Add support for pyb.micros() by using the systick timer.

I also removed trailing spaces from modpyb.c which affected a couple
of lines technically not part of this patch.

Tested using: https://github.com/dhylands/upy-examples/blob/master/micros_test.py

which eventually fails due to wraparound issues (I could fix the test to compensate
but didn't bother)

stmhal/modpyb.c

@@ -187,11 +187,46 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_0(pyb_sync_obj, pyb_sync);
 
 /// \function millis()
 /// Returns the number of milliseconds since the board was last reset.
+///
+/// Note that this may return a negative number. This allows you to always
+/// do:
+///     start = pyb.millis()
+///     ...do some operation...
+///     elapsed = pyb.millis() - start
+///
+/// and as long as the time of your operation is less than 24 days, you'll
+/// always get the right answer and not have to worry about whether pyb.millis()
+/// wraps around.
 STATIC mp_obj_t pyb_millis(void) {
-    return mp_obj_new_int(HAL_GetTick());
+    // We want to "cast" the 32 bit unsigned into a small-int. So we shift it
+    // left by 1 to throw away the top bit, and then shift it right by one
+    // to sign extend.
+    mp_int_t val = HAL_GetTick() << 1;
+    return mp_obj_new_int(val >> 1);
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(pyb_millis_obj, pyb_millis);
 
+/// \function micros()
+/// Returns the number of microseconds since the board was last reset.
+///
+/// Note that this may return a negative number. This allows you to always
+/// do:
+///     start = pyb.micros()
+///     ...do some operation...
+///     elapsed = pyb.micros() - start
+///
+/// and as long as the time of your operation is less than 35 minutes, you'll
+/// always get the right answer and not have to worry about whether pyb.micros()
+/// wraps around.
+STATIC mp_obj_t pyb_micros(void) {
+    // We want to "cast" the 32 bit unsigned into a small-int. So we shift it
+    // left by 1 to throw away the top bit, and then shift it right by one
+    // to sign extend.
+    mp_int_t val = sys_tick_get_microseconds() << 1;
+    return mp_obj_new_int(val >> 1);
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_0(pyb_micros_obj, pyb_micros);
+
 /// \function delay(ms)
 /// Delay for the given number of milliseconds.
 STATIC mp_obj_t pyb_delay(mp_obj_t ms_in) {
@@ -343,6 +378,7 @@ STATIC const mp_map_elem_t pyb_module_globals_table[] = {
     { MP_OBJ_NEW_QSTR(MP_QSTR_USB_VCP), (mp_obj_t)&pyb_usb_vcp_type },
 
     { MP_OBJ_NEW_QSTR(MP_QSTR_millis), (mp_obj_t)&pyb_millis_obj },
+    { MP_OBJ_NEW_QSTR(MP_QSTR_micros), (mp_obj_t)&pyb_micros_obj },
     { MP_OBJ_NEW_QSTR(MP_QSTR_delay), (mp_obj_t)&pyb_delay_obj },
     { MP_OBJ_NEW_QSTR(MP_QSTR_udelay), (mp_obj_t)&pyb_udelay_obj },
     { MP_OBJ_NEW_QSTR(MP_QSTR_sync), (mp_obj_t)&pyb_sync_obj },

stmhal/qstrdefsport.h

@@ -67,6 +67,7 @@ Q(/flash/lib)
 Q(/sd)
 Q(/sd/lib)
 Q(millis)
+Q(micros)
 
 // for file class
 Q(seek)

stmhal/stm32f4xx_it.c

@@ -174,6 +174,11 @@ void PendSV_Handler(void) {
   */
 void SysTick_Handler(void) {
     HAL_IncTick();
+
+    // Read the systick control regster. This has the side effect of clearing
+    // the COUNTFLAG bit, which makes the logic in sys_tick_get_microseconds
+    // work properly.
+    SysTick->CTRL;
 }
 
 /******************************************************************************/

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);
+}

stmhal/systick.h

@@ -26,3 +26,4 @@
 
 void sys_tick_wait_at_least(uint32_t stc, uint32_t delay_ms);
 bool sys_tick_has_passed(uint32_t stc, uint32_t delay_ms);
+uint32_t sys_tick_get_microseconds(void);