backports HWCDC to v2.0.15 (#9462)

* feat: backports HWCDC

* fix: statement order

* feat: Apply suggestions from code review

* Create HWCDC_Events.ino

* Create .skip.esp32

* Create .skip.esp32s2

* Delete libraries/ESP32/examples/HWSerial_Events/.skip.esp32s2

* Delete libraries/ESP32/examples/HWSerial_Events/.skip.esp32

* Delete libraries/ESP32/examples/HWSerial_Events/HWSerial_Events.ino

cores/esp32/HWCDC.cpp

-// Copyright 2015-2020 Espressif Systems (Shanghai) PTE LTD
+// Copyright 2015-2024 Espressif Systems (Shanghai) PTE LTD
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -30,12 +30,14 @@ static RingbufHandle_t tx_ring_buf = NULL;
 static xQueueHandle rx_queue = NULL;
 static uint8_t rx_data_buf[64] = {0};
 static intr_handle_t intr_handle = NULL;
-static volatile bool initial_empty = false;
+static volatile bool connected = false;
 static xSemaphoreHandle tx_lock = NULL;
 
-// workaround for when USB CDC is not connected
-static uint32_t tx_timeout_ms = 0;
-static bool tx_timeout_change_request = false;
+static volatile unsigned long lastSOF_ms;
+static volatile uint8_t SOF_TIMEOUT;
+
+// timeout has no effect when USB CDC is unplugged
+static uint32_t tx_timeout_ms = 100;
 
 static esp_event_loop_handle_t arduino_hw_cdc_event_loop_handle = NULL;
 
@@ -73,21 +75,17 @@ static void hw_cdc_isr_handler(void *arg) {
 
     if (usbjtag_intr_status & USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY) {
         // Interrupt tells us the host picked up the data we sent.
-        if (usb_serial_jtag_ll_txfifo_writable() == 1) {
+        if(!HWCDC::isPlugged()) {
+            connected = false;
+            usb_serial_jtag_ll_clr_intsts_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+            // USB is unplugged, nothing to be done here
+            return;
+        } else {
+            connected = true;
+        }
+        if (tx_ring_buf != NULL && usb_serial_jtag_ll_txfifo_writable() == 1) {
             // We disable the interrupt here so that the interrupt won't be triggered if there is no data to send.
             usb_serial_jtag_ll_disable_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
-            if(!initial_empty){
-                initial_empty = true;
-                // First time USB is plugged and the application has not explicitly set TX Timeout, set it to default 100ms.
-                // Otherwise, USB is still unplugged and the timeout will be kept as Zero in order to avoid any delay in the
-                // application whenever it uses write() and the TX Queue gets full.
-                if (!tx_timeout_change_request) {
-                    tx_timeout_ms = 100;
-                }
-                //send event?
-                //ets_printf("CONNECTED\n");
-                arduino_hw_cdc_event_post(ARDUINO_HW_CDC_EVENTS, ARDUINO_HW_CDC_CONNECTED_EVENT, &event, sizeof(arduino_hw_cdc_event_data_t), &xTaskWoken);
-            }
             size_t queued_size;
             uint8_t *queued_buff = (uint8_t *)xRingbufferReceiveUpToFromISR(tx_ring_buf, &queued_size, 64);
             // If the hardware fifo is avaliable, write in it. Otherwise, do nothing.
@@ -97,7 +95,7 @@ static void hw_cdc_isr_handler(void *arg) {
                 usb_serial_jtag_ll_write_txfifo(queued_buff, queued_size);
                 usb_serial_jtag_ll_txfifo_flush();
                 vRingbufferReturnItemFromISR(tx_ring_buf, queued_buff, &xTaskWoken);
-                usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+                if(connected) usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
                 //send event?
                 //ets_printf("TX:%u\n", queued_size);
                 event.tx.len = queued_size;
@@ -119,18 +117,20 @@ static void hw_cdc_isr_handler(void *arg) {
                 break;
             }
         }
-        //send event?
-        //ets_printf("RX:%u/%u\n", i, rx_fifo_len);
         event.rx.len = i;
         arduino_hw_cdc_event_post(ARDUINO_HW_CDC_EVENTS, ARDUINO_HW_CDC_RX_EVENT, &event, sizeof(arduino_hw_cdc_event_data_t), &xTaskWoken);
+        connected = true;
     }
 
