Add RS485 file handle for mbed crash log redirection

examples/RS485CrashLog/RS485CrashLog.ino

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+/**
+ * This example demonstrates how to redirect the Arduino Mbed crash log output to the RS485 bus.
+ * This is useful for boards that support RS485 but lack easy access to other serial output options (like the Arduino Opta)
+ * 
+ * This example forces a crash in the setup() function to showcase the functionality.
+ * Initial author: Sebastian Romero @sebromero
+ */
+
+#ifdef ARDUINO_ARCH_MBED
+#include "RS485FileHandle.h"
+REDIRECT_STDOUT_TO(&RS485Console) // Redirect mbed crash log output to RS485
+#endif
+
+void setup() {
+  // Force a crash to demonstrate the crash log over RS485
+  volatile int* p = nullptr;
+  *p = 42; // Dereference null pointer to cause a crash
+}
+
+void loop() {
+  // Nothing to do here
+}

src/RS485FileHandle.cpp

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+#include "RS485FileHandle.h"
+
+#ifdef ARDUINO_ARCH_MBED
+
+#include <ArduinoRS485.h>
+#include <errno.h>
+
+RS485FileHandle::RS485FileHandle() {}
+
+void RS485FileHandle::begin() {
+    RS485FileHandle::begin(115200);  // Default baud rate
+}
+
+void RS485FileHandle::begin(int baudRate) {
+    // Ensure idempotent initialization
+    if (!_isInitialized) {
+        const auto bitduration { 1.f / baudRate };
+        const auto wordlen { 9.6f }; // OR 10.0f depending on the channel configuration
+        const auto preDelayBR { bitduration * wordlen * 3.5f * 1e6 };
+        const auto postDelayBR { bitduration * wordlen * 3.5f * 1e6 };
+
+        RS485.begin(baudRate);
+        RS485.setDelays(preDelayBR, postDelayBR);
+        RS485.noReceive();
+        _isInitialized = true;
+    }
+}
+
+ssize_t RS485FileHandle::write(const void* buffer, size_t size) {
+    begin();  // Ensure RS485 is initialized
+
+    // Avoid repeatedly starting transmission if already in progress
+    // as mbed calls write multiple times for a single output operation.
+    // This otherwise results in mangled output on RS485.
+
+    if (!_inTransmission) {
+        RS485.beginTransmission();
+        _inTransmission = true;
+    }
+
+    size_t writtenBytes = RS485.write(static_cast<const uint8_t*>(buffer), size);    
+    return writtenBytes;  // Return the number of bytes written
+}
+
+ssize_t RS485FileHandle::read(void* buffer, size_t size) {
+    // Not implemented for RS485 output redirection
+    return -ENOSYS;
+}
+
+off_t RS485FileHandle::seek(off_t offset, int whence) {
+    return -ESPIPE;  // Not seekable
+}
+
+int RS485FileHandle::close() {
+    if (_inTransmission) {
+        RS485.endTransmission();
+        _inTransmission = false;
+    }
+    RS485.end();
+    _isInitialized = false;
+    return 0;
+}
+
+short RS485FileHandle::poll(short events) const {
+    return POLLOUT;  // Ready to write
+}
+
+int RS485FileHandle::sync() {
+    if (_inTransmission) {
+        RS485.endTransmission();
+        _inTransmission = false;
+    }
+    return 0;
+}
+
+int RS485FileHandle::isatty() const {
+    return true;
+}
+
+// Global instance for stdout redirection
+RS485FileHandle RS485Console;
+
+#endif  // ARDUINO_ARCH_MBED

src/RS485FileHandle.h

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+#ifndef RS485_FILE_HANDLE_H
+#define RS485_FILE_HANDLE_H
+
+#ifdef ARDUINO_ARCH_MBED
+
+#include "mbed.h"
+
+/**
+ * @file RS485FileHandle.h
+ * @brief Declares an mbed FileHandle that redirects stdio to the RS485 bus.
+ * This is used to capture crash logs on Mbed-based boards that support RS485 but don't have other serial output options.
+ */
+
+/**
+ * @brief FileHandle implementation that forwards standard I/O to ArduinoRS485.
+ * The baud rate is fixed at 115200 bps. The internal logic is tuned to ensure correct RS485 transmission behavior in
+ * the context of crash log output.
+ */
+class RS485FileHandle : public mbed::FileHandle {
+public:
+    /**
+     * @brief Constructs a FileHandle that targets the global RS485 interface.
+     */
+    RS485FileHandle();
+
+    /**
+     * @brief Writes a buffer to the RS485 bus.
+     * When the first write occurs, the RS485 interface is initialised automatically.
+     *
+     * @param buffer Data to send.
+     * @param size Number of bytes to write.
+     * @return Bytes written on success, or a negative error code.
+     */
+    virtual ssize_t write(const void* buffer, size_t size) override;
+    /**
+     * @brief Reads data from the RS485 bus.
+     *
+     * @param buffer Destination buffer.
+     * @param size Maximum number of bytes to read.
+     * @return Bytes read on success, or a negative error code.
+     */
+    virtual ssize_t read(void* buffer, size_t size) override;
+    /**
+     * @brief Changes the current position within the stream.
+     *
+     * @param offset Byte offset relative to whence.
+     * @param whence Reference position, defaults to SEEK_SET.
+     * @return New position on success, or -1 on error.
+     */
+    virtual off_t seek(off_t offset, int whence = SEEK_SET) override;
+    /**
+     * @brief Closes the FileHandle and ends any active transmission.
+     *
+     * @return 0 on success, or a negative error code.
+     */
+    virtual int close() override;
+    /**
+     * @brief Reports readiness for the requested events.
+     *
+     * @param events Bitmask of poll events (POLLIN, POLLOUT, ...).
+     * @return Bitmask indicating the ready events.
+     */
+    virtual short poll(short events) const override;
+    /**
+     * @brief Flushes pending data to the RS485 bus.
+     *
+     * @return 0 on success, or a negative error code.
+     */
+    virtual int sync() override;
+    /**
+     * @brief Indicates whether the handle represents a TTY-like device.
+     *
+     * @return Non-zero when treated as a TTY.
+     */
+    virtual int isatty() const;
+
+private:
+    bool _isInitialized = false;
+    bool _inTransmission = false;
+    void begin();
+    void begin(int baudRate);
+};
+
+/**
+ * @brief Global RS485 FileHandle instance used to redirect stdio.
+ */
+extern RS485FileHandle RS485Console;
+
+#endif  // ARDUINO_ARCH_MBED
+
+#endif  // RS485_FILE_HANDLE_H