examples/blinky: add a new example app

examples/blinky/Makefile

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+# name of your application
+APPLICATION = blinky
+
+# If no BOARD is found in the environment, use this default:
+BOARD ?= native
+
+# 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
+
+# Change this to 0 show compiler invocation lines by default:
+QUIET ?= 1
+
+# Use a peripheral timer for the delay, if available
+FEATURES_OPTIONAL += periph_timer
+
+include $(RIOTBASE)/Makefile.include
+
+ifeq (native,$(BOARD))
+  # For the native board, CLOCK_CORECLOCK is undefined. But we need
+  # a valid number to get the example compiled
+  CFLAGS += -DCLOCK_CORECLOCK=1000000000
+endif

examples/blinky/Makefile.board.dep

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+ifneq (,$(filter periph_timer,$(FEATURES_USED)))
+  USEMODULE += ztimer_usec
+endif

examples/blinky/README.md

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+Blinky!
+=======
+
+This is a basic example that blinks an LED, if available. (If no LED is present or configured, it
+will print "Blink!" via stdio instead.)
+
+This is mostly useful for boards without stdio to check if a new port of RIOT works. For that
+reason, this example has only an optional dependency on timer drivers. Hence, this application only
+needs a working GPIO driver and is likely the first milestone when porting RIOT to new MCUs.

examples/blinky/main.c

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+/*
+ * Copyright (C) 2021 Otto-von-Guericke-Universität Magdeburg
+ *
+ * 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       Blinky application
+ *
+ * @author      Marian Buschsieweke <marian.buschsieweke@ovgu.de>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+
+#include "board.h"
+#include "periph_conf.h"
+#include "timex.h"
+#include "ztimer.h"
+
+static void delay(void)
+{
+    if (IS_USED(MODULE_ZTIMER)) {
+        ztimer_sleep(ZTIMER_USEC, 1 * US_PER_SEC);
+    }
+    else {
+        /*
+         * As fallback for freshly ported boards with no timer drivers written
+         * yet, we just use the CPU to delay execution and assume that roughly
+         * 20 CPU cycles are spend per loop iteration.
+         *
+         * Note that the volatile qualifier disables compiler optimizations for
+         * all accesses to the counter variable. Without volatile, modern
+         * compilers would detect that the loop is only wasting CPU cycles and
+         * optimize it out - but here the wasting of CPU cycles is desired.
+         */
+        for (volatile uint32_t i = 0; i < CLOCK_CORECLOCK / 20; i++) { }
+    }
+}
+
+int main(void)
+{
+    while (1) {
+        delay();
+#ifdef LED0_TOGGLE
+        LED0_TOGGLE;
+#else
+        puts("Blink! (No LED present or configured...)");
+#endif
+    }
+
+    return 0;
+}