module exmaple(input a, b, input in[2:0], output d, e, output out[5:0])
I am new to Verilog and trying to understand what input in[2:0] means?
That isn't valid Verilog(IEEE-1364), it is SystemVerilog(IEEE-1800). SV allows ports to be declared as multi-dimensional arrays so in this case in is declared as an array of single bit wires.
Generally if you wanted a vector for a port you would use input [2:0] in which is valid in both Verilog and SystemVerilog. However if your port type cannot be a vector, such as integer or time then you would need to use this method.
Related
I have the following code and I get an error. I am trying to use only one bit out of the four lines in the inputs A, B, and the output, Y. To be clear, I do not want to use the AND operator, I want to use the AND module that I made.
AND_Gate_Quad_SN74AHCT08N and0(.A({3'bzzz,clk_disable}), .B({3'bzzz,clk}), .Y({3'bzzz,clk_buffer}));
What would be a good way to accomplish this?
An output port cannot be connected to numeric literal constant.
There at least three options I can think of
Do a simple connection and live with a port size mismatch warning .y(clk_buffer)
Declare as a 4-bit wire [3:0] clk_buffer and only use clk_buffer[0].
Create a 3-bit dummy_wire and use the connection .y({dummy_wire,clk_buffer}).
Hello I’m new to Verilog and I’ve been confused over this. In some examples the inputs and outputs are declared as wires or regs and on others they are just listed like this:
Module(input a, b, output c)
What is the default data type given to these values?
The default data type for a port in any direction is an unsigned 4-state scalar (1 binary digit). In Verilog, input and inout ports must be and default to the net type wire. output ports also default to wire but can also be specified as a variable reg. (SystemVerilog relaxes some of these rules by allowing input ports to also be declared as variables).
Now that you know what the implicit defaults are, please be a good engineer and never rely on them—be explicit.
This question already has answers here:
What is the difference between reg and wire in a verilog module?
(3 answers)
Closed 2 years ago.
I have a huge confusion in understanding the Reg Data type in verilog. I know that we use reg variable when we need to store a value(s) in this variable. But for example lets consider that we have 4x1 MUX , I see some codes creates the input as
input a
input b
input c
input d
And for the output as
output reg z
So my question why did we use reg data type for the output only ? Why we didn't used it in the input variables ?
Also can I use a shorthand notation for the declaration of these inputs as
input a[4]
Is this right ?
The default type is wire, so input a actually means input wire a. (And you can change the default with default_nettype.)
a[4] isn't really shorthand as the names will be a[0]/a[1]/a[2]/a[3] instead of a/b/c/d, but it will result in the same number of individual nets. Declaring an unpacked type like a[4] may require SystemVerilog, whereas you can use a packed type like [3:0] a for Verilog ports.
Module input ports are essentially continuous assignments from the signal you connect the port to in the module instance, to the port signal in the declaration. You should never make any kind of assignments to the input port from inside the module. Verilog requires any signal that has a continuous assignment to it be declared as a wire.
The output port has the continuous assignment going the other direction, so signal connected in the instance has to be a wire, but inside, the port signal could be a wire or reg, depending on how you want to make an assignment to the port signal.
See this link for more details on wire versus reg.
I need help with a Verilog design I'm doing.
the idea is to do multiplication through repeated addition every time the M bit is set to 1/true. then I need to output that value. The assignment statement F=P; is throwing the error.
This is the error I'm getting
Error (10044): Verilog HDL error at Design2.v(13): expression cannot reference entire array "P"
It is my understanding that I can assign a register to an output if they are the same size. If I'm wrong then how can I implement this?
module Design2(A, N, M, F);
input A[7:0];
input N[3:0];
input M;
reg P[15:0];
output F[15:0];
always #(M) begin
repeat(N) begin
P = P + A;
end
F=P;
end
endmodule
Keeping apart the logic side, I can see two issues with your design.
As the comments pointed out, you have used unpacked arrays at wrong place.
Declaration of output port F.
For the first issue, the design needs a 16-bit vector for all the ports and variables. Here, the design takes a bunch of 8-bits as input A which must be declared as input [7:0] A, which is a packed array. Also, P needs to be a vector of size 16-bit in order to have contiguous operations.
This is to be done so that, when the statement P=P+A executes, the addition operation is done with all the variables taken with their respective sizes (size if P=16 and A=8). The overall addition is to be done with 16-bits, padding zeros in MSB side of A.
