "0.6"
 embassy-futures = "0.1"
 
 # ESP32-S3 (Heltec V3) — requires esp toolchain (espup)

src/board/heltec_v3.rs

@@ -21,8 +21,8 @@ type Iv = GenericSx126xInterfaceVariant<Output<'static>, Input<'static>>;
 type RadioSpiDevice = SpiDevice<'static, NoopRawMutex, SpiBus, Nss>;
 pub type RadioDriver = Sx126x<RadioSpiDevice, Iv, Sx1262>;
 
-// TODO: V3 has CP2102 on UART0 — USB OTG on GPIO19/20 goes nowhere
-pub type UsbDriver = ();
+// V3 has CP2102 on UART0 but also has USB-Serial-JTAG on the ESP32-S3
+pub type UsbDriver = esp_hal::usb_serial_jtag::UsbSerialJtag<'static, esp_hal::Async>;
 
 pub type DisplayI2c = I2c<'static, esp_hal::Async>;
 
@@ -111,8 +111,10 @@ impl Board {
             delay: Delay,
         };
 
-        // ── USB (placeholder — V3 uses CP2102 on UART0) ───────────
-        let usb = UsbParts { driver: () };
+        // ── USB Serial (via USB-Serial-JTAG) ───────────────────────
+        let usb_serial = esp_hal::usb_serial_jtag::UsbSerialJtag::new(p.USB_DEVICE)
+            .into_async();
+        let usb = UsbParts { driver: usb_serial };
 
         // ── Display (SSD1306 OLED on I2C, 0x3C) ───────────────────
         let i2c = I2c::new(p.I2C0, I2cConfig::default())

src/board/heltec_v4.rs

@@ -21,8 +21,7 @@ type Iv = GenericSx126xInterfaceVariant<Output<'static>, Input<'static>>;
 type RadioSpiDevice = SpiDevice<'static, NoopRawMutex, SpiBus, Nss>;
 pub type RadioDriver = Sx126x<RadioSpiDevice, Iv, Sx1262>;
 
-// TODO: USB CDC-ACM (UsbSerialJtag for dev, or full OTG stack for production)
-pub type UsbDriver = ();
+pub type UsbDriver = esp_hal::usb_serial_jtag::UsbSerialJtag<'static, esp_hal::Async>;
 
 pub type DisplayI2c = I2c<'static, esp_hal::Async>;
 
@@ -112,8 +111,10 @@ impl Board {
             delay: Delay,
         };
 
-        // ── USB (placeholder) ──────────────────────────────────────
-        let usb = UsbParts { driver: () };
+        // ── USB Serial (via USB-Serial-JTAG, shares PHY safely) ───
+        let usb_serial = esp_hal::usb_serial_jtag::UsbSerialJtag::new(p.USB_DEVICE)
+            .into_async();
+        let usb = UsbParts { driver: usb_serial };
 
         // ── Display (SSD1315 OLED on I2C, 0x3C) ───────────────────
         // Reset display controller

src/usb/task.rs

@@ -1,7 +1,12 @@
 use embassy_executor::task;
 
 use crate::board::UsbParts;
-use crate::channel::{CommandChannel, DisplayCommandChannel, ResponseChannel};
+use crate::channel::{
+    CommandChannel, DisplayCommand, DisplayCommandChannel, ResponseChannel,
+};
+use crate::protocol::{Command, ErrorCode, Response};
+
+const MAX_FRAME: usize = 512;
 
 #[task]
 pub async fn usb_task(
@@ -11,37 +16,101 @@ pub async fn usb_task(
     display_commands: &'static DisplayCommandChannel,
     has_display: bool,
 ) {
-    let _driver = parts.driver;
-    let _ = commands;
-    let _ = responses;
-    let _ = display_commands;
-    let _ = has_display;
+    cfg_if::cfg_if! {
+        if #[cfg(any(feature = "heltec_v3", feature = "heltec_v4"))] {
+            run_serial(parts.driver, commands, responses, display_commands, has_display).await;
+        } else {
+            // TODO: nRF52840 uses embassy-usb CdcAcmClass — different init path
+            let _ = (parts, commands, responses, display_commands, has_display);
+            loop { embassy_time::Timer::after_secs(3600).await; }
+        }
+    }
+}
 
