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/main.c

@@ -0,0 +1,88 @@
+// Main starting point for SAM3x8e boards.
+//
+// Copyright (C) 2016  Kevin O'Connor <kevin@koconnor.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include "board/misc.h" // console_get_input
+#include "sam3x8e.h" // WDT
+#include "sched.h" // sched_main
+
+
+/****************************************************************
+ * watchdog handler
+ ****************************************************************/
+
+static void
+watchdog_reset(void)
+{
+    WDT->WDT_CR = 0xA5000001;
+}
+DECL_TASK(watchdog_reset);
+
+static void
+watchdog_init(void)
+{
+    uint32_t timeout = 32768 / 2;  // 500ms timeout
+    WDT->WDT_MR = WDT_MR_WDRSTEN | WDT_MR_WDV(timeout) | WDT_MR_WDD(timeout);
+}
+DECL_INIT(watchdog_init);
+
+
+/****************************************************************
+ * irq clearing on fault
+ ****************************************************************/
+
+// Clear the active irq if a shutdown happened in an irq handler
+static void
+clear_active_irq(void)
+{
+    uint32_t psr;
+    asm volatile("mrs %0, psr" : "=r" (psr));
+    if (!(psr & 0x1ff))
+        // Shutdown did not occur in an irq - nothing to do.
+        return;
+    // Clear active irq status
+    psr &= ~0x1ff;
+    psr |= 1<<24; // T-bit
+    uint32_t temp;
+    asm volatile(
+        "  push { %1 }\n"
+        "  adr %0, 1f\n"
+        "  push { %0 }\n"
+        "  push { r0, r1, r2, r3, r12, lr }\n"
+        "  bx %2\n"
+        "1:\n"
+        : "=&r"(temp) : "r"(psr), "r"(0xfffffff9));
+}
+DECL_SHUTDOWN(clear_active_irq);
+
+
+/****************************************************************
+ * misc functions
+ ****************************************************************/
+
+size_t
+alloc_maxsize(size_t reqsize)
+{
+    return reqsize;
+}
+
+void * __visible
+_sbrk(int nbytes)
+{
+    extern char _end;
+    static void *heap_ptr = (void *)&_end;
+    void *pos = heap_ptr;
+    heap_ptr = pos + nbytes;
+    return pos;
+}
+
+// Main entry point
+int
+main(void)
+{
+    SystemInit();
+    sched_main();
+    return 0;
+}