STM32 LPT optimisation

targets/TARGET_STM/lp_ticker.c

@@ -292,15 +292,8 @@ void lp_ticker_init(void)
 
 uint32_t lp_ticker_read(void)
 {
-    uint32_t usecs = 0;
+    uint32_t usecs = rtc_read_us();
-    time_t time = 0;
+    return usecs;
-
-    do {
-        time = rtc_read();
-        usecs = rtc_read_subseconds();
-    } while (time != rtc_read());
-
-    return (time * 1000000) + usecs;
 }
 
 void lp_ticker_set_interrupt(timestamp_t timestamp)

targets/TARGET_STM/rtc_api.c

@@ -36,8 +36,12 @@
 static RTC_HandleTypeDef RtcHandle;
 
 #if DEVICE_LOWPOWERTIMER && !MBED_CONF_TARGET_LOWPOWERTIMER_LPTIM
+
+#define GET_TICK_PERIOD(VALUE) (2048 * 1000000 / VALUE) /* 1s / SynchPrediv value * 2^11 (value to get the maximum precision value with no u32 overflow) */
+
 static void (*irq_handler)(void);
 static void RTC_IRQHandler(void);
+static uint32_t lp_TickPeriod_us = GET_TICK_PERIOD(4095); /* default SynchPrediv value = 4095 */
 #endif /* DEVICE_LOWPOWERTIMER && !MBED_CONF_TARGET_LOWPOWERTIMER_LPTIM */
 
 void rtc_init(void)
@@ -123,6 +127,10 @@ void rtc_init(void)
     RtcHandle.Init.OutPutType     = RTC_OUTPUT_TYPE_OPENDRAIN;
 #endif /* TARGET_STM32F1 */
 
+#if DEVICE_LOWPOWERTIMER && !MBED_CONF_TARGET_LOWPOWERTIMER_LPTIM
+    lp_TickPeriod_us = GET_TICK_PERIOD(RtcHandle.Init.SynchPrediv);
+#endif
+
     if (HAL_RTC_Init(&RtcHandle) != HAL_OK) {
         error("RTC initialization failed");
     }
@@ -309,9 +317,27 @@ static void RTC_IRQHandler(void)
     }
 }
 
-uint32_t rtc_read_subseconds(void)
+uint32_t rtc_read_us(void)
 {
-    return 1000000.f * ((double)((RTC->PRER & RTC_PRER_PREDIV_S) - RTC->SSR) / ((RTC->PRER & RTC_PRER_PREDIV_S) + 1));
+    RTC_TimeTypeDef timeStruct = {0};
+    RTC_DateTypeDef dateStruct = {0};
+
+    RtcHandle.Instance = RTC;
+    HAL_RTC_GetTime(&RtcHandle, &timeStruct, RTC_FORMAT_BIN);
+
+    /* Reading RTC current time locks the values in calendar shadow registers until Current date is read
+    to ensure consistency between the time and date values */
+    HAL_RTC_GetDate(&RtcHandle, &dateStruct, RTC_FORMAT_BIN);
+
+    if (timeStruct.SubSeconds > timeStruct.SecondFraction) {
+        /* SS can be larger than PREDIV_S only after a shift operation. In that case, the correct
+           time/date is one second less than as indicated by RTC_TR/RTC_DR. */
+        timeStruct.Seconds -= 1;
+    }
+    uint32_t RTCTime = timeStruct.Seconds + timeStruct.Minutes * 60 + timeStruct.Hours * 60 * 60;
+    uint32_t Time_us = ((timeStruct.SecondFraction - timeStruct.SubSeconds) * lp_TickPeriod_us) >> 11;
+
+    return (RTCTime * 1000000) + Time_us ;
 }
 
 void rtc_set_wake_up_timer(uint32_t delta)

targets/TARGET_STM/rtc_api_hal.h

@@ -51,11 +51,11 @@ extern "C" {
 #define RTC_CLOCK LSI_VALUE
 #endif
 
-/** Read the subsecond register.
+/** Read RTC time with subsecond precision.
  *
- * @return The remaining time as microseconds (0-999999)
+ * @return Time is microsecond
  */
-uint32_t rtc_read_subseconds(void);
+uint32_t rtc_read_us(void);
 
 /** Program a wake up timer event in delta microseconds.
  *