Reading Heart Rate from the Zephyr HxM BT

The Zephyr HxM BT heart rate monitor may be easily integrated into the meeTrainer application. The API specification for the HxM specifies that it transmits one 60 byte packet every second, and that the heart rate is shipped in one byte with index 12 into this packet. 

   1 # Externally pairing and setup of the SP must be done as root:
   2 # $ rfcomm  connect rfcomm1 00:07:80:5A:35:42 1
   3 
   4 import serial
   5 import struct
   6 import time
   7 
   8 pkg_struct = struct.Struct('60B')
   9 pkg_size = pkg_struct.size
  10 zhxm = serial.Serial('/dev/rfcomm1')
  11 
  12 hr_idx = 12
  13 dist_idx = [50, 51]
  14 speed_idx = [52, 53]
  15 strides_idx = 54
  16 
  17 def read_hr():
  18     zhxm.read(zhxm.inWaiting())
  19     d = pkg_struct.unpack(zhxm.read(pkg_size))
  20     return d[hr_idx]
  21 
  22 
  23 #
  24 # Make a running plot for the past 60 seconds of HR measurements
  25 #
  26 import matplotlib.pyplot as plt
  27 import numpy as np
  28 hrs = read_hr() * np.ones(60,dtype=np.uint8)
  29 plt.ion()
  30 fig = plt.figure()
  31 fig.set
  32 ax = fig.add_subplot(111)
  33 ax.grid(True)
  34 ax.set_yticks(np.arange(40,220,5))
  35 ax.set_ylim(40, 220)
  36 hr_line, = ax.plot(hrs, 'ro-')
  37 while True:
  38     hr = read_hr()
  39     hrs = np.roll(hrs,1)
  40     hrs[0] = hr
  41     hr_line.set_ydata(hrs)
  42     fig.canvas.draw()
  43     time.sleep(0.5)
  44 
  45 zhxm.close()