stmhal: Improve efficiency of SysTick IRQ and HAL_Delay.

SysTick IRQ now increases millisecond counter directly (ie without
calling HAL_IncTick).  Provide our own version of HAL_Delay that does a
wfi while waiting.  This more than halves power consumption when running
a loop containing a pyb.delay call.  It used to be like this, but new
version of HAL library regressed this feature.

1 files changed, 5 insertions, 1 deletions

diff --git a/stmhal/stm32f4xx_it.c b/stmhal/stm32f4xx_it.c
index 4bd13c05cb..74fdf53d1e 100644
--- a/stmhal/stm32f4xx_it.c
+++ b/stmhal/stm32f4xx_it.c
@@ -173,7 +173,11 @@ void PendSV_Handler(void) {
   * @retval None
   */
 void SysTick_Handler(void) {
-    HAL_IncTick();
+    // Instead of calling HAL_IncTick we do the increment here of the counter.
+    // This is purely for efficiency, since SysTick is called 1000 times per
+    // second at the highest interrupt priority.
+    extern __IO uint32_t uwTick;
+    uwTick += 1;
 
     // Read the systick control regster. This has the side effect of clearing
     // the COUNTFLAG bit, which makes the logic in sys_tick_get_microseconds