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John Morley
Joined: 09 Aug 2004
Posts: 97
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| Assembling a 'word' from individual bytes w/o MAKE8/16/32 |
 Posted: Wed Aug 25, 2010 8:52 am |
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Hi All,
I'd like to assemble a 32 bit command 'word' out of 3 individual 16 bit long integers each having a max value of 1023 (10 bits). The 10 bits in the three long integers are used to build the 32 bit command word (30 bits + 2 extra). I think I can do this with bit shifting and bit-wise ANDing, but I can't seem to make it work correctly.
| Code: |
int16 iRedValue;
int16 iGreenValue;
int16 iBlueValue;
int32 iColorCommandWord;
iRedValue = 1023;
iGreenValue = 1023;
iBlueValue = 1023;
iColorCommandWord = ((iRedValue<<6) & (iGreenValue<<6) & (iBlueValue<<6));
fprintf(USB, "%Lu\n\r", iColorCommandWord);
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I know that the above code does not work (and I understand why), I just thought this code clearly illustrates what I'm trying to do. In this example, I want to end up with a 32 bit word that is all '1's' except for the two least significant bits that are both '0's'.
Thanks,
John
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John Morley |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19499
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| Posted: Wed Aug 25, 2010 9:25 am |
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You have 10bit values.
You appear to want to put these into a 32bit word, as:
xxxxxxxxxxyyyyyyyyyyzzzzzzzzzz00
If so, you need to be shifting the first value, by 22bits, not 6, then the second by 12bits, and the third by 2 bits. So you would need:
| Code: |
int16 iRedValue;
int16 iGreenValue;
int16 iBlueValue;
int32 iColorCommandWord;
iRedValue = 1023;
iGreenValue = 1023;
iBlueValue = 1023;
iColorCommandWord = (((int32)iRedValue<<22) & ((int32)iGreenValue<<12) & (iBlueValue<<2));
fprintf(USB, "%Lu\n\r", iColorCommandWord);
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Remember the values are being shifted, before they go into the result, so need to be converted to int32 _first_, otherwise they won't fit (doesn't apply to the last value).
The alternative, is a shift and combine approach:
| Code: |
int16 iRedValue;
int16 iGreenValue;
int16 iBlueValue;
int32 iColorCommandWord;
iRedValue = 1023;
iGreenValue = 1023;
iBlueValue = 1023;
iColorCommandWord = iRedValue;
iColorCommandWord<<=10;
iColorCommandWord &=iGreenValue;
iColorCommandWord<<=10;
iColorCommandWord&=iBlueValue;
iColorCommandWord<<=2;
fprintf(USB, "%Lu\n\r", iColorCommandWord);
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This will actually take less cycles to complete, since there are only a total of 22bits of rotation, rather then 22+12+2.
Best Wishes |
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John Morley
Joined: 09 Aug 2004
Posts: 97
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| Posted: Wed Aug 25, 2010 9:34 am |
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Hi Ttelmah,
Ah, of course! Many thanks for the help!!
John
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John Morley |
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Wayne_
Joined: 10 Oct 2007
Posts: 681
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| Posted: Wed Aug 25, 2010 9:56 am |
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| You need to be oring | the values, not anding & |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19499
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| Posted: Wed Aug 25, 2010 2:30 pm |
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Aargh.
Slap me, I just copied what had been used, and missed this - naughty.....
Best Wishes |
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