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# 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
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