     if (usbjtag_intr_status & USB_SERIAL_JTAG_INTR_BUS_RESET) {
         usb_serial_jtag_ll_clr_intsts_mask(USB_SERIAL_JTAG_INTR_BUS_RESET);
-        initial_empty = false;
-        usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
-        //ets_printf("BUS_RESET\n");
         arduino_hw_cdc_event_post(ARDUINO_HW_CDC_EVENTS, ARDUINO_HW_CDC_BUS_RESET_EVENT, &event, sizeof(arduino_hw_cdc_event_data_t), &xTaskWoken);
+        connected = false;
+    }
+
+    if (usbjtag_intr_status & USB_SERIAL_JTAG_INTR_SOF) {
+        usb_serial_jtag_ll_clr_intsts_mask(USB_SERIAL_JTAG_INTR_SOF);
+        lastSOF_ms = millis();
     }
 
     if (xTaskWoken == pdTRUE) {
@@ -138,26 +138,115 @@ static void hw_cdc_isr_handler(void *arg) {
     }
 }
 
+inline bool HWCDC::isPlugged(void)
+{
+    return (lastSOF_ms + SOF_TIMEOUT) >= millis();
+}
+
+bool HWCDC::isCDC_Connected() 
+{
+    static bool running = false;
+
+    // USB may be unplugged
+     if (!isPlugged()) {
+        connected = false;
+        running = false;
+        SOF_TIMEOUT = 5;       // SOF timeout when unplugged
+        return false;
+    } else {
+        SOF_TIMEOUT = 50;      // SOF timeout when plugged
+    }
+
+    if (connected) {
+        running = false;
+        return true;
+    }
+
+    if (running == false && !connected) { // enables it only once!
+        usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+    }
+    // this will feed CDC TX FIFO to trigger IN_EMPTY 
+    usb_serial_jtag_ll_txfifo_flush();
+    running = true;
+    return false;    
+}
+
+static void flushTXBuffer(const uint8_t *buffer, size_t size)
+{
+    if (!tx_ring_buf) return;
+    UBaseType_t  uxItemsWaiting= 0;
+    vRingbufferGetInfo(tx_ring_buf, NULL, NULL, NULL, NULL, &uxItemsWaiting);
+    size_t freeSpace = xRingbufferGetCurFreeSize(tx_ring_buf);
+    size_t ringbufferLength = freeSpace + uxItemsWaiting;
+
+    if(buffer == NULL) {
+        // just flush the whole ring buffer and exit - used by HWCDC::flush()
+        size_t queued_size = 0;
+        uint8_t *queued_buff = (uint8_t *)xRingbufferReceiveUpTo(tx_ring_buf, &queued_size, 0, ringbufferLength);
+        if (queued_size && queued_buff != NULL) {
+            vRingbufferReturnItem(tx_ring_buf, (void *)queued_buff);
+        }
+        return;
+    }
+    if(size == 0) return; // nothing to do
+    if(freeSpace >= size){
+        // just add the data to the ring buffer and exit
+        if(xRingbufferSend(tx_ring_buf, (void*)buffer, size, 0) != pdTRUE){
+            return;
+        }
+    } else {
+        // how many byte should be flushed to make space for the new data
+        size_t to_flush = size - freeSpace;
+        if(to_flush > ringbufferLength) to_flush = ringbufferLength;
+        size_t queued_size = 0;
+        uint8_t *queued_buff = (uint8_t *)xRingbufferReceiveUpTo(tx_ring_buf, &queued_size, 0, to_flush);
+        if (queued_size && queued_buff != NULL) {
+            vRingbufferReturnItem(tx_ring_buf, (void *)queued_buff);
+        }
+        // now add the new data that fits to the ring buffer
+        uint8_t *bptr = (uint8_t *)buffer;
+        if (size >= ringbufferLength) {
+            size = ringbufferLength;
+            bptr = (uint8_t *)buffer + (size - ringbufferLength);
+        }
+        if(xRingbufferSend(tx_ring_buf, (void *)bptr, size, 0) != pdTRUE){
+            return;
+        }
+    }
+    // flushes CDC FIFO
+    usb_serial_jtag_ll_txfifo_flush();
+}
+
 static void ARDUINO_ISR_ATTR cdc0_write_char(char c) {
+    if(tx_ring_buf == NULL) {
+        return;
+    }
+    if(!HWCDC::isConnected()) {
+        // just pop/push RingBuffer and apply FIFO policy
+        flushTXBuffer((const uint8_t*)&c, 1);
+        return;
+    }
     if(xPortInIsrContext()){
         xRingbufferSendFromISR(tx_ring_buf, (void*) (&c), 1, NULL);
     } else {
         xRingbufferSend(tx_ring_buf, (void*) (&c), 1, tx_timeout_ms / portTICK_PERIOD_MS);
     }
-    usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+    usb_serial_jtag_ll_txfifo_flush();
 }
 
