nRF52: Improve software timer accuracy by keeping timer running while executing jstUtilTimerInterruptHandler (fix #2620)

Author: gfwilliams

ChangeLog

@@ -25,6 +25,7 @@
             Fix potential overflow of locks - favour small memory leak over accidental free if this ever happens (fix #2616)
             micro:bit2: Ensure we don't initialise the I2C1 peripheral (we use software I2C internally)
             Unlock functionCode early in function execution to reduce locks needed during recursion (#2616)
+            nRF52: Improve software timer accuracy by keeping timer running while executing jstUtilTimerInterruptHandler (fix #2620)
 
      2v25 : ESP32C3: Get analogRead working correctly
             Graphics: Adjust image alignment when rotating images to avoid cropping (fix #2535)

src/jshardware.h

@@ -338,7 +338,9 @@ size_t jshFlashGetMemMapAddress(size_t ptr);
 
 /// Start the timer and get it to interrupt once after 'period' (i.e. it should not auto-reload)
 void jshUtilTimerStart(JsSysTime period);
-/// Reschedule the timer (it should already be running) to interrupt after 'period'
+/** Reschedule the timer (it should already be running) to interrupt after 'period'.
+With the timer running this should ideally set the time period from the last time the
+timer triggered (so as to factor out any execution delay in jstUtilTimerInterruptHandler).  */
 void jshUtilTimerReschedule(JsSysTime period);
 /// Stop the timer
 void jshUtilTimerDisable();

src/jstimer.c

@@ -87,9 +87,10 @@ void jstUtilTimerInterruptHandler() {
   /* Note: we're using 32 bit times here, even though the real time counter is 64 bit. We
    * just make sure nothing is scheduled that far in the future */
   if (utilTimerOn) {
-    // TODO: Keep UtilTimer running and then use the value from it
-    // to estimate how long utilTimerPeriod really was
-    // Subtract utilTimerPeriod from all timers' time
+    utilTimerSetTime = jshGetSystemTime();
+    /* We keep UtilTimer running and then jshUtilTimerReschedule reschedules
+    based on the time from when this IRQ was triggered, *not* from the
+    current time. */
     int t = utilTimerTasksTail;
     while (t!=utilTimerTasksHead) {
       utilTimerTasks[t].time -= utilTimerPeriod;
@@ -218,7 +219,6 @@ void jstUtilTimerInterruptHandler() {
     // re-schedule the timer if there is something left to do
     if (utilTimerTasksTail != utilTimerTasksHead) {
       utilTimerPeriod = utilTimerTasks[utilTimerTasksTail].time;
-      utilTimerSetTime = jshGetSystemTime();
       if (utilTimerPeriod<0) utilTimerPeriod=0;
       jshUtilTimerReschedule(utilTimerPeriod);
     } else {

targets/nrf5x/jshardware.c

@@ -626,7 +626,6 @@ const nrf_drv_twis_t *jshGetTWIS(IOEventFlags device) {
 #endif
 
 void TIMER1_IRQHandler(void) {
-  nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_CLEAR);
   nrf_timer_event_clear(NRF_TIMER1, NRF_TIMER_EVENT_COMPARE0);
   jstUtilTimerInterruptHandler();
 }
@@ -2776,17 +2775,43 @@ void jshUtilTimerReschedule(JsSysTime period) {
     period = NRF_TIMER_MAX;
   }
   //jsiConsolePrintf("Sleep for %d %d -> %d\n", (uint32_t)(t>>32), (uint32_t)(t), (uint32_t)(period));
-  if (utilTimerActive) nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_STOP);
-  nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_CLEAR);
-  nrf_timer_cc_write(NRF_TIMER1, NRF_TIMER_CC_CHANNEL0, (uint32_t)period);
-  if (utilTimerActive) nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_START);
+
+  /* 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.
+  */
+  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);
+  } else {
+    // timer is off, it'll be cleared to literally just set the 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) {
   jshUtilTimerReschedule(period);
   if (!utilTimerActive) {
     utilTimerActive = true;
+    nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_CLEAR);
     nrf_timer_task_trigger(NRF_TIMER1, NRF_TIMER_TASK_START);
   }
 }