Python Wiegand 34bits – ignore last 2 bits

Question

I’m trying to read a code from a QR reader with python.
Have used pigpio example from here http://abyz.me.uk/rpi/pigpio/examples.html#Python_wiegand_py

When I scan a QR it returns bits=34 value=12835750879 which is too long, the code should be 10 digit only.
The QR sends 34 bits, and the guy that installed it said I need to ignore the last 2 bits, to take only 0-31, thing is I don’t really understand the code, I come from PHP, which has nothing to do with bits and memory.

This is the part of the code that deals with the reading, I think somewhere in the _cb function.

Can somebody help me understand the code better so I can go into the right direction?

   def __init__(self, pi, gpio_0, gpio_1, callback, bit_timeout=5):

      """
      Instantiate with the pi, gpio for 0 (green wire), the gpio for 1
      (white wire), the callback function, and the bit timeout in
      milliseconds which indicates the end of a code.

      The callback is passed the code length in bits and the value.
      """

      self.pi = pi
      self.gpio_0 = gpio_0
      self.gpio_1 = gpio_1

      self.callback = callback

      self.bit_timeout = bit_timeout

      self.in_code = False

      self.pi.set_mode(gpio_0, pigpio.INPUT)
      self.pi.set_mode(gpio_1, pigpio.INPUT)

      self.pi.set_pull_up_down(gpio_0, pigpio.PUD_UP)
      self.pi.set_pull_up_down(gpio_1, pigpio.PUD_UP)

      self.cb_0 = self.pi.callback(gpio_0, pigpio.FALLING_EDGE, self._cb)
      self.cb_1 = self.pi.callback(gpio_1, pigpio.FALLING_EDGE, self._cb)

   def _cb(self, gpio, level, tick):

      """
      Accumulate bits until both gpios 0 and 1 timeout.
      """

      if level < pigpio.TIMEOUT:

         if self.in_code == False:
            self.bits = 1
            self.num = 0

            self.in_code = True
            self.code_timeout = 0
            self.pi.set_watchdog(self.gpio_0, self.bit_timeout)
            self.pi.set_watchdog(self.gpio_1, self.bit_timeout)
         else:
            self.bits += 1
            self.num = self.num << 1

         if gpio == self.gpio_0:
            self.code_timeout = self.code_timeout & 2 # clear gpio 0 timeout
         else:
            self.code_timeout = self.code_timeout & 1 # clear gpio 1 timeout
            self.num = self.num | 1

      else:

         if self.in_code:

            if gpio == self.gpio_0:
               self.code_timeout = self.code_timeout | 1 # timeout gpio 0
            else:
               self.code_timeout = self.code_timeout | 2 # timeout gpio 1

            if self.code_timeout == 3: # both gpios timed out
               self.pi.set_watchdog(self.gpio_0, 0)
               self.pi.set_watchdog(self.gpio_1, 0)
               self.in_code = False
               self.callback(self.bits, self.num)

Asked By: alex

||

Source

Answer 1

To remove the two last bits, you can convert the integer self.num to a binary string with bin(), trim off two characters, then convert back to integer.

Here is an example:

num = 2**34 - 1 # 17179869183
num_b = bin(num) # '0b1111111111111111111111111111111111'
num = int(num_b[:-2], base=2) # 4294967295

So you want to use int(bin(self.num)[:-2], 2) somewhere in your code where you retrieve the complete QR code.

Answered By: Guimoute

Python Wiegand 34bits – ignore last 2 bits

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