 HWCDC::HWCDC() {
-
+    lastSOF_ms = 0;
+    SOF_TIMEOUT = 5;
 }
 
 HWCDC::~HWCDC(){
     end();
 }
 
+// It should return <true> just when USB is plugged and CDC is connected.
 HWCDC::operator bool() const
 {
-    return initial_empty;
+    return HWCDC::isCDC_Connected();
 }
 
 void HWCDC::onEvent(esp_event_handler_t callback){
@@ -168,6 +257,16 @@ void HWCDC::onEvent(arduino_hw_cdc_event_t event, esp_event_handler_t callback){
     arduino_hw_cdc_event_handler_register_with(ARDUINO_HW_CDC_EVENTS, event, callback, this);
 }
 
+void HWCDC::deinit() 
+{
+    // Setting USB D+ D- pins
+        // Force the host to re-enumerate (BUS_RESET)
+        pinMode(USB_DM_GPIO_NUM, OUTPUT_OPEN_DRAIN);
+        pinMode(USB_DP_GPIO_NUM, OUTPUT_OPEN_DRAIN);
+        digitalWrite(USB_DM_GPIO_NUM, LOW);
+        digitalWrite(USB_DP_GPIO_NUM, LOW);
+}
+
 void HWCDC::begin(unsigned long baud)
 {
     if(tx_lock == NULL) {
@@ -185,20 +284,35 @@ void HWCDC::begin(unsigned long baud)
             log_e("HW CDC TX Buffer error");
         }    
     }
+
+    // the HW Serial pins needs to be first deinited in order to allow `if(Serial)` to work :-(
+    deinit();
+    delay(10);     // USB Host has to enumerate it again
+
+    // Configure PHY
+    // USB_Serial_JTAG use internal PHY
+    USB_SERIAL_JTAG.conf0.phy_sel = 0;
+    // Disable software control USB D+ D- pullup pulldown (Device FS: dp_pullup = 1)
+    USB_SERIAL_JTAG.conf0.pad_pull_override = 0;
+    // Enable USB D+ pullup
+    USB_SERIAL_JTAG.conf0.dp_pullup = 1;
+    // Enable USB pad function
+    USB_SERIAL_JTAG.conf0.usb_pad_enable = 1;
     usb_serial_jtag_ll_disable_intr_mask(USB_SERIAL_JTAG_LL_INTR_MASK);
-    usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY | USB_SERIAL_JTAG_INTR_SERIAL_OUT_RECV_PKT | USB_SERIAL_JTAG_INTR_BUS_RESET);
+    usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY | USB_SERIAL_JTAG_INTR_SERIAL_OUT_RECV_PKT
+                                     | USB_SERIAL_JTAG_INTR_BUS_RESET | USB_SERIAL_JTAG_INTR_SOF);
     if(!intr_handle && esp_intr_alloc(ETS_USB_SERIAL_JTAG_INTR_SOURCE, 0, hw_cdc_isr_handler, NULL, &intr_handle) != ESP_OK){
         isr_log_e("HW USB CDC failed to init interrupts");
         end();
         return;
     }
-    usb_serial_jtag_ll_txfifo_flush();
 }
 
