Adding USB MSD implementation to the HAL: Now, when you plug the Native USB port of the Arduino DUE to your PC, it will be detected as a composite USB device: One of the devices is a USB CDC (Serial port over USB) with native USB handshake (no more overflows!!) (set SERIAL_PORT to -1 to use it as main serial port) and also, a MSD (USB Mass Storage Device) will be detected, and the SD card present on the socket will be reflected as a Removable disk on your PC)

Marlin/src/HAL/HAL_DUE/usb/usb_task.c

@@ -0,0 +1,186 @@
+/**
+ * \file
+ *
+ * \brief Main functions for USB composite example
+ *
+ * Copyright (c) 2011-2015 Atmel Corporation. All rights reserved.
+ *
+ * \asf_license_start
+ *
+ * \page License
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * 3. The name of Atmel may not be used to endorse or promote products derived
+ *    from this software without specific prior written permission.
+ *
+ * 4. This software may only be redistributed and used in connection with an
+ *    Atmel microcontroller product.
+ *
+ * THIS SOFTWARE IS PROVIDED BY ATMEL "AS IS" AND ANY EXPRESS OR IMPLIED
+ * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT ARE
+ * EXPRESSLY AND SPECIFICALLY DISCLAIMED. IN NO EVENT SHALL ATMEL BE LIABLE FOR
+ * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ * \asf_license_stop
+ *
+ */
+/*
+ * Support and FAQ: visit <a href="http://www.atmel.com/design-support/">Atmel Support</a>
+ */
+
+#ifdef ARDUINO_ARCH_SAM
+
+#include "conf_usb.h"
+#include "udc.h"
+
+static volatile bool main_b_msc_enable = false;
+static volatile bool main_b_cdc_enable = false;
+
+void HAL_init(void) {
+	udd_disable();
+	UDD_SetStack(&USBD_ISR);
+
+	// Start USB stack to authorize VBus monitoring
+	udc_start();
+}
+
+void HAL_idletask(void)
+{
+
+	// Attend SD card access from the USB MSD -- Priotize access to improve speed
+	int delay = 2;
+	while (main_b_msc_enable && --delay > 0 ) {
+		if (udi_msc_process_trans()) {
+			delay = 10000;
+		}
+		
+		/* Reset the watchdog, just to be sure */
+		REG_WDT_CR = WDT_CR_WDRSTT | WDT_CR_KEY(0xA5);
+	}
+}
+
+/*! \brief Example of extra USB string management
+ * This feature is available for single or composite device
+ * which want implement additional USB string than
+ * Manufacture, Product and serial number ID.
+ *
+ * return true, if the string ID requested is know and managed by this functions
+ */
+bool usb_task_extra_string(void)
+{
+	static uint8_t udi_cdc_name[] = "CDC interface";
+	static uint8_t udi_msc_name[] = "MSC interface";
+
+	struct extra_strings_desc_t{
+		usb_str_desc_t header;
+		le16_t string[Max(sizeof(udi_cdc_name)-1, sizeof(udi_msc_name)-1)];
+	};
+	static UDC_DESC_STORAGE struct extra_strings_desc_t extra_strings_desc = {
+		.header.bDescriptorType = USB_DT_STRING
+	};
+
+	uint8_t i;
+	uint8_t *str;
+	uint8_t str_lgt=0;
+
+	// Link payload pointer to the string corresponding at request
+	switch (udd_g_ctrlreq.req.wValue & 0xff) {
+	case UDI_CDC_IAD_STRING_ID:
+		str_lgt = sizeof(udi_cdc_name)-1;
+		str = udi_cdc_name;
+		break;
+	case UDI_MSC_STRING_ID:
+		str_lgt = sizeof(udi_msc_name)-1;
+		str = udi_msc_name;
+		break;
+	default:
+		return false;
+	}
+
+	if (str_lgt!=0) {
+		for( i=0; i<str_lgt; i++) {
+			extra_strings_desc.string[i] = cpu_to_le16((le16_t)str[i]);
+		}
+		extra_strings_desc.header.bLength = 2+ (str_lgt)*2;
+		udd_g_ctrlreq.payload_size = extra_strings_desc.header.bLength;
+		udd_g_ctrlreq.payload = (uint8_t *) &extra_strings_desc;
+	}
+
+	// if the string is larger than request length, then cut it
+	if (udd_g_ctrlreq.payload_size > udd_g_ctrlreq.req.wLength) {
+		udd_g_ctrlreq.payload_size = udd_g_ctrlreq.req.wLength;
+	}
+	return true;
+}
+
+bool usb_task_msc_enable(void)
+{
+	main_b_msc_enable = true;
+	return true;
+}
+
+void usb_task_msc_disable(void)
+{
+	main_b_msc_enable = false;
+}
+
+bool usb_task_msc_isenabled(void)
+{
+	return main_b_msc_enable;
+}
+
+bool usb_task_cdc_enable(uint8_t port)
+{
+	main_b_cdc_enable = true;
+	return true;
+}
+
+void usb_task_cdc_disable(uint8_t port)
+{
+	main_b_cdc_enable = false;
+}
+
+bool usb_task_cdc_isenabled(void)
+{
+	return main_b_cdc_enable;
+}
+
+/*! \brief Called by CDC interface
+ * Callback running when CDC device have received data
+ */
+void usb_task_cdc_rx_notify(uint8_t port)
+{
+}
+
+/*! \brief Configures communication line
+ *
+ * \param cfg      line configuration
+ */
+void usb_task_cdc_config(uint8_t port, usb_cdc_line_coding_t * cfg)
+{
+}
+
+void usb_task_cdc_set_dtr(uint8_t port, bool b_enable)
+{
+	if (b_enable) {
+	} else {
+	}
+}
+
+#endif