c compound assignment increment only right bits

Question

I am new to C and working with a serial device and I have this line:

short DA, DacData;    
DA=0xFFF;
DacData = ((channel&0x03)<<14)|((serialA&0x03)<<12)|(DA&0x6AA);

How do I increment only the last part of DacData (DA&0x6AA) in my loop? Thanks in advance!

@StoryTeller Will that change the values of the ((channel&0x03)<<14)|((serialA&0x03)<<12) part? — Takuzo Kimura, Jan 26 '17 at 06:47
This question is rather weird! `DA&0x6AA` is equivalent to `DA&0b11010101010`, which is just masking out the zero bits. Are you sure the result of incrementing is what you want? If you really just want to set the last bit, use `DA & 0x6AA | 0x1` so the next guy won't be scratching his head for 10 minutes. Since you mask off the lsb each time, the increment can never carry out of the lsb. — Jim Garrison, Jan 26 '17 at 07:26
@JimGarrison thanks for your response! basically, the 0x6AA part is for outputting a certain voltage from a DAC, and I want to keep increasing the voltage but I didn't really know how. would masking off the lsb the way you said accomplish this? — Takuzo Kimura, Jan 26 '17 at 11:11
Take a value for the rightmost 11 bits, say 257 (`0x101`) and apply the mask `0x6aa`. The result is `0x000`. Is that what you want? — Jim Garrison, Jan 26 '17 at 17:11

score 0 · Answer 1 · answered Jan 26 '17 at 06:40

0

Try this:

DacData = ((channel&0x03)<<14)|((serialA&0x03)<<12) | ( (DA&0x6AA) + 1 );

Take into account that there is wrap around...

answered Jan 26 '17 at 06:40

Israel Unterman

Thank you! Was thinking the same, didnt know if it was the right wayI can check if this works when I get to my lab. Will accept it as an answer if it does! Thanks – Takuzo Kimura Jan 26 '17 at 06:45
@TakuzoKimura - That will affect the other "parts" in case of overflow as well – StoryTeller - Unslander Monica Jan 26 '17 at 06:49
@Spider-main Sorry for the noob question but what do you mean by wrap around? – Takuzo Kimura Jan 26 '17 at 06:49
@StoryTeller I only want to increment the rightmost 12 bits (they control voltage on my DAC), but it will increment from the right, correct? as long as I stop it at (DA&0xFFF) it should be ok? – Takuzo Kimura Jan 26 '17 at 06:51

1 Answers1