Support boards that auto-reset on serial open (DTR->RST), e.g. STM32W…

src/connection/nodejs_serial_connection.js

@@ -4,10 +4,14 @@ class NodeJSSerialConnection extends SerialConnection {
 
     /**
      * @param path serial port to connect to, e.g: "/dev/ttyACM0" or "/dev/cu.usbmodem14401"
+     * @param bootDelayMs ms to wait after the port opens before handshaking. Set to
+     *        ~2500 for boards that reset on connect (DTR -> RST, e.g. Wio-E5/STM32WL
+     *        devboards). Default 0 preserves the original behaviour.
      */
-    constructor(path) {
+    constructor(path, bootDelayMs = 0) {
         super();
         this.serialPortPath = path;
+        this.bootDelayMs = bootDelayMs;
     }
 
     async connect() {
@@ -23,7 +27,26 @@ class NodeJSSerialConnection extends SerialConnection {
         });
 
         this.serialPort.on("open", async () => {
-           await this.onConnected();
+
+            // De-assert DTR/RTS. On boards that wire DTR -> RST (via a cap), opening
+            // the port asserts DTR and resets the MCU; hold a steady de-asserted state
+            // to avoid further spurious resets. Harmless on native-USB boards.
+            try {
+                this.serialPort.set({ dtr: false, rts: false });
+            } catch(e) {
+                // set() unsupported / failed; ignore
+            }
+
+            // Wait for the device to finish booting before handshaking. The MCU was
+            // reset by the DTR assertion on open; the original immediate onConnected()
+            // raced that boot (~1.2-1.9 s measured on a Wio-E5) and the handshake was
+            // lost -> connection timeout. Default bootDelayMs 0 = original behaviour.
+            if(this.bootDelayMs > 0){
+                await new Promise((resolve) => setTimeout(resolve, this.bootDelayMs));
+            }
+
+            await this.onConnected();
+
         });
 
         this.serialPort.on("close", () => {

src/connection/web_serial_connection.js

@@ -2,7 +2,7 @@ import SerialConnection from "./serial_connection.js";
 
 class WebSerialConnection extends SerialConnection {
 
-    constructor(serialPort) {
+    constructor(serialPort, bootDelayMs = 0) {
 
         super();
 
@@ -16,14 +16,25 @@ class WebSerialConnection extends SerialConnection {
             this.onDisconnected();
         });
 
-        // fire connected callback after constructor has returned
+        // fire connected callback after constructor has returned.
+        //
+        // bootDelayMs lets boards that reset on connect finish booting before we
+        // send the handshake. Some boards (e.g. STM32WL / Seeed Wio-E5 devboards)
+        // wire DTR -> RST through a cap, so opening the port asserts DTR and resets
+        // the MCU. The original setTimeout(0) raced that boot -> handshake (deviceQuery)
+        // was sent into a rebooting MCU -> never answered -> connection timeout.
+        // Measured boot-to-ready on a Wio-E5 repeater: ~1.2-1.9 s, so ~2.5 s is safe.
+        // Default 0 preserves the original behaviour for native-USB boards.
         setTimeout(async () => {
             await this.onConnected();
-        }, 0);
+        }, bootDelayMs);
 
     }
 
-    static async open() {
+    static async open(options = {}) {
+
+        const baudRate = options.baudRate ?? 115200;
+        const bootDelayMs = options.bootDelayMs ?? 0; // set to ~2500 for DTR-reset boards (Wio-E5)
 
         // ensure browser supports web serial
         if(!navigator.serial){
@@ -38,10 +49,24 @@ class WebSerialConnection extends SerialConnection {
 
         // open port
         await serialPort.open({
-            baudRate: 115200,
+            baudRate: baudRate,
         });
 
-        return new WebSerialConnection(serialPort);
+        // De-assert DTR/RTS after opening.
+        //
+        // Chrome's Web Serial asserts DTR on open(); on boards that wire DTR -> RST
+        // (via a cap) that pulses a reset. We can't suppress the open-time assertion
+        // (the API gives no pre-open signal control), but holding a steady de-asserted
+        // state afterwards avoids further spurious resets mid-session. This is harmless
+        // on native-USB boards (no reset wire). setSignals may be unsupported on some
+        // platforms, so failures are ignored.
+        try {
+            await serialPort.setSignals({ dataTerminalReady: false, requestToSend: false });
+        } catch(e) {
+            // setSignals unsupported / failed; ignore
+        }
+
+        return new WebSerialConnection(serialPort, bootDelayMs);
 
     }