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hi evryone, is there any best way to check if the difference between two integers is greater than a threshold i find the code witten by me is quite big..need ur inputs thanks
if((unsigned(a)>unsigned(b))and((unsigned(a)unsigned(b))>=500)) or ((unsigned(a)<unsigned(b)) and ((unsigned(b)unsigned(a))>=500))then my code end if; 
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Absolute value of difference of absolute integer values? Depending on your compiler it may recognize the pattern abs(abs(a)abs(b)) and do some sign bit optimization. If you need to be certain of clock cycles: do it in assembler, otherwise trust the compiler.
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It seems as if you could simply subtract b from a and then somehow ignore the possible negative sign if the first value was smaller than the second. Check only if the difference is bigger than 500.
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nick wrote: > i find the code witten by me is quite big.. What means "big" here? > check if the difference between two integers Are those two integers unsigned integers (aka naturals)? nick wrote: > if((unsigned(a)>unsigned(b))and((unsigned(a)unsigned(b))>=500)) or > ((unsigned(a)<unsigned(b)) and ((unsigned(b)unsigned(a))>=500))then > my code > end if; With integers simply this:
if abs(ab)>=500 then ... end if; 
Boris O. wrote: > If you need to be certain of clock cycles: do it in assembler We are in real hardware here. It is done in one clock cycle... ;) HG S. wrote: > and then somehow ignore the possible negative sign Have a closer look at the chapter "twos complement and integers". There is no such thing like a "negative sign" on a integer. All of the bits are "signs". So you cannot "cut away" or "simply ignore" the sign. Look at this "8bitinteger": 2 = 00000010 1 = 00000001 0 = 00000000 1 = 11111111 2 = 11111110 See it: you don't see a "negative sign"
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Edited by Moderator
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As i read your code again now it seems it lacks something. Does it work the way it is now ? To me it seems you have to implement more code for this task, especially for the case that the subtraction causes a rollover. Simple asking if the first value is bigger than the subtracted one does not always mean that the subtraction of 500 does not cause a rollover. But it may be that the higher language compiler has a solution for this problem and counts that all in (the "bigger than"routine for example could automaticalle handle rollovers).
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Edited by User
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Lothar M. wrote: > Are those two integers unsigned integers (aka naturals)? they are declared as standard logic vectors.
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nick wrote: > they are declared as standard logic vectors. Then why do you write somewhat about "integers"? An integer is a predefined data type in VHDL. Never ever call a std_logic_vector an integer! Ok so you must use the numeric_std.all and write it that way:
if abs(unsigned(a)unsigned(b))>=500 then ... end if; 
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Edited by Moderator
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Lothar M. wrote: > if abs(unsigned(a)unsigned(b))>=500 then > ... > end if; but abs works only with signed operand so.. if abs(signed(a)signed(b))>=500 then ... end if;