tests/sys/busy_wait: add test for busy wait

tests/sys/busy_wait/Makefile

@@ -0,0 +1,5 @@
+include ../Makefile.sys_common
+
+USEMODULE += ztimer_usec
+
+include $(RIOTBASE)/Makefile.include

tests/sys/busy_wait/Makefile.ci

@@ -0,0 +1,3 @@
+BOARD_INSUFFICIENT_MEMORY := \
+    atmega8 \
+    #

tests/sys/busy_wait/main.c

@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) 2024 ML!PA Consulting GmbH
+ *
+ * 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 tests
+ * @{
+ *
+ * @file
+ * @brief   Busy Wait loop Test Application
+ *
+ *          This can be used to determine `CPU_CYCLES_PER_LOOP` by
+ *          comparing the time the busy wait loop took with the
+ *          actual µsec timer.
+ *
+ * @author  Benjamin Valentin <benjamin.valentin@ml-pa.com>
+ * @}
+ */
+
+#include <stdio.h>
+#include "busy_wait.h"
+#include "ztimer/stopwatch.h"
+
+static inline void _measure_interval(ztimer_stopwatch_t *clock, unsigned usec)
+{
+    unsigned usec_real;
+
+    printf("waiting for %u µs…\n", usec);
+    ztimer_stopwatch_start(clock);
+    busy_wait_us(usec);
+    usec_real = ztimer_stopwatch_measure(clock);
+    printf("took %u µs (diff: %d µs)\n", usec_real, (int)usec_real - usec);
+}
+
+int main(void)
+{
+    ztimer_stopwatch_t clock;
+    ztimer_stopwatch_init(ZTIMER_USEC, &clock);
+
+    _measure_interval(&clock, 10);
+    _measure_interval(&clock, 100);
+    _measure_interval(&clock, 1000);
+    _measure_interval(&clock, 10000);
+
+    return 0;
+}