\\ Design Block
module full_adder(A,B,C_in,S,C_out);
input A,B,C_in;
output S,C_out;
wire f1,f2,f3;
xor(S,A,B,C_in);
and(f1,A,B);
and(f2,B,C_in);
and(f3,A,C_in);
or(C_out,f1,f2,f3);
endmodule
\\Stimulus Block
`timescale 1ns/1ns
module full_adder_tb;
reg clk;
reg [2:0] count;
wire S,C;
initial
begin
clk=1;
count=3'b111;
end
always #5 clk=~clk;
always @(posedge clk)
count=count+1;
full_adder MUT(count[2],count[1],count[0],S,C);
endmodule
Don't confuse RTL schematic with implementation.3- Why Vivado didn't generate 1 3-input XOR gate, but instead generated 2 2-input XOR gate? isn't it practical?
1.) for FPGA's, prefer using just A + B. Also prefer using named connections vs positional connections for instances.
2.) the second number is the precision that you can use for delays, etc...
3.) for the elaboration/synthesis, it doesn't really matter. After implementation it will be a single LUT (or something special, see below). A single LUT6 can replace several gates in a design.
In this case, there might be a difference between "+" and creating the full adders. FPGA's now have "Fast-Carry Chains", which are special pieces of logic in addition to the LUTs. The dedicated routing for the carry chains allow much higher performance than using general-purpose routing.
assign {C_out,Sum}=A+B+C_in;
`timescale 1ns/1ps;
Don't confuse RTL schematic with implementation.
it is concatenation of C_out with Sum. C_out is on the left because it is the carry out of the add operations and therefore resides in the MSB position (in this case bit-1 of [1:0] result bits)Code:assign {C_out,Sum}=A+B+C_in;
Unfortunately the built-in gate primitives don't have port names, they are defined by their ordering...About the seconf part of your reply, what did you mean by: "Also prefer using named connections vs positional connections for instances"? Could you elaborate more? As I stated elsewhere, I'm still new to this field, and I'm not familiar with everything, yet.
and(f1,A,B);
my_and(.O(f1), .I1(A), .I2(B));
my_and(.I2(B), .O(f1), .I1(A));
Look at this page2- Could you elaborate more on this point, too? I know for example that # is used to specify the number of time units, which is 1 ns in my example, but how to use the other 1 ns? Also, is it better to increase the precision as
Code:`timescale 1ns/1ps;
because it's probably easier to deal with 2-input XOR than a multi-bit XOR gate as all you need to do in parsing is pick up each new input bit until you run out of inputs. I imagine there are probably no gates that have more than 2-input versions in the library as you can daisy chain the gates to make the multi-input gate.3- I get that, but I was wondering since Verilog allows xor(Out,in1,in2,...), which is to say, N-input XOR gate for n>=2, why Vivado didn't follow the same rule?
the schematic is not the output of a synthesis tool, the synthesis tool generates a bunch of logic equations that are reduced using boolean transformations which are then mapped into the technology library. For a Xilinx design the technology schematic will no longer look anything like gates, but will be things like LUTs and FF primitives.To my understanding, logic gates are the result of synthesis of a synthesizable RTL code, which is used (I mean the schematic) for implementation. So, you map your schematic to implementation. Am I right?
When I used the + operation I got the attached schematic diagram. I guess in this case Vivado treats a full adder as a cascade of two half adders, where half adders are considered as black boxes.
Actually, I'm using Vivado just out of curiosity. I use ModelSim for simulation and functional verification, but sometimes I use Vivado for simulation as well and look at the schematic diagrams it produces.
Actually, I'm using Vivado just out of curiosity. I use ModelSim for simulation and functional verification, but sometimes I use Vivado for simulation as well and look at the schematic diagrams it produces.
As FvM has mentioned earlier, for the time, keep simulation + functional verification separate from synthesis.
ModelSim is used for simulation/functional-verification (I am not sure if it can produce you a schematic; I don't use it).
As for Vivado, it is a complete suite. It contains simulation + synthesis + PnR engines, which helps you to translate a design from RTL to bit-stream. Many engineers use a separate simulation s/w (ModelSim/VCS), separate synth s/w (Synopsys Synplify) and just do the final PnR with Vivado. It depends on the amount of $ you have at your disposal.
Could you please tell me a little more about Synonsys? Is it used only for synthesis?
`timescale 1ns / 1ps
//////////////////////////////////////////////////////////////////////////////////
// SingleStage.v
//
//////////////////////////////////////////////////////////////////////////////////
module singleStage (
input a,
input b,
input cin,
output s,
output cout );
assign {cout,s} = a + b + cin;
endmodule
`timescale 1ns / 1ps
////////////////////////////////////////////////////////////////////////////////
// Module Name: Y:/Desktop/Xilinx Stuff/Projects/NBitAdder/test.v
//
////////////////////////////////////////////////////////////////////////////////
module test;
// Inputs
reg a;
reg b;
reg cin;
// Outputs
wire s;
wire cout;
integer i;
// Instantiate the Unit Under Test (UUT)
SingleStage uut (
.a(a),
.b(b),
.cin(cin),
.s(s),
.cout(cout)
);
initial begin
// Initialize Inputs
a = 0;
b = 0;
cin = 0;
end
always @ ( a, b, cin )
begin
// generate truth table
for ( i = 0; i < 8; i = i + 1 )
// every 10 ns set a, b, and cin to the binary rep. of i
#10 {a, b, cin} = i;
// stop 10ns after last change of inputs
#10 $stop;
end
endmodule
no, it just changes the values of a, b, cin -- the inputs to the single instance of "singleStage".
SingleStage uut (
a,
b,
c,
s,
cout
);
The first thing that caught my attention is that the instantiation happens before the final always block, as if any change in a, b, and cin in the final always block makes a new instantiation automatically.
An additional comment to your previous question.
The order of continuous assignments, module instantiations and procedural blocks in a Verilog design is irrelevant. Statement order only matters inside a procedural block (e.g. always block) or subroutine.
You are still missing basic understanding how Verilog works, should have a text book or tutorial at hand when browsing arbitrary designs.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?