[SOLVED] how to calculate log2(n) in verilog

UFK · May 10, 2010

Hiii
can someone please help me to calculate log base 2 in Verilog. What i need to do is to simply calculate the log base to of a variable n. Please help

RBB · May 11, 2010

Code:

function integer clogb2;
   input [31:0] value;
   integer 	i;
   begin
      clogb2 = 0;
      for(i = 0; 2**i < value; i = i + 1)
	clogb2 = i + 1;
   end
endfunction

UFK · May 11, 2010

Thanks alot RBB. I hope my code starts behaving well now. Thanks again.

UFK · May 28, 2010

Dear RBB

The function code you gave me is good but i cant call it. Im simply trying to calculate the log base 2 of 26 n save it to a register n_var. Please correct this code for me. Its as follows

module sample11(n_var);

output [15:0]n_var;
n_var = clogb2(26);

function integer clogb2;
input [31:0] value;
integer i;
begin
clogb2 = 0;
for(i = 0; 2**i < value; i = i + 1)
clogb2 = i + 1;
end
endfunction

endmodule

Thanks alot

meher81 · May 28, 2010

Code:

module sample11(v,n_var);

input  [15:0]v;
output [15:0]n_var;

wire [15:0] v;
reg [15:0]n_var;
reg [15:0]value;

always @ (v)
n_var = clogb2(v);



function integer clogb2;
input [31:0] value;
integer i;
begin
clogb2 = 0;
for(i = 0; 2**i < value; i = i + 1)
clogb2 = i + 1;
end
endfunction

endmodule

it is a simple testbench:

Code:

module test();
  
  reg [15:0] v;
  reg i;
  wire [15:0] n_var;
  
  sample11 s1(v,n_var);
 
 initial
  v = 2; 
  
  initial
  #2000 $finish;
   
 always
  begin
      #5 v = v+1;
  end
  
  always @ (v)
  $monitor ("log%d = %d", v, n_var);
  
endmodule

UFK · May 29, 2010

Dear meher81

Thankyou very much for your code. It simulates great. I just have to figure out a thing or two in it. Thanks alot.

Im stuck in another little piece of code now. I wondr if you can help me with it. Its supposed to scan an array for 16 elements and calculate the maximum value. Its as follows:

module maxvalue(b_array);

input [0:15] b_array;
integer i;
reg [0:15] mxm;

initial
begin
mxm = 16'b0;

for (i=0;i<15;i=i+1)
begin
if (b_array > mxm)
mxm = b_array;
end
end

endmodule

I just cant figure out how to write the testbench for it. I gave initial values to the b_array in the tb as follows:

module max11_tb();

// Inputs
reg [0:15] b_array;

// Instantiate the Unit Under Test (UUT)
maxvalue uut (
.b_array(b_array)
);

initial begin
// Initialize Inputs
b_array [0]= 13;
b_array [1]= 1;
b_array [2]= 2;
b_array [3]=14;
b_array [4]=12;
b_array [5]=0;
b_array [6]=-1;
b_array [7]=21;
b_array [8]=12;
b_array [9]=2;
b_array [10]=-4;
b_array [11]=0;
b_array [12]=9;
b_array [13]=13;
b_array [14]=10;
b_array [15]=-2;

// Wait 100 ns for global reset to finish
#100;

// Add stimulus here

end

endmodule

It isnt giving me the largest value as expected.
Please let me know if you can find a solution to it. Thanks alot

meher81 · Jun 14, 2010

Dear UFK
Verilog doesn't allow an I/O port to be a 2-D array.
but if it it is necessary, you could flatten your array into a vector and pass that through the port.

Code:

module maxvalue(
                b_array,
                mxm,
                reset
                );

input  [255:0] b_array;
input reset;
output [15:0] mxm;


reg  [15:0] array [0:15];
integer i;
reg [15:0] mxm;


always @ (b_array)
	begin
   if (reset)
     mxm = 0;

  	{array[15],array[14],array[13],array[12],array[11],array[10],array[9],array[8],array[7],array[6],array[5],array[4],array[3],array[2],array[1],array[0]}=b_array;	
  	
	 for (i=0;i<16;i=i+1)	
		 	if (array[i] > mxm)
		 	mxm = array [i];
	end

endmodule

simple test module

Code:

module max11_tb();

reg  [255:0] b_array;
wire [15:0] max;
reg reset;

maxvalue uut (
			 .b_array(b_array),
			 .mxm (max),
			 .reset (reset)
			 );
			 
initial
  begin
    reset = 1;
   #10 reset = 0;
  end
 

initial
  begin
    #7 b_array = 255'd1;
    #10 b_array = 255'd2;
    #15 b_array = 255'd197879804;
    #20 b_array = 255'd200000000000000000003;
  end

always @ (b_array)
	$monitor  ($time, "maximum value component = %d", max );

endmodule

UFK · Jun 14, 2010

Dear meher81

Thanks alot for your help. You just made my life easier

UFK · Jun 19, 2010

Dear meher81

I am using the code u helped me with to scan values in a bram and then calculate maximum value. I stored 16 values in a bram by a coe file. Now im trying to run the following code by invoking the bram (spiht_mem). Since i already gave the values in the coe file i have no clue what to write in the testbench. Can u please help me? Thankyou in advance.

