cpu/sam3: add opt to use external 32khz oscillator

cpu/sam3/cpu.c

@@ -30,6 +30,11 @@
 #define MORKEY              (0x37)
 /** @} */
 
+/**
+ * @brief   Key for writing the SUPC control register
+ */
+#define SUPCKEY             (0xa5)
+
 /**
  * @brief   Start-up time for external crystal (will be multiplied by 8)
  */
@@ -91,6 +96,13 @@ void cpu_init(void)
     /* wait for master clock to be ready */
     while (!(PMC->PMC_SR & PMC_SR_MCKRDY));
 
+    /* setup the SCLK: switch to external oscillator if applicable */
+#if CLOCK_SCLK_XTAL
+    /* enable external oscillator */
+    SUPC->SUPC_CR = (SUPC_CR_KEY(SUPCKEY) | SUPC_CR_XTALSEL);
+    while (!(SUPC->SUPC_SR & SUPC_SR_OSCSEL_CRYST)) {}
+#endif
+
     /* initialize stdio prior to periph_init() to allow use of DEBUG() there */
     stdio_init();