apps: heart: Introduce simple app for the heart rate sensor

The heart rate analysis step is still a work in progress but the current
app allows us to visualize the the results of the signal conditioning.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

1 files changed, 139 insertions, 0 deletions

diff --git a/wasp/apps/heart.py b/wasp/apps/heart.py
new file mode 100644
index 0000000..d88efd4
--- /dev/null
+++ b/wasp/apps/heart.py
@@ -0,0 +1,139 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+# Copyright (C) 2020 Daniel Thompson
+
+import wasp
+import machine
+
+class Biquad():
+    """Direct Form II Biquad Filter"""
+
+    def __init__(self, b0, b1, b2, a1, a2):
+        self._coeff = (b0, b1, b2, a1, a2)
+        self._v1 = 0
+        self._v2 = 0
+
+    def step(self, x):
+        c = self._coeff
+        v1 = self._v1
+        v2 = self._v2
+
+        v = x - (c[3] * v1) - (c[4] * v2)
+        y = (c[0] * v) + (c[1] * v1) + (c[2] * v2)
+
+        self._v2 = v1
+        self._v1 = v
+        return y
+
+class PTAGC():
+    """Peak Tracking Automatic Gain Control
+    
+    In order for the correlation checks to work correctly we must
+    aggressively reject spikes caused by fast DC steps. Setting a
+    threshold based on the median is very effective at killing
+    spikes but needs an extra 1k for sample storage which isn't
+    really plausible for a microcontroller.
+    """
+    def __init__(self, start, decay, threshold):
+        self._peak = start
+        self._decay = decay
+        self._boost = 1 / decay
+        self._threshold = threshold
+
+    def step(self, spl):        
+        # peak tracking
+        peak = self._peak
+        if abs(spl) > peak:
+            peak *= self._boost
+        else:
+            peak *= self._decay
+        self._peak = peak
+
+        # rejection filter (clipper)
+        threshold = self._threshold
+        if spl > (peak * threshold) or spl < (peak * -threshold):
+            return 0
+        
+        # booster
+        spl = 100 * spl / (2 * peak)
+        
+        return spl
+
+class HeartApp():
+    """Heart Rate Sensing application.
+
+    """
+    NAME = 'Heart'
+
+    def foreground(self):
+        """Activate the application."""
+        wasp.watch.hrs.enable()
+
+        # There is no delay after the enable because the redraw should
+        # take long enough it is not needed
+        draw = wasp.watch.drawable
+        draw.fill()
+        draw.string('PPG graph', 0, 6, width=240)
+
+        self._hpf = Biquad(0.87518309, -1.75036618,  0.87518309, -1.73472577,  0.7660066)
+        self._agc = PTAGC(20, 0.971, 2)
+        self._lpf = Biquad(0.10873253, 0.21746505, 0.10873253, -0.76462555,  0.19955565)
+
+        self._x = 0
+        self._offset = wasp.watch.hrs.read_hrs()
+
+        wasp.system.request_tick(1000 // 8)
+
+    def background(self):
+        wasp.watch.hrs.disable()
+        del self._hpf
+        del self._agc
+        del self._lpf
+
+    def _tick(self, ticks):
+        """Notify the application that its periodic tick is due."""
+        spl = wasp.watch.hrs.read_hrs()
+        spl -= self._offset
+        spl = self._hpf.step(spl)
+        spl = self._agc.step(spl)
+        spl = self._lpf.step(spl)
+
+        color = 0xffc0
+
+        # If the maths goes wrong lets show it in the chart!
+        if spl > 100 or spl < -100:
+            color = 0xffff
+        if spl > 104 or spl < -104:
+            spl = 0
+        spl = int(spl) + 104
+
+        x = self._x
+
+        draw = wasp.watch.drawable
+        draw.fill(0, x, 32, 1, 208-spl)
+        draw.fill(color, x, 239-spl, 1, spl)
+
+        x += 2
+        if x >= 240:
+            x = 0
+        self._x = x
+
+    def tick(self, ticks):
+        """This is an outrageous hack but, at present, the RTC can only
+        wake us up every 125ms so we implement sub-ticks using a regular
+        timer to ensure we can read the sensor at 24Hz.
+        """
+        t = machine.Timer(id=1, period=8000000)
+        t.start()
+        self._tick(1)
+        wasp.system.keep_awake()
+
+        while t.time() < 41666:
+            pass
+        self._tick(1)
+
+        while t.time() < 83332:
+            pass
+        self._tick(1)
+
+        t.stop()
+        del t