[Time] delayMicroseconds() accuracy

delayMicroseconds() should not rely to getCurrentMicros()
and should only compute required ticks for the delay
requested.

This allow to use it even if interrupts are disabled.
Example for OneWire library.

Fixes #412

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/wiring_time.h

@@ -69,8 +69,20 @@ static inline void delayMicroseconds(uint32_t us)
 
   while ((int32_t)dwt_getCycles() - start < cycles);
 #else
-  uint32_t start = getCurrentMicros();
-  while (getCurrentMicros() - start < us);
+  __IO uint32_t currentTicks = SysTick->VAL;
+  /* Number of ticks per millisecond */
+  const uint32_t tickPerMs = SysTick->LOAD + 1;
+  /* Number of ticks to count */
+  const uint32_t nbTicks = ((us - ((us > 0) ? 1 : 0)) * tickPerMs) / 1000;
+  /* Number of elapsed ticks */
+  uint32_t elapsedTicks = 0;
+  __IO uint32_t oldTicks = currentTicks;
+  do {
+    currentTicks = SysTick->VAL;
+    elapsedTicks += (oldTicks < currentTicks) ? tickPerMs + oldTicks - currentTicks :
+                    oldTicks - currentTicks;
+    oldTicks = currentTicks;
+  } while (nbTicks > elapsedTicks);
 #endif
 }