The code is

module maxvalue( b_array,mxm,reset,addrA,clkA);

input [255:0] b_array;
input reset;
output [15:0] mxm;
input clkA;
input [15:0] addrA;

Spiht_mem M1 (
.clka(clkA),
.wea(), // Bus [0 : 0]
.addra(addrA), // Bus [15 : 0]
.dina(), // Bus [15 : 0]
.douta(array)); // Bus [15 : 0]

reg [15:0] array [0:15];
integer i;
reg [15:0] mxm;

always @ (b_array)
begin
if (reset)
mxm = 0;
//[15:0] array[0:15]= b_array;
{array[15],array[14],array[13],array[12],array[11],array[10],array[9],array[8],array[7],array[6],array[5],array[4],array[3],array[2],array[1],array[0]}=b_array;

for (i=0;i<16;i=i+1)
if (array > mxm)
mxm = array ;
end

endmodule

meher81 · Jul 1, 2010

I don't know what you want.
I guess that you have stored some values in a file (as bram.dat) and want to apply them to your code, which detect maximum value. yes?

UFK · Jul 1, 2010

Hii meher81

Yes that is exactly what i want. I was testing my algorithm to work for 16 values initially but i wanted my code to work for as many values as i want, say for a 128 x128 image which means that if i load 16384 decimal values in my bram as a .coe file it should call the values from the bram and then scan for the maximum decimal value.
Btw if i call values from the bram using (.dout(w1[0:b]) where w1 is a wire do u think it will work?

Part of my code is like this

reg [31:0] LIS5a [1023:0];
.
.
.
FiveA M1 (.douta(LIS5a));

here FiveA is my bram in which i have loaded a .coe file of 1024 decimal values. I want to call those values so i tried moving them directly in to the reg LIS5a instead of using wire w1. Any suggestions or comments?

Thanks alot for all the help

meher81 · Jul 1, 2010

Dear UFK
you can do this in a testbench. at first, store file content in an array using $readmemb. after that you can do, with the array elements, what you want.

UFK · Jul 9, 2010

meher81 said:
Dear UFK
you can do this in a testbench. at first, store file content in an array using $readmemb. after that you can do, with the array elements, what you want.

Dear meher81

I did what u suggested and everything worked brilliantly, until i realized that $readmemh will not take negative hex values from my text file. Im back at square one now !

((( Do u have a solution to this problem?
Previously i tried using a bram, but i couldnt figure out how to move its .coe file contents in to my array.
Please help me.
And thanks alot for everything

meher81 · Jul 9, 2010

Dear UFK

you should change your negative numbers to 2's complement format. (perhaps with MATLAB or manually!)

UFK · Jul 9, 2010

Hii meher81

I did change it to negative format (e.g -1 to FFFFFFFF) and it still isnt quite working. Please do me this big favour. Im posting my code, test bench and txt file. Can u plz run it and see the results? I know ull know wat im doing wrong.

module max_val(mxm,
reset
);
input reset;
output [31:0] mxm;
reg [31:0] Mem [0:3];

integer i;
reg [31:0] mxm;

initial $readmemh("D15.txt",Mem);

always @ *
begin
if (reset)
mxm = 0;

for (i=0;i<=3;i=i+1)
if (Mem > mxm)
mxm = Mem ;
end
endmodule

Testbench

module max_val_tb;

// Inputs
reg reset;

// Outputs
wire [31:0] mxm;

// Instantiate the Unit Under Test (UUT)
max_val uut (
.mxm(mxm),
.reset(reset)
);

initial begin
// Initialize Inputs
reset = 0;

#100;

reset = 1;

end

endmodule

Click to expand...

and the text file

//D15 =

29B
043
FFFFFE72
FFFFFEE7

//667
//67
//-398
//-281

Click to expand...

wat i figured is that if my text file has the following numbers in hex form i.e -10,-11,-12,10,11,12, the code simply ignores the positive values and computes the maximum of the negative values, for instance in this case it will return -10 which is the max.