 void HWCDC::end()
 {
-    //Disable tx/rx interrupt.
+    //Disable/clear/free tx/rx interrupt.
     usb_serial_jtag_ll_disable_intr_mask(USB_SERIAL_JTAG_LL_INTR_MASK);
+    usb_serial_jtag_ll_clr_intsts_mask(USB_SERIAL_JTAG_LL_INTR_MASK);
     esp_intr_free(intr_handle);
     intr_handle = NULL;
     if(tx_lock != NULL) {
@@ -211,13 +325,13 @@ void HWCDC::end()
         esp_event_loop_delete(arduino_hw_cdc_event_loop_handle);
         arduino_hw_cdc_event_loop_handle = NULL;
     }
+    deinit();
+    setDebugOutput(false);
+    connected = false;
 }
 
 void HWCDC::setTxTimeoutMs(uint32_t timeout){
     tx_timeout_ms = timeout;
-    // it registers that the user has explicitly requested to use a value as TX timeout
-    // used for the workaround with unplugged USB and TX Queue Full that causes a delay on every write()
-    tx_timeout_change_request = true;
 }
 
 /*
@@ -260,35 +374,63 @@ size_t HWCDC::write(const uint8_t *buffer, size_t size)
     if(xSemaphoreTake(tx_lock, tx_timeout_ms / portTICK_PERIOD_MS) != pdPASS){
         return 0;
     }
-    size_t max_size = xRingbufferGetMaxItemSize(tx_ring_buf);
-    size_t space = xRingbufferGetCurFreeSize(tx_ring_buf);
-    size_t to_send = size, so_far = 0;
-
-    if(space > size){
-        space = size;
-    }
-    // Non-Blocking method, Sending data to ringbuffer, and handle the data in ISR.
-    if(xRingbufferSend(tx_ring_buf, (void*) (buffer), space, 0) != pdTRUE){
-        size = 0;
+    if(!isCDC_Connected()) {
+        // just pop/push RingBuffer and apply FIFO policy
+        flushTXBuffer(buffer, size);
     } else {
-        to_send -= space;
-        so_far += space;
-        // Now trigger the ISR to read data from the ring buffer.
-        usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+        size_t space = xRingbufferGetCurFreeSize(tx_ring_buf);
+        size_t to_send = size, so_far = 0;
 
