[Nuvoton] Add nu_busy_wait_us

nu_busy_wait_us is a replacement for wait_us when intermediary us ticker layer is disabled.

Author: ccli8

targets/TARGET_NUVOTON/nu_timer.c

@@ -17,6 +17,8 @@
 #include "nu_timer.h"
 #include "mbed_power_mgmt.h"
 #include "mbed_critical.h"
+#include "us_ticker_api.h"
+#include "mbed_assert.h"
 
 void nu_countdown_init(struct nu_countdown_ctx_s *ctx, us_timestamp_t interval_us)
 {
@@ -45,3 +47,21 @@ void nu_countdown_free(struct nu_countdown_ctx_s *ctx)
     sleep_manager_unlock_deep_sleep();
     core_util_critical_section_exit();
 }
+
+
+void nu_busy_wait_us(uint32_t us)
+{
+    const uint32_t bits = us_ticker_get_info()->bits;
+    const uint32_t mask = (1 << bits) - 1;
+    MBED_ASSERT(us_ticker_get_info()->frequency == 1000000);
+    uint32_t prev = us_ticker_read();
+    while (1) {
+        const uint32_t cur = us_ticker_read();
+        const uint32_t elapsed = (cur - prev) & mask;
+        if (elapsed > us) {
+            break;
+        }
+        us -= elapsed;
+        prev = cur;
+    }
+}

targets/TARGET_NUVOTON/nu_timer.h

@@ -60,6 +60,20 @@ void nu_countdown_init(struct nu_countdown_ctx_s *ctx, us_timestamp_t interval_u
 bool nu_countdown_expired(struct nu_countdown_ctx_s *ctx);
 void nu_countdown_free(struct nu_countdown_ctx_s *ctx);
 
+
+/* Replacement for wait_us when intermediary us ticker layer is disabled
+ *
+ * Use of wait_us directly from the low power ticker causes the system to deadlock during
+ * the sleep test because the sleep test disables the intermediary us ticker layer during
+ * the test.
+ *
+ * To prevent this lockup, nu_busy_wait_us is created to replace wait_us, which uses the us ticker
+ * directly rather than go though the intermediary us ticker layer.
+ *
+ * During wait period through nu_busy_wait_us, CPU would be busy spinning. 
+ */
+void nu_busy_wait_us(uint32_t us);
+
 #ifdef __cplusplus
 }
 #endif