-    // TODO: build embassy_usb CDC-ACM class from driver
-    // TODO: COBS-framed postcard ser/de loop:
-    //   select! {
-    //       bytes = cdc_read() => {
-    //           let cmd: Command = deserialize(bytes);
-    //           match cmd {
-    //               Command::DisplayOn => route_display(DisplayCommand::On),
-    //               Command::DisplayOff => route_display(DisplayCommand::Off),
-    //               other => commands.send(other).await,
-    //           }
-    //       }
-    //       resp = responses.receive() => serialize Response → cdc_write()
-    //   }
-    //
-    // Display command routing:
-    //   async fn route_display(cmd, display_commands, has_display, responses) {
-    //       if has_display {
-    //           display_commands.send(cmd).await;
-    //           responses.send(Response::Ok).await;
-    //       } else {
-    //           responses.send(Response::Error(ErrorCode::NoDisplay)).await;
-    //       }
-    //   }
+#[cfg(any(feature = "heltec_v3", feature = "heltec_v4"))]
+async fn run_serial(
+    serial: crate::board::UsbDriver,
+    commands: &'static CommandChannel,
+    responses: &'static ResponseChannel,
+    display_commands: &'static DisplayCommandChannel,
+    has_display: bool,
+) {
+    use embassy_futures::select::{select, Either};
+    use embedded_io_async::{Read, Write};
+    use postcard::accumulator::{CobsAccumulator, FeedResult};
 
+    let (mut rx, mut tx) = serial.split();
+    let mut cobs_buf: CobsAccumulator<MAX_FRAME> = CobsAccumulator::new();
+    let mut read_buf = [0u8; 64];
+    let mut write_buf = [0u8; MAX_FRAME];
+
     loop {
-        embassy_time::Timer::after_secs(1).await;
+        match select(rx.read(&mut read_buf), responses.receive()).await {
+            // ── Bytes from host ────────────────────────────────────
+            Either::First(result) => {
+                let n = match result {
+                    Ok(0) | Err(_) => continue,
+                    Ok(n) => n,
+                };
+
+                let mut window = &read_buf[..n];
+                'cobs: while !window.is_empty() {
+                    window = match cobs_buf.feed::<Command>(window) {
+                        FeedResult::Consumed => break 'cobs,
+                        FeedResult::OverFull(remaining) => {
+                            defmt::warn!("COBS frame too large");
+                            remaining
+                        }
+                        FeedResult::DeserError(remaining) => {
+                            defmt::warn!("COBS deserialize error");
+                            remaining
+                        }
+                        FeedResult::Success { data: cmd, remaining } => {
+                            route_command(
+                                cmd, commands, responses, display_commands, has_display,
+                            )
+                            .await;
+                            remaining
+                        }
+                    };
+                }
+            }
+            // ── Response to host ───────────────────────────────────
+            Either::Second(response) => {
+                if let Ok(buf) = postcard::to_slice_cobs(&response, &mut write_buf) {
+                    let _ = tx.write_all(buf).await;
+                }
+            }
+        }
+    }
+}
+
+async fn route_command(
+    cmd: Command,
+    commands: &CommandChannel,
+    responses: &ResponseChannel,
+    display_commands: &DisplayCommandChannel,
+    has_display: bool,
+) {
+    match cmd {
+        Command::DisplayOn => {
+            if has_display {
+                display_commands.send(DisplayCommand::On).await;
+                responses.send(Response::Ok).await;
+            } else {
+                responses.send(Response::Error(ErrorCode::NoDisplay)).await;
+            }
+        }
+        Command::DisplayOff => {
+            if has_display {
+                display_commands.send(DisplayCommand::Off).await;
+                responses.send(Response::Ok).await;
+            } else {
+                responses.send(Response::Error(ErrorCode::NoDisplay)).await;
+            }
+        }
+        other => {
+            commands.send(other).await;
+        }
     }
 }