boards: arm: document and enable the DAC for the Arduino Zero

The previous commits added the SAM0 DAC driver.  Now document and
enable it, including enabling it in the DAC sample.

Signed-off-by: Michael Hope <mlhx@google.com>

boards/arm/arduino_zero/arduino_zero.dts

@@ -89,6 +89,10 @@
 	#pwm-cells = <1>;
 };
 
+&dac0 {
+	status = "okay";
+};
+
 &flash0 {
 	partitions {
 		compatible = "fixed-partitions";

boards/arm/arduino_zero/arduino_zero.yaml

@@ -11,6 +11,7 @@ toolchain:
 supported:
   - adc
   - counter
+  - dac
   - dma
   - gpio
   - hwinfo

boards/arm/arduino_zero/doc/index.rst

@@ -53,6 +53,8 @@ features:
 +-----------+------------+------------------------------------------+
 | USB       | on-chip    | USB device                               |
 +-----------+------------+------------------------------------------+
+| DAC       | on-chip    | Digital to analogue converter            |
++-----------+------------+------------------------------------------+
 
 Other hardware features are not currently supported by Zephyr.
 
@@ -101,6 +103,12 @@ with a host PC.  See the :ref:`usb-samples` sample applications for
 more, such as the :ref:`usb_cdc-acm` sample which sets up a virtual
 serial port that echos characters back to the host PC.
 
+DAC
+===
+
+The SAMD21 MCU has a single channel DAC with 10 bits of resolution. On the
+Arduino Zero, the DAC is available on pin A0.
+
 Programming and Debugging
 *************************
 

boards/arm/arduino_zero/pinmux.c

@@ -75,6 +75,10 @@ static int board_pinmux_init(struct device *dev)
 	pinmux_pin_set(muxa, 24, PINMUX_FUNC_G);
 #endif
 
+#if DT_NODE_HAS_STATUS(DT_NODELABEL(dac0), okay) && defined(CONFIG_DAC_SAM0)
+	/* DAC on PA02 */
+	pinmux_pin_set(muxa, 2, PINMUX_FUNC_B);
+#endif
 	return 0;
 }
 

samples/drivers/dac/sample.yaml

@@ -3,7 +3,7 @@ sample:
 tests:
   sample.drivers.dac:
     tags: DAC
-    platform_whitelist: frdm_k64f nucleo_l073rz nucleo_l152re twr_ke18f
+    platform_whitelist: arduino_zero frdm_k64f nucleo_l073rz nucleo_l152re twr_ke18f
     depends_on: dac
     harness: console
     harness_config:

samples/drivers/dac/src/main.c

@@ -21,6 +21,10 @@
 #define DAC_DEVICE_NAME		DT_LABEL(DT_NODELABEL(dac0))
 #define DAC_CHANNEL_ID		0
 #define DAC_RESOLUTION		12
+#elif defined(CONFIG_BOARD_ARDUINO_ZERO)
+#define DAC_DEVICE_NAME		DT_LABEL(DT_NODELABEL(dac0))
+#define DAC_CHANNEL_ID		0
+#define DAC_RESOLUTION		10
 #else
 #error "Unsupported board."
 #endif