machine/rp2040: add basic support for ADC

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

src/machine/machine_rp2040_adc.go

@@ -0,0 +1,61 @@
+// +build rp2040
+
+package machine
+
+import (
+	"device/rp"
+)
+
+func InitADC() {
+	// reset ADC
+	rp.RESETS.RESET.SetBits(rp.RESETS_RESET_ADC)
+	rp.RESETS.RESET.ClearBits(rp.RESETS_RESET_ADC)
+	for !rp.RESETS.RESET_DONE.HasBits(rp.RESETS_RESET_ADC) {
+	}
+
+	// enable ADC
+	rp.ADC.CS.Set(rp.ADC_CS_EN)
+
+	waitForReady()
+}
+
+// Configure configures a ADC pin to be able to be used to read data.
+func (a ADC) Configure(config ADCConfig) {
+	switch a.Pin {
+	case GP26, GP27, GP28, GP29:
+		a.Pin.Configure(PinConfig{Mode: PinAnalog})
+	default:
+		// invalid ADC
+		return
+	}
+}
+
+func (a ADC) Get() uint16 {
+	rp.ADC.CS.SetBits(uint32(a.getADCChannel()) << rp.ADC_CS_AINSEL_Pos)
+	rp.ADC.CS.SetBits(rp.ADC_CS_START_ONCE)
+
+	waitForReady()
+
+	// rp2040 uses 12-bit sampling, so scale to 16-bit
+	return uint16(rp.ADC.RESULT.Get() << 4)
+}
+
+func waitForReady() {
+	for !rp.ADC.CS.HasBits(rp.ADC_CS_READY) {
+	}
+}
+
+func (a ADC) getADCChannel() uint8 {
+	switch a.Pin {
+	case GP26:
+		return 0
+	case GP27:
+		return 1
+	case GP28:
+		return 2
+	case GP29:
+		return 3
+	default:
+		return 0
+	}
+}