-        while(to_send){
-            if(max_size > to_send){
-                max_size = to_send;
-            }
-            // Blocking method, Sending data to ringbuffer, and handle the data in ISR.
-            if(xRingbufferSend(tx_ring_buf, (void*) (buffer+so_far), max_size, tx_timeout_ms / portTICK_PERIOD_MS) != pdTRUE){
-                size = so_far;
-                break;
-            }
-            so_far += max_size;
-            to_send -= max_size;
+        if(space > size){
+            space = size;
+        }
+        // Non-Blocking method, Sending data to ringbuffer, and handle the data in ISR.
+        if(space > 0 && xRingbufferSend(tx_ring_buf, (void*) (buffer), space, 0) != pdTRUE){
+            size = 0;
+        } else {
+            to_send -= space;
+            so_far += space;
             // Now trigger the ISR to read data from the ring buffer.
-            usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+            usb_serial_jtag_ll_txfifo_flush();
+            if(connected) usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+            // tracks CDC trasmission progress to avoid hanging if CDC is unplugged while still sending data
+            size_t last_toSend = to_send; 
+            uint32_t tries = tx_timeout_ms; // waits 1ms per sending data attempt, in case CDC is unplugged
+            while(connected && to_send){
+                space = xRingbufferGetCurFreeSize(tx_ring_buf);
+                if(space > to_send){
+                    space = to_send;
+                }
+                // Blocking method, Sending data to ringbuffer, and handle the data in ISR.
+                if(xRingbufferSend(tx_ring_buf, (void*) (buffer+so_far), space, tx_timeout_ms / portTICK_PERIOD_MS) != pdTRUE){
+                    size = so_far;
+                    log_w("write failed due to ring buffer full - timeout");
+                    break;
+                }
+                so_far += space;
+                to_send -= space;
+                // Now trigger the ISR to read data from the ring buffer.
+                usb_serial_jtag_ll_txfifo_flush();
+                if(connected) usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+                if(last_toSend == to_send) {
+                    // no progress in sending data... USB CDC is probably unplugged
+                    tries--;
+                    delay(1);
+                } else {
+                    last_toSend = to_send;
+                    tries = tx_timeout_ms; // reset the timeout
+                }
+                if (tries == 0) {  // CDC isn't connected anymore...
+                    size = so_far;
+                    log_w("write failed due to waiting USB Host - timeout");
+                    connected = false;
+                }
+            }
+        }
+        // CDC was diconnected while sending data ==> flush the TX buffer keeping the last data
+        if(to_send && !usb_serial_jtag_ll_txfifo_writable()) {
+            connected = false;
+            flushTXBuffer(buffer + so_far, to_send);
         }
     }
     xSemaphoreGive(tx_lock);
@@ -308,15 +450,34 @@ void HWCDC::flush(void)
     if(xSemaphoreTake(tx_lock, tx_timeout_ms / portTICK_PERIOD_MS) != pdPASS){
         return;
     }
-    UBaseType_t uxItemsWaiting = 0;
-    vRingbufferGetInfo(tx_ring_buf, NULL, NULL, NULL, NULL, &uxItemsWaiting);
-    if(uxItemsWaiting){
-        // Now trigger the ISR to read data from the ring buffer.
-        usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
-    }
-    while(uxItemsWaiting){
-        delay(5);
+    if(!isCDC_Connected()) {
+        flushTXBuffer(NULL, 0);
+    } else {
+        UBaseType_t uxItemsWaiting = 0;
         vRingbufferGetInfo(tx_ring_buf, NULL, NULL, NULL, NULL, &uxItemsWaiting);
+        if(uxItemsWaiting){
+            // Now trigger the ISR to read data from the ring buffer.
+            usb_serial_jtag_ll_txfifo_flush();
+            if(connected) {
+                usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+            }
+        }
+        uint32_t tries = tx_timeout_ms; // waits 1ms per ISR sending data attempt, in case CDC is unplugged
+        while(connected && tries && uxItemsWaiting){
+            delay(1);
+            UBaseType_t lastUxItemsWaiting = uxItemsWaiting;
+            vRingbufferGetInfo(tx_ring_buf, NULL, NULL, NULL, NULL, &uxItemsWaiting);
+            if (lastUxItemsWaiting == uxItemsWaiting) {
+                tries--;
+            }
+            if(connected) {
+                usb_serial_jtag_ll_ena_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
+            }
+        }
+        if (tries == 0) {  // CDC isn't connected anymore...
+            connected = false;
+            flushTXBuffer(NULL, 0); // flushes all TX Buffer
+        }
     }
     xSemaphoreGive(tx_lock);
 }
@@ -407,4 +568,4 @@ HWCDC USBSerial;
 #endif
 #endif
 
-#endif /* CONFIG_TINYUSB_CDC_ENABLED */
+#endif /* CONFIG_IDF_TARGET_ESP32C3 */

cores/esp32/HWCDC.h

@@ -42,6 +42,10 @@ typedef union {
 
 class HWCDC: public Stream
 {
+private:
+    static void deinit();
+    static bool isCDC_Connected();
+
 public:
     HWCDC();
     ~HWCDC();
@@ -64,6 +68,11 @@ public:
     size_t write(const uint8_t *buffer, size_t size);
     void flush(void);
     
