1 files changed, 36 insertions, 0 deletions
diff --git a/stmhal/systick.c b/stmhal/systick.c
index 196f1fbcae..660b777fc0 100644
--- a/stmhal/systick.c
+++ b/stmhal/systick.c
@@ -27,6 +27,10 @@
 #include <stm32f4xx_hal.h>
 #include "mpconfig.h"
 #include "misc.h"
+#include "nlr.h"
+#include "qstr.h"
+#include "obj.h"
+#include "irq.h"
 #include "systick.h"
 
 bool sys_tick_has_passed(uint32_t start_tick, uint32_t delay_ms) {
@@ -41,3 +45,35 @@ void sys_tick_wait_at_least(uint32_t start_tick, uint32_t delay_ms) {
         __WFI(); // enter sleep mode, waiting for interrupt
     }
 }
+
+// The SysTick timer counts down at 168 MHz, so we can use that knowledge
+// to grab a microsecond counter.
+//
+// We assume that HAL_GetTickis returns milliseconds.
+uint32_t sys_tick_get_microseconds(void) {
+    mp_int_t enabled = disable_irq();
+    uint32_t counter = SysTick->VAL;
+    uint32_t milliseconds = HAL_GetTick();
+    uint32_t status  = SysTick->CTRL;
+    enable_irq(enabled);
+
+  // It's still possible for the countflag bit to get set if the counter was
+  // reloaded between reading VAL and reading CTRL. With interrupts  disabled
+  // it definitely takes less than 50 HCLK cycles between reading VAL and
+  // reading CTRL, so the test (counter > 50) is to cover the case where VAL
+  // is +ve and very close to zero, and the COUNTFLAG bit is also set.
+  if ((status & SysTick_CTRL_COUNTFLAG_Msk) && counter > 50) {
+    // This means that the HW reloaded VAL between the time we read VAL and the
+    // time we read CTRL, which implies that there is an interrupt pending
+    // to increment the tick counter.
+    milliseconds++;
+  }
+  uint32_t load = SysTick->LOAD;
+  counter = load - counter; // Convert from decrementing to incrementing
+
+  // ((load + 1) / 1000) is the number of counts per microsecond.
+  //
+  // counter / ((load + 1) / 1000) scales from the systick clock to microseconds
+  // and is the same thing as (counter * 1000) / (load + 1)
+  return milliseconds * 1000 + (counter * 1000) / (load + 1);
+}