nRF52: Further improve software timer accuracy

Author: ssievert42

targets/nrf5x/jshardware.c

@@ -1096,12 +1096,18 @@ void jshSetSystemTime(JsSysTime time) {
 
 /// Convert a time in Milliseconds to one in ticks.
 JsSysTime jshGetTimeFromMilliseconds(JsVarFloat ms) {
-  return (JsSysTime) (ms * (SYSCLK_FREQ / 1000.0));
+  // round to nearest system tick
+  JsVarFloat ticksFloat = ms * ((JsVarFloat)SYSCLK_FREQ / 1000.0);
+  JsSysTime ticks = (JsSysTime)ticksFloat;
+  if ((ticksFloat - (JsVarFloat)ticks) >= 0.5) {
+    ticks += 1;
+  }
+  return ticks;
 }
 
 /// Convert ticks to a time in Milliseconds.
 JsVarFloat jshGetMillisecondsFromTime(JsSysTime time) {
-  return time * (1000.0 / SYSCLK_FREQ);
+  return (JsVarFloat)time * (1000.0 / (JsVarFloat)SYSCLK_FREQ);
 }
 
 void jshInterruptOff() {
@@ -2762,13 +2768,14 @@ bool jshSleep(JsSysTime timeUntilWake) {
   return true;
 }
 
-bool utilTimerActive = false;
+volatile bool utilTimerActive = false;
+volatile uint32_t utilTimerTriggerTime = 0;
 
 /// Reschedule the timer (it should already be running) to interrupt after 'period'
 void jshUtilTimerReschedule(JsSysTime period) {
   if (period < JSSYSTIME_MAX / NRF_TIMER_FREQ) {
     period = period * NRF_TIMER_FREQ / (long long)SYSCLK_FREQ;
-    if (period < 1) period=1;
+    if (period < 0) period=0;
     if (period > NRF_TIMER_MAX) period=NRF_TIMER_MAX;
   } else {
     // it's too big to do maths on... let's just use the maximum period
@@ -2779,35 +2786,45 @@ void jshUtilTimerReschedule(JsSysTime period) {
   /* Setting the timer is complicated because the compare register only compares for equality,
   so if we set the compare register even 1 less than the current timer it won't fire for 2^32 microsec
 
-  That would be fine but we're not ever allowed to totally disable interrupts so we have to check *after*
-  we set it just to make sure it hasn't overflowed and if so to redo it.
+  That would be fine but we're not ever allowed to totally disable the timer so we have to check *after*
+  we set it just to make sure it hasn't overflowed and if so go ahead and trigger the interrupt ourselves.
   */
   if (utilTimerActive) { // Reschedule an active timer...
-    // Find out what our last trigger time was
-    uint32_t lastCC = nrf_timer_cc_read(NRF_TIMER1, NRF_TIMER_CC_CHANNEL0);
     // schedule timer to trigger at the last time we triggered PLUS our period
-    uint32_t thisCC = lastCC + period;
-    bool needsReschedule;
-    do {
-      // set up the timer
-      nrf_timer_cc_write(NRF_TIMER1, NRF_TIMER_CC_CHANNEL0, (uint32_t)thisCC);
-      needsReschedule = false;
-      // Check that the timer hasn't already passed this value? Reschedule it 2us in the future
-      NRF_TIMER1->TASKS_CAPTURE[1] = 1; // get current timer value
-      uint32_t current = NRF_TIMER1->CC[1];
-      if (((int32_t)thisCC - (int32_t)current) < 2) { // it it's closer than 2us (or has already passed!)
-        thisCC = current+2; // reschedule into the future
-        needsReschedule = true;
-      }
-    } while (needsReschedule);
+    uint32_t thisCC = utilTimerTriggerTime + period;
+    // set up the timer
+    nrf_timer_cc_write(NRF_TIMER1, NRF_TIMER_CC_CHANNEL0, (uint32_t)thisCC);
+    // Check that the timer hasn't already passed this value?
+    NRF_TIMER1->TASKS_CAPTURE[1] = 1; // get current timer value
+    uint32_t current = NRF_TIMER1->CC[1];
+    if (((int32_t)thisCC - (int32_t)current) <= -(int32_t)(NRF_TIMER_MAX  >> 2)) { // if we can't keep up and are already fairly far behind
+      // skip ahead to avoid falling victim to timer wraparound
+      // and potentially forgetting to schedule stuff altogether for some time
+      thisCC = current;
+      nrf_timer_cc_write(NRF_TIMER1, NRF_TIMER_CC_CHANNEL0, (uint32_t)current);
+    }
+    utilTimerTriggerTime = thisCC;
+    if (((int32_t)thisCC - (int32_t)current) < 2) { // if it's closer than 2us (or has already passed!)
+      // make sure that the timer doesn't trigger the interrupt shortly afterwards
+      nrf_timer_cc_write(NRF_TIMER1, NRF_TIMER_CC_CHANNEL0, (uint32_t)(current - 1));
+      // and manually trigger the interrupt now
+      NVIC_SetPendingIRQ(TIMER1_IRQn);
+    }
   } else {
     // timer is off, it'll be cleared to literally just set the period
+    utilTimerTriggerTime = period;
     nrf_timer_cc_write(NRF_TIMER1, NRF_TIMER_CC_CHANNEL0, (uint32_t)period);
   }
 }
 
 /// Start the timer and get it to interrupt after 'period'
 void jshUtilTimerStart(JsSysTime period) {
+  if (utilTimerActive) {
+    // schedule at the current time + period
+    // by pretending that the last time we wanted to be called at is now
+    NRF_TIMER1->TASKS_CAPTURE[1] = 1; // get current timer value
+    utilTimerTriggerTime = NRF_TIMER1->CC[1];
+  }
   jshUtilTimerReschedule(period);
   if (!utilTimerActive) {
     utilTimerActive = true;