machine/samd21: implement ADC

Signed-off-by: Ron Evans <ron@hybridgroup.com>

src/runtime/runtime_atsamd21.go

@@ -24,6 +24,7 @@ func init() {
 	initRTC()
 	initSERCOMClocks()
 	initUSBClock()
+	initADCClock()
 
 	// connect to USB CDC interface
 	machine.UART0.Configure(machine.UARTConfig{})
@@ -355,3 +356,14 @@ func initUSBClock() {
 		sam.GCLK_CLKCTRL_CLKEN)
 	waitForSync()
 }
+
+func initADCClock() {
+	// Turn on clock for ADC
+	sam.PM.APBCMASK |= sam.PM_APBCMASK_ADC_
+
+	// Put Generic Clock Generator 0 as source for Generic Clock Multiplexer for ADC.
+	sam.GCLK.CLKCTRL = sam.RegValue16((sam.GCLK_CLKCTRL_ID_ADC << sam.GCLK_CLKCTRL_ID_Pos) |
+		(sam.GCLK_CLKCTRL_GEN_GCLK0 << sam.GCLK_CLKCTRL_GEN_Pos) |
+		sam.GCLK_CLKCTRL_CLKEN)
+	waitForSync()
+}