tests-mbed_drivers-lp_timer: change delay method

The test sometimes fails on NRF51_DK (test case: "Test: LowPowerTimer - time measurement 1 ms.") in morph tests.

The test verifies if LowPowerTimer class correctly counts elapsed time. Sometimes we got measured ~1600 us for delay 1000 us (delta 550 us).
The delay is performed using `wait_us()` function which for delays greater than or equal to 1 ms (our case) calls `Thread::wait((uint32_t)ms);`. This causes rescheduling and potentially can put board into sleep (deep sleep mode is disabled by `wait_us()`). For our test purposes we don't need rescheduling/sleep since this actions takes extra time and have influence on the time measurement accuracy.
The solution is to implement function for delay which is based on busy loop and uses us ticker. It has been verified that this solves the problem. With this fix when measurement of 1 ms is repeated 1000 times we got usually measured time equal to ~1080 us, and sometimes ~1300us (checked that this is caused by systick interrupt handling). Since this is test for drivers layer and the results are acceptable I decided to not disabling systick in the test).

Author: mprse

TESTS/mbed_drivers/lp_timer/main.cpp

@@ -55,6 +55,18 @@ extern uint32_t SystemCoreClock;
 #define DELTA_MS(delay_ms) (1 + ((delay_ms) * US_PER_MSEC / 20 / US_PER_MSEC))
 #define DELTA_S(delay_ms) (0.000500f + (((float)(delay_ms)) / MSEC_PER_SEC / 20))
 
+void busy_wait_us(int us)
+{
+    const ticker_data_t *const ticker = get_us_ticker_data();
+    uint32_t start = ticker_read(ticker);
+    while ((ticker_read(ticker) - start) < (uint32_t)us);
+}
+
+void busy_wait_ms(int ms)
+{
+    busy_wait_us(ms * US_PER_MSEC);
+}
+
 /* This test verifies if low power timer is stopped after
  * creation.
  *
@@ -74,7 +86,7 @@ void test_lptimer_creation()
 
     /* Wait 10 ms.
      * After that operation timer read routines should still return 0. */
-    wait_ms(10);
+    busy_wait_ms(10);
 
     /* Check results. */
     TEST_ASSERT_EQUAL_FLOAT(0, lp_timer.read());
@@ -102,7 +114,7 @@ void test_lptimer_time_accumulation()
     lp_timer.start();
 
     /* Wait 10 ms. */
-    wait_ms(10);
+    busy_wait_ms(10);
 
     /* Stop the timer. */
     lp_timer.stop();
@@ -116,15 +128,15 @@ void test_lptimer_time_accumulation()
     /* Wait 50 ms - this is done to show that time elapsed when
      * the timer is stopped does not have influence on the
      * timer counted time. */
-    wait_ms(50);
+    busy_wait_ms(50);
 
     /* ------ */
 
     /* Start the timer. */
     lp_timer.start();
 
     /* Wait 20 ms. */
-    wait_ms(20);
+    busy_wait_ms(20);
 
     /* Stop the timer. */
     lp_timer.stop();
@@ -145,7 +157,7 @@ void test_lptimer_time_accumulation()
     lp_timer.start();
 
     /* Wait 30 ms. */
-    wait_ms(30);
+    busy_wait_ms(30);
 
     /* Stop the timer. */
     lp_timer.stop();
@@ -159,15 +171,15 @@ void test_lptimer_time_accumulation()
     /* Wait 50 ms - this is done to show that time elapsed when
      * the timer is stopped does not have influence on the
      * timer time. */
-    wait_ms(50);
+    busy_wait_ms(50);
 
     /* ------ */
 
     /* Start the timer. */
     lp_timer.start();
 
     /* Wait 1 sec. */
-    wait_ms(1000);
+    busy_wait_ms(1000);
 
     /* Stop the timer. */
     lp_timer.stop();
@@ -196,7 +208,7 @@ void test_lptimer_reset()
     lp_timer.start();
 
     /* Wait 10 ms. */
-    wait_ms(10);
+    busy_wait_ms(10);
 
     /* Stop the timer. */
     lp_timer.stop();
@@ -214,7 +226,7 @@ void test_lptimer_reset()
     lp_timer.start();
 
     /* Wait 20 ms. */
-    wait_ms(20);
+    busy_wait_ms(20);
 
     /* Stop the timer. */
     lp_timer.stop();
@@ -241,13 +253,13 @@ void test_lptimer_start_started_timer()
     lp_timer.start();
 
     /* Wait 10 ms. */
-    wait_ms(10);
+    busy_wait_ms(10);
 
     /* Now start timer again. */
     lp_timer.start();
 
     /* Wait 20 ms. */
-    wait_ms(20);
+    busy_wait_ms(20);
 
     /* Stop the timer. */
     lp_timer.stop();
@@ -274,7 +286,7 @@ void test_lptimer_float_operator()
     lp_timer.start();
 
     /* Wait 10 ms. */
-    wait_ms(10);
+    busy_wait_ms(10);
 
     /* Stop the timer. */
     lp_timer.stop();
@@ -302,7 +314,7 @@ void test_lptimer_time_measurement()
     lp_timer.start();
 
     /* Wait <wait_val_us> us. */
-    wait_us(wait_val_us);
+    busy_wait_us(wait_val_us);
 
     /* Stop the timer. */
     lp_timer.stop();