sam3x8e: Add initial support for Arduino Due boards

This adds basic support for running on the Atmel SAM3x8e
micro-controllers that are found in the Arudino Due boards.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sam3x8e/timer.c

@@ -0,0 +1,142 @@
+// SAM3x8e timer interrupt scheduling
+//
+// Copyright (C) 2016  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include "autoconf.h" // CONFIG_CLOCK_FREQ
+#include "board/misc.h" // timer_from_us
+#include "command.h" // shutdown
+#include "irq.h" // irq_disable
+#include "sam3x8e.h" // TC0
+#include "sched.h" // sched_timer_kick
+
+
+/****************************************************************
+ * Low level timer code
+ ****************************************************************/
+
+// Return the number of clock ticks for a given number of microseconds
+uint32_t
+timer_from_us(uint32_t us)
+{
+    return us * (CONFIG_CLOCK_FREQ / 1000000);
+}
+
+// IRQ handler
+void __visible
+TC0_Handler(void)
+{
+    TC0->TC_CHANNEL[0].TC_SR; // clear irq pending
+    irq_disable();
+    sched_timer_kick();
+    irq_enable();
+}
+
+static void
+timer_set(uint32_t value)
+{
+    TC0->TC_CHANNEL[0].TC_RA = value;
+}
+
+static void
+timer_init(void)
+{
+    TcChannel *tc = &TC0->TC_CHANNEL[0];
+    // Reset the timer
+    tc->TC_CCR = TC_CCR_CLKDIS;
+    tc->TC_IDR = 0xFFFFFFFF;
+    tc->TC_SR;
+    // Enable it
+    PMC->PMC_PCER0 = 1 << ID_TC0;
+    tc->TC_CMR = TC_CMR_WAVE | TC_CMR_WAVSEL_UP | TC_CMR_TCCLKS_TIMER_CLOCK1;
+    tc->TC_IER = TC_IER_CPAS;
+    NVIC_SetPriority(TC0_IRQn, 1);
+    NVIC_EnableIRQ(TC0_IRQn);
+    timer_set(1);
+    tc->TC_CCR = TC_CCR_CLKEN | TC_CCR_SWTRG;
+}
+DECL_INIT(timer_init);
+
+// Called by main code once every millisecond.  (IRQs disabled.)
+void
+timer_periodic(void)
+{
+}
+
+// Return the current time (in absolute clock ticks).
+uint32_t
+timer_read_time(void)
+{
+    return TC0->TC_CHANNEL[0].TC_CV;
+}
+
+#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)
+{
+    uint32_t cur = timer_read_time();
+    uint32_t mintime = cur + TIMER_MIN_TICKS;
+    if (sched_is_before(mintime, next)) {
+        timer_set(next);
+        return 0;
+    }
+    timer_set(mintime);
+    return 1;
+}
+
+static uint32_t timer_repeat;
+#define TIMER_MAX_REPEAT 400
+#define TIMER_MAX_NEXT_REPEAT 150
+
+#define TIMER_MIN_TRY_TICKS timer_from_us(1)
+#define TIMER_DEFER_REPEAT_TICKS timer_from_us(2)
+
+// 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 next)
+{
+    uint32_t now = timer_read_time();
+    int32_t diff = next - now;
+    if (diff > (int32_t)TIMER_MIN_TRY_TICKS)
+        // Schedule next timer normally.
+        goto done;
+
+    // Next timer is in the past or near future - can't reschedule to it
+    uint32_t tr = timer_repeat-1;
+    if (likely(tr)) {
+        irq_enable();
+        timer_repeat = tr;
+        while (diff >= 0) {
+            // Next timer is in the near future - wait for time to occur
+            now = timer_read_time();
+            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 < (int32_t)(-timer_from_us(1000)))
+        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);