Velocity Reviews > problem with bcd and a number

problem with bcd and a number

nephish
Guest
Posts: n/a

 08-04-2011
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.

MRAB
Guest
Posts: n/a

 08-04-2011
On 04/08/2011 19:26, 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.

The value is MSB * 100 + (LSB >> 4) * 10 + (LSB & 0xF)

Ethan Furman
Guest
Posts: n/a

 08-04-2011
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
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

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~

Christoph Hansen
Guest
Posts: n/a

 08-04-2011
MRAB schrieb:

> The value is MSB * 100 + (LSB>> 4) * 10 + (LSB& 0xF)

i would say

(MSB >> 4)*100 + (MSB & 0xF)*10 + (LSB >> 4)

but who knows

Ethan Furman
Guest
Posts: n/a

 08-04-2011
MRAB wrote:
> On 04/08/2011 19:26, 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.

>
> The value is MSB * 100 + (LSB >> 4) * 10 + (LSB & 0xF)

Not according to the docs -- msb has two digits, lsb has one and garbage.

~Ethan~

Peter Pearson
Guest
Posts: n/a

 08-05-2011
On Thu, 04 Aug 2011 21:52:45 +0200, Christoph Hansen <(E-Mail Removed)> wrote:
> MRAB schrieb:
>
>> The value is MSB * 100 + (LSB>> 4) * 10 + (LSB& 0xF)

>
> i would say
>
> (MSB >> 4)*100 + (MSB & 0xF)*10 + (LSB >> 4)
>
> but who knows

I concur. I think the documentation is trying to say that the
low-order nibble of the LSB is garbage.

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