How to reverse the order of bits in VHDL?

[voho]

Hi all,

I have a vector A(15 downto 0)=0000 0000_000F FFFF
how to do in VHDL in order to obtain B(15 downto 0)= FFFF F000_0000 0000

Thank's in advance

 

[master.ro]

Re: complement in vhdl??

Please say again what do you want to do because you do some mistakes.
First you say that the vector has 16 bits and you give to it a 64 bits value.

Second , by the way you wrote there I think that you want to reverse the order of the bits in the vector.

you must write :

B(15)<=A(0);
B(14)<=A(1);
B(13)<=A(2);
B(12)<=A(3);
...................
B(1)<=A(14);
B(0)<=A(15);

 

[voho]

Re: complement in vhdl??

Hi All,

Of course A(63 downto 0) and B(63 downto 0).
Thank's master.ro for your help.

Regards

 

[master.ro]

Re: complement in vhdl??

it's not clear yet what do you want: to reverse the order of the bits as you say earlyer or to calculate the complement?

if you want the complement :

B(63 downto 0) <= not A (63 downto 0);

or

B(63 downto 0) <= not A (63 downto 0) + '1';

 

[Ethelthefrog]

Re: complement in vhdl??

To reverse the order of bits without having to do it long-hand

process(A)
begin
    for i in A'range loop
        B(B'left - i) <= A(i);
    end loop
end process

This should be synthesisable. My use of B'left assumes that A and B are both defined with the same range (63 downto 0 in your case).

 

[Manny_Calavera]

Re: complement in vhdl??

You don't need to loop or anything, this simple statement will do:

B(0 to 63) <= A(63 downto 0);

Cheers.