calculate 64 bit unsigned

Altera_Forum · ‎07-11-2014

hi,

i've implemented a nco megafunction with a 64 bit phase accumulator in order to obtain very high precision output frequencies.

Now to be able to program externally (by microcontroller for example) the phase increment value and also to make my nco fully programmable, i need to write vhdl code for this function:

Phase_Inc = Fout*2^N/F_clk.

The parameters for the fuction should be Fout, F_clk and N (number of bits). The output should be a std_logic_vector(63 downto 0) in my case.

Could you help me?

Thanks

Altera_Forum · ‎07-11-2014

--- Quote Start ---

hi,

i've implemented a nco megafunction with a 64 bit phase accumulator in order to obtain very high precision output frequencies.

Now to be able to program externally (by microcontroller for example) the phase increment value and also to make my nco fully programmable, i need to write vhdl code for this function:

Phase_Inc = Fout*2^N/F_clk.

The parameters for the fuction should be Fout, F_clk and N (number of bits). The output should be a std_logic_vector(63 downto 0) in my case.

Could you help me?

Thanks

--- Quote End ---

try this. you need numeric_std library


function get_phase_inc(Fout,clk,N : natural) return std_logic_vector is 
variable phase : integer;
begin
  phase := Fout*2**N/clk;
  return std_logic_vector(to_unsigned(phase,N));
end function;
signal test : std_logic_vector(7 downto 0);
...
test <= get_phase_inc(10,100,8);

Altera_Forum · ‎07-11-2014

the problem arises when N is 64 as a integer is not enough to represent it.

How could i resolve?

Altera_Forum · ‎07-11-2014

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the problem arises when N is 64 as a integer is not enough to represent it.

How could i resolve?

--- Quote End ---

Indeed..

try this since Fo*2^64/clk = Fo*2^32/clk * 2*32/clk.


function phase_inc(Fout,clk, N : natural) return std_logic_vector is 
variable phase : integer;
variable temp1,temp2:unsigned(N/2-1 downto 0);
begin
  phase := Fout*(2**N/2)/clk;
  temp1 := to_unsigned(phase,32);
  phase := (2**N/2)/clk;
  temp2 := to_unsigned(phase,32);
  return std_logic_vector(temp1*temp2);
end function;

Altera_Forum · ‎07-11-2014

actually you need to go lower than 2^32 as temp1,temp2 exceed 2^32, try 2^16

Altera_Forum · ‎07-11-2014

I finally see no way to get value without losing resolution as there is no support for 64 bits range

anyway 64 bits phase accum is too drastic, 24 bits is ok generally

Altera_Forum · ‎07-11-2014

looks like this will work (not a function)


constant fo:unsigned(31 downto 0) := x"11110000";
constant fclk : unsigned(31 downto 0) := x"ffff0000";
constant temp: unsigned(95 downto 0) := fo & x"0000000000000000";
constant phase : unsigned(95 downto 0) := temp/fclk;

then choose 64 bits of phase

Altera_Forum · ‎07-12-2014

ok I'll try this way then I'll let you know...

i don't understand the constant temp....

So if my numbers is:

Fo=4050Hz

Fclk=250Khz

N=64

how could i put them in your code?

Thanks

Altera_Forum · ‎07-12-2014

--- Quote Start ---

ok I'll try this way then I'll let you know...

i don't understand the constant temp....

So if my numbers is:

Fo=4050Hz

Fclk=250Khz

N=64

how could i put them in your code?

Thanks

--- Quote End ---

just insert the binary value(in hex) of 4050 for Fo and 250000 for Fclk. Keep everything else same (2**64 is implied by the zeros on temp)

Altera_Forum · ‎07-12-2014

It works thanks