USBUS: add minimal working example

examples/usbus_minimal/Makefile

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+# name of your application
+APPLICATION = usbus_minimal
+
+# If no BOARD is found in the environment, use this default:
+BOARD ?= samr21-xpro
+
+# This has to be the absolute path to the RIOT base directory:
+RIOTBASE ?= $(CURDIR)/../..
+
+# Comment this out to disable code in RIOT that does safety checking
+# which is not needed in a production environment but helps in the
+# development process:
+DEVELHELP ?= 1
+
+USEMODULE += usbus
+
+# USB device vendor and product ID
+USB_VID ?= 1209
+USB_PID ?= 0001
+
+# Change this to 0 show compiler invocation lines by default:
+QUIET ?= 1
+
+CFLAGS += -DUSB_CONFIG_VID=0x$(USB_PID) -DUSB_CONFIG_PID=0x$(USB_VID)
+
+include $(RIOTBASE)/Makefile.include
+
+ifeq ($(USB_VID):$(USB_PID), 1209:0001)
+  $(shell $(COLOR_ECHO) "$(COLOR_RED)Private testing pid.codes USB VID/PID used!, do not use it outside of test environments!$(COLOR_RESET)" 1>&2)
+  $(shell $(COLOR_ECHO) "$(COLOR_RED)MUST NOT be used on any device redistributed, sold or manufactured, VID/PID is not unique!$(COLOR_RESET)" 1>&2)
+endif

examples/usbus_minimal/README.md

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+# usbus_minimal example
+
+This is a minimalistic example for RIOT's USB stack. The application will
+initialize and start the USB stack. The stack is started without any USB
+handlers, it should show up as an empty USB device on the host.
+
+RIOT doesn't own any USB vendor and product ID. To compile this example, add
+your own vendor and product ID to the makefile:
+
+```
+CFLAGS += -DUSB_CONFIG_VID=0xYOURVID -DUSB_CONFIG_PID=0xYOURPID
+```
+
+The example demonstrates basic USB communication between a host and a RIOT
+based USB peripheral. Tools such as `lsusb` should display the device and
+detailed information about the device such as descriptor strings.

examples/usbus_minimal/main.c

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+/*
+ * Copyright (C) 2019 Koen Zandberg
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     examples
+ * @{
+ *
+ * @file
+ * @brief       Example application for demonstrating the RIOT USB stack
+ *
+ * @author      Koen Zandberg <koen@bergzand.net>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+#include "usb/usbus.h"
+
+static char _stack[USBUS_STACKSIZE];
+
+static usbus_t usbus;
+/* TODO: remove as soon as we have decent auto_init */
+#include "periph_conf.h"
+#include "sam_usb.h"
+
+int main(void)
+{
+    puts("RIOT USB stack example application");
+
+    /* TODO: remove as soon as we have decent auto_init */
+    usbdev_t *usbdev_ctx = usbdev_get_ctx(0);
+    /* start usb stack */
+    usbus_init(&usbus, usbdev_ctx);
+    usbus_create(_stack, sizeof(_stack), USBUS_PRIO, USBUS_TNAME, &usbus);
+
+    /* start shell */
+    puts("Started USB stack!");
+    return 0;
+}