Fix issue ARMmbed#816

NUCLEO_F103RB is using a 16-bit timer as a internal ticker but the
mBed ticker needs a 32-bit timer implementation, so the upper part
of that 32-bit timer is being calculated in software.

Software bug has been fixed - Continous HIGH/LOW voltage levels
could be observerd for 65ms due to 16-bit timer overflow.

Now current value of TIM_MST->CNT is stored in cnt_val and is
updated in interrupt context only. This avoids master timer
overflow without SlaveCounter update.

Change-Id: I09cc262083b66e16affea14c4d2126287519b477

libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32F1/TARGET_NUCLEO_F103RB/hal_tick.c

@@ -43,9 +43,10 @@ void set_compare(uint16_t count);
 extern volatile uint32_t SlaveCounter;
 extern volatile uint32_t oc_int_part;
 extern volatile uint16_t oc_rem_part;
+extern volatile uint16_t cnt_val;
 
 void timer_irq_handler(void) {
- uint16_t cval = TIM_MST->CNT;
+ cnt_val= TIM_MST->CNT;
 
  TimMasterHandle.Instance = TIM_MST;
 
@@ -64,7 +65,7 @@ void timer_irq_handler(void) {
  } else {
  if (oc_int_part > 0) {
  set_compare(0xFFFF);
- oc_rem_part = cval; // To finish the counter loop the next time
+ oc_rem_part = cnt_val; // To finish the counter loop the next time
  oc_int_part--;
  } else {
  us_ticker_irq_handler();

libraries/mbed/targets/hal/TARGET_STM/TARGET_STM32F1/us_ticker.c

@@ -38,6 +38,7 @@ static int us_ticker_inited = 0;
 volatile uint32_t SlaveCounter = 0;
 volatile uint32_t oc_int_part = 0;
 volatile uint16_t oc_rem_part = 0;
+volatile uint16_t cnt_val = 0;
 
 void set_compare(uint16_t count)
 {
@@ -58,24 +59,15 @@ void us_ticker_init(void)
 
 uint32_t us_ticker_read()
 {
- uint32_t counter, counter2;
+ uint32_t counter;
  if (!us_ticker_inited) us_ticker_init();
- // A situation might appear when Master overflows right after Slave is read and before the
- // new (overflowed) value of Master is read. Which would make the code below consider the
- // previous (incorrect) value of Slave and the new value of Master, which would return a
- // value in the past. Avoid this by computing consecutive values of the timer until they
- // are properly ordered.
+
+ //Current value of TIM_MST->CNT is stored in cnt_val and is't
+ //updated in interrupt context
  counter = (uint32_t)(SlaveCounter << 16);
- counter += TIM_MST->CNT;
- while (1) {
- counter2 = (uint32_t)(SlaveCounter << 16);
- counter2 += TIM_MST->CNT;
- if (counter2 > counter) {
- break;
- }
- counter = counter2;
- }
- return counter2;
+ counter += cnt_val;
+
+ return counter;
 }
 
 void us_ticker_set_interrupt(timestamp_t timestamp)