microphone: PDM MEMS microphone support using I2S interface

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

microphone/microphone.go

@@ -0,0 +1,173 @@
+// Package microphone implements a driver for a PDM microphone.
+// For example, the Adafruit PDM MEMS breakout board (https://www.adafruit.com/product/3492)
+//
+// Datasheet: https://cdn-learn.adafruit.com/assets/assets/000/049/977/original/MP34DT01-M.pdf
+//
+package microphone // import "tinygo.org/x/drivers/microphone"
+
+import (
+	"machine"
+	"math"
+)
+
+const (
+	defaultSampleRate        = 22000
+	quantizeSteps            = 64
+	msForSPLSample           = 50
+	defaultSampleCountForSPL = (defaultSampleRate / 1000) * msForSPLSample
+	defaultGain              = 9.0
+	defaultRefLevel          = 0.00002
+)
+
+// Device wraps an I2S connection to a PDM microphone device.
+type Device struct {
+	bus machine.I2S
+
+	// data buffer used for SPL sound pressure level samples
+	data []int32
+
+	// buf buffer used for sinc filter
+	buf []uint32
+
+	// SampleCountForSPL is number of samples aka size of data buffer to be used
+	// for sound pressure level measurement.
+	// Once Configure() is called, changing this value has no effect.
+	SampleCountForSPL int
+
+	// Gain setting used to calculate sound pressure level
+	Gain float64
+
+	// ReferenceLevel setting used to calculate sound pressure level.
+	ReferenceLevel float64
+}
+
+// New creates a new microphone connection. The I2S bus must already be
+// configured.
+//
+// This function only creates the Device object, it does not touch the device.
+func New(bus machine.I2S) Device {
+	return Device{
+		bus:               bus,
+		SampleCountForSPL: defaultSampleCountForSPL,
+		Gain:              defaultGain,
+		ReferenceLevel:    defaultRefLevel,
+	}
+}
+
+// Configure the microphone.
+func (d *Device) Configure() {
+	d.data = make([]int32, d.SampleCountForSPL)
+	d.buf = make([]uint32, (quantizeSteps / 16))
+}
+
+// Read the raw microphone data.
+func (d *Device) Read(r []int32) (int, error) {
+	count := len(r)
+
+	// get the next group of samples
+	machine.I2S0.Read(d.buf)
+
+	if len(r) > len(d.buf) {
+		count = len(d.buf)
+	}
+	for i := 0; i < count; i++ {
+		r[i] = int32(d.buf[i])
+	}
+
+	return count, nil
+}
+
+// ReadWithFilter reads the microphone and filters the buffer using the sinc filter.
+func (d *Device) ReadWithFilter(r []int32) (int, error) {
+	// read/filter the samples
+	var sum uint16
+	for i := 0; i < len(r); i++ {
+
+		// get the next group of samples
+		machine.I2S0.Read(d.buf)
+
+		// filter
+		sum = applySincFilter(d.buf)
+
+		// adjust to 10 bit value
+		s := int32(sum >> 6)
+
+		// make it close to 0-offset signed
+		s -= 512
+
+		r[i] = s
+	}
+
+	return len(r), nil
+}
+
+// GetSoundPressure returns the sound pressure in milli-decibels.
+func (d *Device) GetSoundPressure() (int32, int32) {
+	// read/filter the samples
+	d.ReadWithFilter(d.data)
+
+	// remove offset
+	var avg int32
+	for i := 0; i < len(d.data); i++ {
+		avg += d.data[i]
+	}
+	avg /= int32(len(d.data))
+
+	for i := 0; i < len(d.data); i++ {
+		d.data[i] -= avg
+	}
+
+	// get max value
+	var maxval int32
+	for i := 0; i < len(d.data); i++ {
+		v := d.data[i]
+		if v < 0 {
+			v = -v
+		}
+		if maxval < v {
+			maxval = v
+		}
+	}
+
+	// calculate SPL
+	spl := float64(maxval) / 1023.0 * d.Gain
+	spl = 20 * math.Log10(spl/d.ReferenceLevel)
+
+	return int32(spl * 1000), maxval
+}
+
+// sinc filter for 44 khz with 64 samples
+// each value matches the corresponding bit in the 8-bit value
+// for that sample.
+//
+// For more information: https://en.wikipedia.org/wiki/Sinc_filter
+//
+var sincfilter = [quantizeSteps]uint16{
+	0, 2, 9, 21, 39, 63, 94, 132,
+	179, 236, 302, 379, 467, 565, 674, 792,
+	920, 1055, 1196, 1341, 1487, 1633, 1776, 1913,
+	2042, 2159, 2263, 2352, 2422, 2474, 2506, 2516,
+	2506, 2474, 2422, 2352, 2263, 2159, 2042, 1913,
+	1776, 1633, 1487, 1341, 1196, 1055, 920, 792,
+	674, 565, 467, 379, 302, 236, 179, 132,
+	94, 63, 39, 21, 9, 2, 0, 0,
+}
+
+// applySincFilter uses the sinc filter to process a single set of sample values.
+func applySincFilter(samples []uint32) (result uint16) {
+	var sample uint16
+	pos := 0
+	for j := 0; j < len(samples); j++ {
+		// takes only the low order 16-bits
+		sample = uint16(samples[j] & 0xffff)
+		for i := 0; i < 16; i++ {
+			if (sample & 0x1) > 0 {
+				result += sincfilter[pos]
+				pos++
+			}
+			sample >>= 1
+		}
+	}
+
+	return
+}