Make tick.c atomic (#181)

* Make tick.c atomic

* Replace tabs with spaces

opendps/tick.c

@@ -27,7 +27,8 @@
 #include <nvic.h>
 #include "tick.h"
 
-static volatile uint64_t tick_ms;
+static volatile uint32_t ticks_lower;
+static volatile uint32_t ticks_upper;
 
 /**
  * @brief Initialize the systick module
@@ -51,20 +52,24 @@ void systick_init(void)
  * @param delay time in milliseconds
  * @retval none
  */
-
 void delay_ms(uint32_t delay)
 {
- uint64_t start = tick_ms;
- while (tick_ms < start + delay) ; 
+ uint32_t start = ticks_lower;
+
+ while ((ticks_lower - start) < delay)
+ {}
 }
 
 /**
  * @brief Get systick
- * @retval number of milliseconcs since powerup
+ * @retval number of milliseconds since powerup
  */
 uint64_t get_ticks(void)
 {
- return tick_ms;
+ systick_interrupt_disable();
+ uint64_t ticks = ((uint64_t) ticks_upper << 32) | ((uint64_t) ticks_lower);
+ systick_interrupt_enable();
+ return ticks;
 }
 
 /**
@@ -73,6 +78,8 @@ uint64_t get_ticks(void)
  */
 void sys_tick_handler(void)
 {
- tick_ms++;
+ ticks_lower++;
+ if (ticks_lower == 0) // If an overflow has occured
+ ticks_upper++;
 }