Add method to clear the systick value, that is undefined at startup.

include/libopencm3/cm3/systick.h

@@ -123,6 +123,7 @@ void systick_interrupt_disable(void);
 void systick_counter_enable(void);
 void systick_counter_disable(void);
 uint8_t systick_get_countflag(void);
+void systick_clear(void);
 
 uint32_t systick_get_calib(void);
 

lib/cm3/systick.c

@@ -46,6 +46,10 @@
  * The counter is set to the reload value when the counter starts and after it
  * reaches zero.
  *
+ * @note The systick counter value might be undefined upon startup. To get
+ * predictable behavior, it is a good idea to set or clear the counter after
+ * set reload. @seealso systick_clear
+ *
  * @param[in] value uint32_t. 24 bit reload value.
  */
 
@@ -175,6 +179,17 @@ uint8_t systick_get_countflag(void)
 	return (STK_CSR & STK_CSR_COUNTFLAG) ? 1 : 0;
 }
 
+/*---------------------------------------------------------------------------*/
+/** @brief SysTick Clear counter Value.
+ *
+ * The counter value is cleared. Useful for well defined startup.
+ */
+
+void systick_clear(void)
+{
+	STK_CVR = 0;
+}
+
 /*---------------------------------------------------------------------------*/
 /** @brief SysTick Get Calibration Value
  *