Verilog: How to Take the Absolute Value
In Verilog I Have an Array of Binary Values. How Do I Take the Absolute Value of the Subtracted Values? Verilog Code: Module Aaa(Clk); Input Clk; Reg [7:0] a...
In verilog I have an array of binary values. How do I take the absolute value of the subtracted values ?
Verilog code:
module aaa(clk);
input clk;
reg [7:0] a [1:9];
reg [7:0] s [1:9];
always@(posedge clk)
begin
s[1] = a[1] - a[2];
s[2] = a[2] - a[3];
s[3] = a[1] + a[3];
end
endmodule
I want my s[1] and s[2] values to be always positive. How can I do it in synthesisable verilog?
I have tried using signed reg, but it shows an error.
3 Answers
Regardless of whether the number is signed or not twos complement is still used which correctly performs addition and subtraction at the bit level.
If a number is to be interpreted as signed the MSB can be used to tell if it is positive (0) or negative (1)
To absolute the number just invert based on the MSB:
reg [31:0] ans ; // Something else drives this value
reg [31:0] abs_ans; // Absolute version of ans
// invert (absolute value)
always @* begin
if (ans[31] == 1'b1) begin
abs_ans = -ans;
end
else begin
abs_ans = ans;
end
end
NB: using = because it is a combinatorial block, if using a flip-flop (edge trigger) use <= as @TzachiNoy has mentioned.
This should do the work:
s[1] <= (a[1]>a[2])?(a[1]-a[2]):(a[2]-a[1]);
Note: you should always use '<=' in clocked always blocks.
Just following the answer from @Morgan, and because I already had a module in my system that performed this operation, here is my contribution:
module Mod(
input signed [11:0] i,
output signed [11:0] o
);
assign o = i[11] ? -i : i; // This does all the magic
endmodule
And here is a testbench:
module tb;
reg signed [11:0] i;
wire signed [11:0] o;
Mod M(i,o);
integer t;
initial begin
for (t = -10; t < 10; t = t + 1) begin
#1
i <= t;
$display("i = %d, o = %d", i, o);
end
end
endmodule
The output is:
i = x, o = x
i = -10, o = 10
i = -9, o = 9
i = -8, o = 8
i = -7, o = 7
i = -6, o = 6
i = -5, o = 5
i = -4, o = 4
i = -3, o = 3
i = -2, o = 2
i = -1, o = 1
i = 0, o = 0
i = 1, o = 1
i = 2, o = 2
i = 3, o = 3
i = 4, o = 4
i = 5, o = 5
i = 6, o = 6
i = 7, o = 7
i = 8, o = 8
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