+    static bool isPlugged(void);
+    inline static bool isConnected(void)
+    {
+        return isCDC_Connected();
+    }
     inline size_t read(char * buffer, size_t size)
     {
         return read((uint8_t*) buffer, size);

libraries/ESP32/examples/HWCDC_Events/.skip.esp32

+

libraries/ESP32/examples/HWCDC_Events/.skip.esp32s2

+

libraries/ESP32/examples/HWCDC_Events/HWCDC_Events.ino

+/*
+ * This Example demonstrates how to receive Hardware Serial Events
+ * This USB interface is available for the ESP32-S3 and ESP32-C3
+ * 
+ * It will log all events and USB status (plugged/unplugged) into UART0
+ * Any data read from UART0 will be sent to the USB CDC
+ * Any data read from USB CDC will be sent to the UART0
+ * 
+ * A suggestion is to use Arduino Serial Monitor for the UART0 port
+ * and some other serial monitor application for the USB CDC port
+ * in order to see the exchanged data and the Hardware Serial Events
+ * 
+ */
+
+#ifndef ARDUINO_USB_MODE
+#error This ESP32 SoC has no Native USB interface
+#elif ARDUINO_USB_MODE == 0
+#warning This sketch should be used when USB is in Hardware CDC and JTAG mode
+void setup(){}
+void loop(){}
+#else
+
+// Makes it work always using Serial0 as UART0 and USBSerial as the HW Serial USB CDC port
+#if ARDUINO_USB_CDC_ON_BOOT
+// HardwareSerial::Serial0 is declared but HWCDC::USBSerial not
+// Serial is the HWCDC USB CDC port
+#define USBSerial Serial
+#else
+// HWCDC::USBSerial is declared but HardwareSerial::Serial0 not
+// Serial is HardwareSerial UART0
+#define Serial0 Serial   // redefine the symbol Serial0 to the default Arduino
+#endif
+
+// USB Event Callback Function that will log CDC events into UART0
+static void usbEventCallback(void* arg, esp_event_base_t event_base, int32_t event_id, void* event_data) {
+  if (event_base == ARDUINO_HW_CDC_EVENTS) {
+    switch (event_id) {
+      case ARDUINO_HW_CDC_CONNECTED_EVENT:
+        Serial0.println("CDC EVENT:: ARDUINO_HW_CDC_CONNECTED_EVENT");
+        break;
+      case ARDUINO_HW_CDC_BUS_RESET_EVENT:
+        Serial0.println("CDC EVENT:: ARDUINO_HW_CDC_BUS_RESET_EVENT");
+        break;
+      case ARDUINO_HW_CDC_RX_EVENT:
+        Serial0.println("\nCDC EVENT:: ARDUINO_HW_CDC_RX_EVENT");
+        // sends all bytes read from USB Hardware Serial to UART0
+        while (USBSerial.available()) Serial0.write(USBSerial.read());
+        break;
+      case ARDUINO_HW_CDC_TX_EVENT:
+        Serial0.println("CDC EVENT:: ARDUINO_HW_CDC_TX_EVENT");
+        break;
+
+      default:
+        break;
+    }
+  }
+}
+
+const char* _hwcdc_status[] = {
+  " USB Plugged but CDC is NOT connected\r\n",
+  " USB Plugged and CDC is connected\r\n",
+  " USB Unplugged and CDC is NOT connected\r\n",
+  " USB Unplugged BUT CDC is connected :: PROBLEM\r\n",
+};
+
+const char* HWCDC_Status() {
+  int i = USBSerial.isPlugged() ? 0 : 2;
+  if(USBSerial.isConnected()) i += 1;
+  return _hwcdc_status[i];
+}
+
+void setup() {
+  USBSerial.onEvent(usbEventCallback);
+  USBSerial.begin();
+
+  Serial0.begin(115200);
+  Serial0.setDebugOutput(true);
+  Serial0.println("Starting...");
+}
+
+void loop() {
+  static uint32_t counter = 0;
+  
+  Serial0.print(counter);
+  Serial0.print(HWCDC_Status());
+
+  if (USBSerial) {
+    USBSerial.printf("  [%ld] connected\n\r", counter);
+  }
+  // sends all bytes read from UART0 to USB Hardware Serial
+  while (Serial0.available()) USBSerial.write(Serial0.read());
+  delay(1000);
+  counter++;
+}
+#endif