microphone: PDM MEMS microphone support using I2S interface

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

examples/microphone/main.go

@@ -0,0 +1,37 @@
+// Example using the i2s hardware interface on the Adafruit Circuit Playground Express
+// to read data from the onboard MEMS microphone.
+//
+// Uses ideas from the https://github.com/adafruit/Adafruit_CircuitPlayground repo.
+//
+package main
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/microphone"
+)
+
+const (
+	defaultSampleRate        = 22000
+	quantizeSteps            = 64
+	msForSPLSample           = 50
+	defaultSampleCountForSPL = (defaultSampleRate / 1000) * msForSPLSample
+)
+
+func main() {
+	machine.I2S0.Configure(machine.I2SConfig{
+		Mode:           machine.I2SModePDM,
+		AudioFrequency: defaultSampleRate * quantizeSteps / 16,
+		ClockSource:    machine.I2SClockSourceExternal,
+		Stereo:         true,
+	})
+
+	mic := microphone.New(machine.I2S0)
+	mic.SampleCountForSPL = defaultSampleCountForSPL
+	mic.Configure()
+
+	for {
+		spl, maxval := mic.GetSoundPressure()
+		println("C", spl, "max", maxval)
+	}
+}