Hence, convert all the variables to packed array as follows:
input [7:0] A;
input [3:0] N;
input M;
reg [15:0] P;
output reg [15:0] F;
For the second issue, the design assigns output in a procedural always block.
wire elements must be continuously driven by something, and cannot store a value. Henceforth, they are assigned values using continuous assignment statements.
reg can be used to create registers in procedural blocks. Thus, it can store some value.
When an input port is connected, it has to be driven through continuous assignments from parent module, hence input ports are always wire. While output from a module can be driven continuously through a wire, or can be driven through procedural assignments through reg. Following image shows the port connection rules.
To accomplish this in current design, the output port must be declared as output reg [15:0] F. This is the cause of compilation error shown.
Either way, you can have output [15:0] F with no reg and make continuous assignment to F as follows. This will synthesize F to wire:
output [15:0] F;
assign F=P;
SystemVerilog adds a logic datatype to remove confusion between usage of wire and reg declaration. A logic can be driven by both continuous assignment or blocking/non blocking assignment.
// Either continuous assignment
output logic [15:0] F;
assign F=P;
// Or procedural assignment
output logic [15:0] F;
// Inside always block
F=P;
For more information on packed and unpacked array, refer SystemVerilog Arrays link. Regarding port declarations, refer Wire and Net pdf. Refer SystemVerilog IEEE 1800-2012 for more information on logic datatype.
When are we supposed to use reg and when are we supposed to use wire in a verilog module?
I have also noticed sometimes that a output is declared again as a reg. E.g reg Q in a D flip flop. I have read this somewhere - "The target output of procedural assignment statements must be of reg data type."
What are procedural assignment statements?
I have thoroughly googled this but was not able to find a clear explanation.
Wire:-
Wires are used for connecting different elements. They can be treated as physical wires. They can be read or assigned. No values get stored in them. They need to be driven by either continuous assign statement or from a port of a module.
Reg:-
Contrary to their name, regs don't necessarily correspond to physical registers. They represent data storage elements in Verilog/SystemVerilog. They retain their value till next value is assigned to them (not through assign statement). They can be synthesized to FF, latch or combinatorial circuit. (They might not be synthesizable !!!)
Wires and Regs are present from Verilog timeframe. SystemVerilog added a new data type called logic to them. So the next question is what is this logic data type and how it is different from our good old wire/reg.
Logic:-
As we have seen, reg data type is bit mis-leading in Verilog. System Verilog's logic data type addition is to remove the above confusion. The idea behind is having a new data type called logic which at least doesn't give an impression that it is hardware synthesizable. Logic data type doesn't permit multiple drivers. It has a last assignment wins behavior in case of multiple assignments (which implies it has no hardware equivalence). Reg/Wire data types give X if multiple drivers try to drive them with different values. Logic data type simply assigns the last assignment value. The next difference between reg/wire and logic is that logic can be both driven by assign block, output of a port and inside a procedural block like this
logic a;
assign a = b ^ c; // wire style
always (c or d) a = c + d; // reg style
MyModule module(.out(a), .in(xyz)); // wire style
Procedural blocks refers to always, always_ff, always_comb, always_latch, initial etc. blocks. While procedural assignment statements refers to assigning values to reg, integer etc., but not wires(nets).
wire elements must be continuously driven by something, and cannot store a value. Henceforth, they are assigned values using continuous assignment statements.
reg can be used to create registers in procedural blocks. Thus, it can store some value.
reg elements can be used as output within an actual module declaration. But,reg elements cannot be connected to the output port of a module instantiation.
Thus, a reg can drive a wire as RHS of an assign statement. On the other way round, a wire can drive a reg in as RHS of a procedural block.
For clear idea about declaration of reg or wire, refer the image below:
So, whenever inferring to sequential logic, which stores/holds some value, declare that variable/port as reg. Here, Q is a reg inside a module, but while instantiating this module inside some other module, then this port must be connected to a wire.
Remember, wire can only infer to combinational logic, while reg can infer to either combinational or sequential logic.
Dave's blog is a good source for detailed information. For further information, refer to synthesizing difference and Verilog wire-reg links.
Simple difference between reg and wire is, the reg is used in combinational or sequential circuit in verilog and wire is used in combinational circuit
reg is used to store a value but wire is continuely driven some thing and wire is connected to outport when module initialization but reg is con not connected