Show battery voltage on splash screen

Author: ysoldak

Diff for: src/extras_nano-33-ble.go

@@ -2,6 +2,14 @@
 
 package main
 
+import "errors"
+
+var errNoBattery = errors.New("no battery")
+
 func initExtras() {
 	// nop
 }
+
+func batteryVoltage() (float64, error) {
+	return 0, errNoBattery
+}

Diff for: src/extras_xiao-ble.go

@@ -15,9 +15,25 @@ import "machine"
 
 var (
 	pinChargeCurrent = machine.P0_13
+	pinRead          = machine.P0_14
+	pinVoltage       = machine.P0_31
+
+	adc machine.ADC
 )
 
 func initExtras() {
 	pinChargeCurrent.Configure(machine.PinConfig{Mode: machine.PinOutput})
 	pinChargeCurrent.Low() // enable charging at high current, 100mA
+
+	// Shall keep this low while reading voltage
+	pinRead.Configure(machine.PinConfig{Mode: machine.PinOutput})
+
+	// Battery sensor pin
+	adc = machine.ADC{Pin: pinVoltage}
+	adc.Configure(machine.ADCConfig{})
+}
+
+func batteryVoltage() (float64, error) {
+	pinRead.Low()
+	return float64(adc.Get()) / 7050, nil
 }

Diff for: src/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"math"
+	"strconv"
 	"time"
 
 	"github.com/ysoldak/HeadTracker/src/display"
@@ -60,7 +61,12 @@ func init() {
 
 func main() {
 
-	d.AddText(0, "Head Tracker")
+	batVolts, err := batteryVoltage()
+	batString := ""
+	if err == nil {
+		batString = strconv.FormatFloat(batVolts, 'f', 2, 64) + "V"
+	}
+	d.AddText(0, "Head Tracker "+batString)
 	d.AddText(1, Version+" @ysoldak")
 
 	// warm up IMU (1 sec)