Maybe ud know a solution.
And thanks a million for always coming to my rescue

meher81 · Jul 10, 2010

Code:

module max_val (
				mxm,
				in,
				reset
				);
input reset;
input  [ 31 : 0 ] in;

output [ 31 : 0 ] mxm;

wire   [ 31 : 0 ] in;

integer k;
integer i;
integer  mxm;

always @ (in or reset)
	begin
		if (reset)
			mxm = 0;
		else 
			k=in;
		if (k>mxm)
			mxm <= k;
	end

endmodule

Testbench:

Code:

module max_val_tb;

wire [31:0] mxm;

reg reset,clk;
reg [31:0] Mem [0:3];

integer i;
integer input_data;

max_val uut (
			.mxm(mxm),
			.in(input_data),
			.reset(reset)
			);

initial $readmemh("D15.txt",Mem);

initial 
	begin
		reset = 1;
		#2 reset = 0;
	end

initial
	clk =0;

always
	#5 clk=~clk;

initial 
	i=0;

always @ (posedge clk)
	begin
		input_data = Mem[i];
		i = i+1;
		$monitor ("max = %d, input=%d", mxm, input_data);
	end

endmodule

Added after 24 minutes:

I did that same as you, but in this way, after the reset, mxm is set to zero. therefore this code neglect negative numbers. to solve that, you can do same as below:

Code:

module max_val (
				mxm,
				in,
				reset
				);
input reset;
input  [ 31 : 0 ] in;

output [ 31 : 0 ] mxm;

wire   [ 31 : 0 ] in;

integer k;
integer i;
integer  mxm;

always @ (in or reset)
	begin
		if (reset)
			mxm = 32'h80000001;
		else 
			k=in;
		if (k>mxm)
			mxm <= k;
	end

endmodule

Testbench:

Code:

module max_val_tb;

wire [31:0] mxm;

reg reset,clk;
reg [31:0] Mem [0:3];

integer i,m;
integer input_data;

max_val uut (
			.mxm(mxm),
			.in(input_data),
			.reset(reset)
			);

initial $readmemh("D15.txt",Mem);

initial 
	begin
		reset = 1;
		#2 reset = 0;
	end

initial
	clk =0;

always
	#5 clk=~clk;

initial 
	i=0;

always @ (posedge clk)
	begin
		input_data = Mem[i];
		i = i+1;
		m=mxm;
		$monitor ("max = %d, input=%d", m, input_data);
	end

endmodule

UFK · Jul 14, 2010

Dear meher

Maybe you could solve another one of my neverending problems.
Im posting my code below. If possible, Please run it and see if it can be altered to function the way i want. My code isnt reading negative hex values. It only gives correct results for positive hex values. Can anything be done about it?

1. I have two arrays LIP and LSP. readmem will put memory values into LIP.

2. The code is supposed to read the four values from LIP and compare with the parameter T.

3. If greater than T, then move values to LSP.

4. If less than T, then leave it in LIP

5. The bit counters seem to function fine.

My code is

module test();

reg [31:0] Signbit_ctr;
reg [31:0] Bit_ctr0;
reg [31:0] Bit_ctr1;

reg[31:0] T=32'h00001000; //decimal 4096

integer i;
integer k;

reg [31:0] LIP [0:3];
reg [31:0] LSP [0:1023];

initial $readmemh("D16.txt",LIP);

initial
begin

Signbit_ctr = 0;
Bit_ctr0 = 0;
Bit_ctr1 = 0;

end

always @ (LIP)
begin
for(i=0;i<=3;i=i+1)

if (LIP >= T)

begin

LSP = LIP;

Bit_ctr1 = Bit_ctr1 + 1;
Signbit_ctr = Signbit_ctr + 1;

end
else
begin

Bit_ctr0 = Bit_ctr0 + 1;
$display("%d:%h",i,LSP);

end

end
endmodule

and the text file D16 is

12F2
FFFFFD3E
FFFFFF78
FF

Thankyou so much

UFK · Nov 1, 2010

Dear meher81
I need you help.
Im posting a code which u wrote for me a few months ago. Its a part of my encoder. It just wont synthesize. Maybe you can help.

The code is

module Log_max(v,n_var,T);

input [31:0]v;
output [31:0]n_var;
output reg [31:0] T=0;

wire [31:0] v;
reg [31:0]n_var;
reg [3:0] base = 2;

always @ (v)
n_var = CLogB2(v);

always@(*)
begin
T = base ** n_var;
end

//ceil of the log base 2
function integer CLogB2;
input [31:0] Depth;
integer i;
begin
i = Depth;
for(CLogB2 = 0; i > 1; CLogB2 = CLogB2 + 1)
i = i >> 1;
end
endfunction
endmodule

Testbench

module Log_max_tb;

// Inputs
reg [31:0] v;

// Outputs
wire [31:0] n_var;
wire [31:0] T;

// Instantiate the Unit Under Test (UUT)
Log_max uut (
.v(v),
.n_var(n_var),
.T(T)
);

initial begin
// Initialize Inputs
v = 32'h00000B43;

end

endmodule

It gives correct result but cannot synthesize. Please help
Thankyou

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[SOLVED] how to calculate log2(n) in verilog

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