1 files changed, 171 insertions, 0 deletions
diff --git a/src/avr/timer.c b/src/avr/timer.c
new file mode 100644
index 00000000..0b248552
--- /dev/null
+++ b/src/avr/timer.c
@@ -0,0 +1,171 @@
+// AVR timer interrupt scheduling code.
+//
+// Copyright (C) 2016  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include <avr/interrupt.h> // TCNT1
+#include "command.h" // shutdown
+#include "irq.h" // irq_save
+#include "sched.h" // sched_timer_kick
+#include "timer.h" // timer_from_ms
+
+
+/****************************************************************
+ * Low level timer code
+ ****************************************************************/
+
+// Return the number of clock ticks for a given number of milliseconds
+uint32_t
+timer_from_ms(uint32_t ms)
+{
+    return ms * (F_CPU / 1000);
+}
+
+static inline uint16_t
+timer_get(void)
+{
+    return TCNT1;
+}
+
+static inline void
+timer_set(uint16_t next)
+{
+    OCR1A = next;
+}
+
+static inline void
+timer_set_clear(uint16_t next)
+{
+    OCR1A = next;
+    TIFR1 = 1<<OCF1A;
+}
+
+ISR(TIMER1_COMPA_vect)
+{
+    sched_timer_kick();
+}
+
+static void
+timer_init(void)
+{
+    // no outputs
+    TCCR1A = 0;
+    // Normal Mode
+    TCCR1B = 1<<CS10;
+    // enable interrupt
+    TIMSK1 = 1<<OCIE1A;
+}
+DECL_INIT(timer_init);
+
+
+/****************************************************************
+ * 32bit timer wrappers
+ ****************************************************************/
+
+static uint32_t timer_last;
+
+// Return the 32bit current time given the 16bit current time.
+static __always_inline uint32_t
+calc_time(uint32_t last, uint16_t cur)
+{
+    union u32_u16_u calc;
+    calc.val = last;
+    if (cur < calc.lo)
+        calc.hi++;
+    calc.lo = cur;
+    return calc.val;
+}
+
+// Called by main code once every millisecond.  (IRQs disabled.)
+void
+timer_periodic(void)
+{
+    timer_last = calc_time(timer_last, timer_get());
+}
+
+// Return the current time (in absolute clock ticks).
+uint32_t
+timer_read_time(void)
+{
+    uint8_t flag = irq_save();
+    uint16_t cur = timer_get();
+    uint32_t last = timer_last;
+    irq_restore(flag);
+    return calc_time(last, cur);
+}
+
+#define TIMER_MIN_TICKS 100
+
+// Set the next timer wake time (in absolute clock ticks).  Caller
+// must disable irqs.  The caller should not schedule a time more than
+// a few milliseconds in the future.
+uint8_t
+timer_set_next(uint32_t next)
+{
+    uint16_t cur = timer_get();
+    if ((int16_t)(OCR1A - cur) < 0 && !(TIFR1 & (1<<OCF1A)))
+        // Already processing timer irqs
+        try_shutdown("timer_set_next called during timer dispatch");
+    uint32_t mintime = calc_time(timer_last, cur + TIMER_MIN_TICKS);
+    if (sched_is_before(mintime, next)) {
+        timer_set_clear(next);
+        return 0;
+    }
+    timer_set_clear(mintime);
+    return 1;
+}
+
+static uint8_t timer_repeat;
+#define TIMER_MAX_REPEAT 40
+#define TIMER_MAX_NEXT_REPEAT 15
+
+#define TIMER_MIN_TRY_TICKS 60 // 40 ticks to exit irq; 20 ticks of progress
+#define TIMER_DEFER_REPEAT_TICKS 200
+
+// Similar to timer_set_next(), but wait for the given time if it is
+// in the near future.
+uint8_t
+timer_try_set_next(uint32_t target)
+{
+    uint16_t next = target, now = timer_get();
+    int16_t diff = next - now;
+    if (diff > TIMER_MIN_TRY_TICKS)
+        // Schedule next timer normally.
+        goto done;
+
+    // Next timer is in the past or near future - can't reschedule to it
+    uint8_t tr = timer_repeat-1;
+    if (likely(tr)) {
+        irq_enable();
+        timer_repeat = tr;
+        irq_disable();
+        while (diff >= 0) {
+            // Next timer is in the near future - wait for time to occur
+            now = timer_get();
+            irq_enable();
+            diff = next - now;
+            irq_disable();
+        }
+        return 0;
+    }
+
+    // Too many repeat timers from a single interrupt - force a pause
+    timer_repeat = TIMER_MAX_NEXT_REPEAT;
+    next = now + TIMER_DEFER_REPEAT_TICKS;
+    if (diff < (int16_t)(-timer_from_ms(1)))
+        goto fail;
+
+done:
+    timer_set(next);
+    return 1;
+fail:
+    shutdown("Rescheduled timer in the past");
+}
+
+static void
+timer_task(void)
+{
+    timer_repeat = TIMER_MAX_REPEAT;
+}
+DECL_TASK(timer_task);