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I receive byte data from a RS-232 device with Serial.read();. Now I need to slice this down in bit-pieces:

  • value1: 15bits
  • value2: 7bits
  • value3: 9bits
  • value4: 12bits
  • and so on

Data from the stream (switch them around for better understanding):

  • Byte 1: 00101100 => 00110100 (least significant byte first)
  • Byte 2: 10011011 => 11011001
  • Byte 3: 00110011 => 11001100
  • Byte 4: 11101000 => 00010111
  • Byte 5: 01001100 => 00110010
  • Byte 6: 01100111 => 11100110

Now the least significant is first.

  • value1: 00110100.1101100[1] => 6956
    Need to cut of the last bit of the second byte
  • value2: 1.110011[00] => 103
    Add the last bit of the second byte and cut off the last two bits of the third byte
  • value3: 00.0001011[1] => 416
    Add the last two bits of the third byte and cut off the last bit of the fourth byte
  • value4: 1.00110010.111[00110] => 3737
    Add the last bit of the fourth byte and cut off the last 5 bits of the fifth byte

Now the question: How do I realize this from programming side. I'm lost.

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    If the records are byte aligned you could use a struct with bit-fields and let the compiler do the work. tutorialspoint.com/cprogramming/c_bit_fields.htm May 22, 2016 at 10:11
  • Another alternative is to define a BitStream class that operates on a Stream (Serial). Typical member function would be uint16_t read(uint8_t bits). The class would have to buffer any remaining bits for the next read, etc. drdobbs.com/bitstream-parsing-in-c/184402014 May 22, 2016 at 10:13
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    I'm voting to close this question as off-topic because it's a year old question with no answers.
    – user31481
    Oct 24, 2017 at 14:51
  • @LookAlterno But this one has an answer. It is/was edited into the question. I have now put that as an answer.
    – gre_gor
    Oct 25, 2017 at 13:19

1 Answer 1

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Note: This is an answer that OP originally edited into his question.


I gather the data into an array of bytes data[125].

Then I gather the relevant bytes per value and stack them together:

long var = (data[4]<<8) + (data[3]);

I shift them so the relevant bytes start first

var=var>>3;

Afterwards I mask them with the RIGHT mask

var=var & 0xFF;

For the example in my question:

long var = (data[1]<<8) + (data[0]); // I need two bytes  
var=var>>1; // shift them to the right one bit  
var=var & 0x7FFF; // mask it with "0111 1111 1111 1111"
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  • 1
    Good work gre_gor!
    – sa_leinad
    Oct 25, 2017 at 14:03

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