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Date: Thu, 04 Aug 2011 12:44:41 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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To: nephish <nephish@gmail.com>
Subject: Re: problem with bcd and a number
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nephish wrote:
> Hey all,
> 
> I have been trying to get my head around how to do something, but i am
> missing how to pull it off.
> I am reading a packet from a radio over a serial port.
> 
> i have " two bytes containing the value i need.  The first byte is the
> LSB, second is MSB.  Both bytes are BCD-encoded, with the LSB
> containing digits zX and MSB containing digits xy.  The system speed
> is then xyz%, where 100% means maximum speed and would be given by
> bytes 00(LSB) 10(MSB)."
> 
> that is a quote from the documentation.
> Anyway, i am able to parse out the two bytes i need, but don't know
> where to go from there.
> 
> thanks for any tips on this.

As I recall, when you cut a byte in half you get two nibbles.  You need 
to translate your nibbles into numbers, then multiply and add -- 
something like this:

8<--- bcd.py ----------------------------------------------------------
def bcd_to_int(msb, lsb):
     """msb has two decimal digits, lsb has one
     e.g. msb has xy=10, lsb has zX = 00, actual number (xyz)
     is 100"""
     ones = lsb >> 4
     tens = msb & 0x0f
     hundreds = msb >> 4
     if ones > 9 or tens > 9 or hundreds > 9:
         raise ValueError(
               "invalid BCD digits in %02x %02x" % (msb, lsb)
               )
     tens *= 10
     hundreds *= 100
     answer = hundreds + tens + ones
     return answer

if __name__ == '__main__':
     import unittest

     class Test_BCD(unittest.TestCase):
         def test_valid(self):
             msb = 0x09
             lsb = 0x34  # 4 will be discarded as garbage
             self.assertEqual(bcd_to_int(msb, lsb), 93)
         def test_invalid(self):
             msb = 0x1a
             lsb = 0x10
             self.assertRaises(ValueError, bcd_to_int, msb, lsb)

     unittest.main()
8<---------------------------------------------------------------------

Hope this helps.

~Ethan~