Ensured micros() doesn't return a smaller value on millisecond bound

src/main/drivers/system.c

@@ -35,6 +35,7 @@
 static uint32_t usTicks = 0;
 // current uptime for 1kHz systick timer. will rollover after 49 days. hopefully we won't care.
 static volatile uint32_t sysTickUptime = 0;
+static volatile uint32_t sysTickValStamp = 0;
 // cached value of RCC->CSR
 uint32_t cachedRccCsrValue;
 
@@ -57,6 +58,7 @@ void SysTick_Handler(void)
 {
     ATOMIC_BLOCK(NVIC_PRIO_MAX) {
         sysTickUptime++;
+        sysTickValStamp = SysTick->VAL;
         sysTickPending = 0;
         (void)(SysTick->CTRL);
     }
@@ -108,12 +110,7 @@ uint32_t micros(void)
     do {
         ms = sysTickUptime;
         cycle_cnt = SysTick->VAL;
-        /*
-         * If the SysTick timer expired during the previous instruction, we need to give it a little time for that
-         * interrupt to be delivered before we can recheck sysTickUptime:
-         */
-        asm volatile("\tnop\n");
-    } while (ms != sysTickUptime);
+    } while (ms != sysTickUptime || cycle_cnt >= sysTickValStamp);
 
     return (ms * 1000) + (usTicks * 1000 - cycle_cnt) / usTicks;
 }