lib: Make sure that PWM continues to work even if it misses an IRQ.

If the soft PWM driver misses an IRQ then the next CC value (computed as
CC[x] += delay) could already be behind the timer counter value, and hence
the next compare IRQ won't fire until the timer counter value wraps around.

Example code which demonstrates the bug (by having a long neopixel write
that stops IRQs):

    import microbit
    import music
    import neopixel

    np = neopixel.NeoPixel(microbit.pin0, 64)

    while True:
        music.play(music.BA_DING, wait=False)
        microbit.sleep(1000)
        np.clear()
        microbit.sleep(2000)

Prior to this commit the music would only play every 16 seconds or so,
which is the time taken for the PWM fast ticker to wrap around.

Fixes issue #600.

Signed-off-by: Damien George <damien@micropython.org>

source/lib/ticker.c

@@ -37,6 +37,23 @@
 #define LowPriority_IRQn SWI4_IRQn
 #define LowPriority_IRQHandler SWI4_IRQHandler
 
+// Ticker is configured to TIMER_BITMODE_BITMODE_24Bit.
+#define TICKER_MASK 0xffffff
+
+// Schedule the next CC value, taking care of the case where an IRQ could have been
+// missed, so CC[x]+dt is already behind the timer counter value.  In such a case
+// schedule it as COUNTER[x]+dt.
+#define TICKER_SCHEDULE_NEXT_CC(reg_id, dt) \
+    do { \
+        uint32_t next_cc = ticker->CC[reg_id] + dt; \
+        ticker->TASKS_CAPTURE[reg_id] = 1; \
+        uint32_t cc = ticker->CC[reg_id]; \
+        if (((next_cc - cc) & TICKER_MASK) > (TICKER_MASK >> 1)) { \
+            next_cc = cc + dt; \
+        } \
+        ticker->CC[reg_id] = next_cc; \
+    } while (0)
+
 // The number of milliseconds that have passed since the ticker was initialised
 volatile uint32_t ticker_ticks_ms;
 
@@ -90,19 +107,23 @@ void FastTicker_IRQHandler(void) {
     ticker_callback_ptr *call = callbacks;
     if (ticker->EVENTS_COMPARE[0]) {
         ticker->EVENTS_COMPARE[0] = 0;
-        ticker->CC[0] += call[0]()*MICROSECONDS_PER_TICK;
+        uint32_t dt = call[0]() * MICROSECONDS_PER_TICK;
+        TICKER_SCHEDULE_NEXT_CC(0, dt);
     }
     if (ticker->EVENTS_COMPARE[1]) {
         ticker->EVENTS_COMPARE[1] = 0;
-        ticker->CC[1] += call[1]()*MICROSECONDS_PER_TICK;
+        uint32_t dt = call[1]() * MICROSECONDS_PER_TICK;
+        TICKER_SCHEDULE_NEXT_CC(1, dt);
     }
     if (ticker->EVENTS_COMPARE[2]) {
         ticker->EVENTS_COMPARE[2] = 0;
-        ticker->CC[2] += call[2]()*MICROSECONDS_PER_TICK;
+        uint32_t dt = call[2]() * MICROSECONDS_PER_TICK;
+        TICKER_SCHEDULE_NEXT_CC(2, dt);
     }
     if (ticker->EVENTS_COMPARE[3]) {
         ticker->EVENTS_COMPARE[3] = 0;
-        ticker->CC[3] += MICROSECONDS_PER_MACRO_TICK;
+        uint32_t dt = MICROSECONDS_PER_MACRO_TICK;
+        TICKER_SCHEDULE_NEXT_CC(3, dt);
         ticker_ticks_ms += MILLISECONDS_PER_MACRO_TICK;
         NVIC_SetPendingIRQ(SlowTicker_IRQn);
     }