NGE101 – Norgo wireless energy meter (part 3)

Before decoding the data from the NGE101, it was necessary to find out the duration of the pulses it sends. To do this I needed a small sketch that could output the exact duration of the received  pulses.

This is the code I came up with, it will print a stream of bits and there durations in microseconds.

uint8_t lastinput = LOW;
uint32_t lastinputat;

void setup()
  pinMode(2, INPUT);
  lastinputat = micros();

void loop()
  uint8_t input = digitalRead(2);
  if(input != lastinput)
    uint32_t now = micros();
    uint32_t dt = now - lastinputat;
    lastinput = input;
    lastinputat = now;
    Serial.print(input, 10);
    Serial.print(' ');
    Serial.println(dt, 10);

The output looks something like this:
1 1068
0 472
1 488
0 416
1 568

The durations were not as consistent as I had hoped they would be, so to get a better idea of what I was dealing with, I wrote a python gui application to graph the durations.

import wx 
import re 
import serial 
import sys 
import time 
import math 

WIN_WIDTH = 590 # pixels 

class TestFrame(wx.Frame): 
  def __init__(self): 
    wx.Frame.__init__(self, None, title='test', size=(WIN_WIDTH, WIN_HEIGHT)) 

    self.line_re = re.compile('([01]) *([0-9]+)') 

    self.ser = serial.Serial('/dev/ttyACM0', 115200*8, timeout=0.000) 
    self.last = None 
    self.recvqueue = '' 
    self.durations = [[0]*WIN_WIDTH, [0]*WIN_WIDTH] 

    self.panel = wx.Panel(self, size=(WIN_WIDTH, WIN_HEIGHT)) 
    self.panel.Bind(wx.EVT_PAINT, self.on_paint) 

    self.timer = wx.Timer(self.panel, 123) 
    wx.EVT_TIMER(self.panel, 123, self.on_timer) 

  def on_paint(self, event): 
    dc = wx.PaintDC(self.panel) 

    dc.SetPen(wx.Pen('red', 1)) 
    dc.DrawLine(0, WIN_HEIGHT/2, WIN_WIDTH, WIN_HEIGHT/2) 
    for i in range(0, WIN_WIDTH, 1): 
      if math.floor(t/0.001) != math.floor((t-DURATION_PER_PIXEL)/0.001): 
        dc.DrawLine(i, 0, i, WIN_HEIGHT) 
      elif math.floor(t/0.0001) != math.floor((t-DURATION_PER_PIXEL)/0.0001): 
        dc.DrawLine(i, WIN_HEIGHT/2-8, i, WIN_HEIGHT/2+8) 

    dc.SetPen(wx.Pen('black', 1)) 
    for i in range(WIN_WIDTH): 
      dc.DrawLine(i, WIN_HEIGHT/2+1, i, WIN_HEIGHT/2+1+self.durations[0][i]) 
      dc.DrawLine(i, WIN_HEIGHT/2-1, i, WIN_HEIGHT/2-1-self.durations[1][i]) 

  def on_timer(self, event): 
    redraw = False 

    while True: 
      line = self.ser.readline() 
      if not line: break 
      m = self.line_re.match(line) 
      if not m: continue 

      bit = int(m.group(1)) 
      duration = float(m.group(2))/1000000.0 
      print bit, duration 

      self.durations[bit][min(int(duration/DURATION_PER_PIXEL), WIN_WIDTH-1)] += 1 
      redraw = True 

    if redraw: 

app = wx.App(False) 
frame = TestFrame() 


The result looked like this. The window covers a durations of 3 milliseconds, so there is a tall vertical line per millisecond.

It's now possible to see that the durations fall into 4 groups:

  • short high pulses (logic level 1) in the range 400-600 milliseconds.

  • short low pulses (logic level 0) in the range 300-500 milliseconds.

  • long high pulses in the range 900-1100 milliseconds.

  • long low pulses in the range 800-1000 milliseconds.

I suspect the difference in timing from the high and low pulses is because the sender has an extremely slow cpu, and it spends a few more cycles when sending a 1 than sending a 0. A bit of goggling suggest that 32.768 kHz is very common speed for low powered embedded micro controllers. At that speed, the time difference (0.15 ms) between high and low pulses are just around 5 cycles.

Visualizing the data as a pulse train, using __, _ for low pulses, and --, - for high pulses, a pattern start to revel itself. Here are 4 bursts of data:




The script used to print these pulse trains:

import serial
import sys
import time
serdev = '/dev/ttyACM0'
ser = serial.Serial(serdev, 115200*8)
def decode(bit, duration):
  if bit==1 and duration>=400 and duration<=600: return '-'
  if bit==1 and duration>=900 and duration<=1100: return '--'
  if bit==0 and duration>=300 and duration<=500: return '_'
  if bit==0 and duration>=800 and duration<=1000: return '__'
  return None
while True:
  line = ser.readline().strip()
  pulse = decode(int(line[0]), int(line[2:]))
  if pulse: sys.stdout.write(pulse)
  else: sys.stdout.write('\n')  sys.stdout.flush()
A examination of the pulse trains reveals that short pulses always come in pairs! This suggest that the data is likely to be Manchester encoded, or more likely Differential Manchester encoded. After applying a differential Manchester decoder to the data, we see some very promising patterns:
(100 ms delay)
(43 s delay)





The data is being send it bursts with around 43 seconds between them. In each burst there are two frames, separated by around 100 ms. As can be seen, every second burst have a longer frame than usual, the other bursts just appear to be repeating the same frame twice. The differential Manchester decoder:
lastshort = False
while True:
  line = ser.readline().strip()
  pulse = decode(int(line[0]), int(line[2:]))
  if pulse in ('_', '-'):
    if lastshort:
    lastshort = not lastshort
  elif pulse in ('__', '--'):
    if lastshort:
      print 'ERROR'
      lastshort = False
    lastshort = False
    sys.stdout.write('\n')  sys.stdout.flush()
That's it for now, next time I will try to make some sense of the newly discovered bits...

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