DAC support

Hardware Arduino Due
29

@jgmdavies

This modification seem to fix this buggy behaviour:

diff --git a/hardware/arduino/sam/cores/arduino/wiring_analog.c b/hardware/arduino/sam/cores/arduino/wiring_analog.c
index 58bd87a..97f50e8 100644
--- a/hardware/arduino/sam/cores/arduino/wiring_analog.c
+++ b/hardware/arduino/sam/cores/arduino/wiring_analog.c
@@ -243,8 +243,8 @@ void analogWrite(uint32_t ulPin, uint32_t ulValue) {
 
                        // Write user value
                        ulValue = mapResolution(ulValue, _writeResolution, DACC_RESOLUTION);
-                       while ((dacc_get_interrupt_status(DACC_INTERFACE) & DACC_ISR_TXRDY) == 0);
                        dacc_write_conversion_data(DACC_INTERFACE, ulValue);
+                       while ((dacc_get_interrupt_status(DACC_INTERFACE) & DACC_ISR_EOC) == 0);
                        return;
                }
        }

also with this change should desired value be on output pin by the time analogWrite() exits. Opinions?

V.