Verilog code for ALU with 8 Operations; Verilog code for ALU (16 Operations ) DESIGN AND IMPLEMENTATION OF ALU USING FPGA SPARTAN 2; REGISTERS. Verilog code for 4-bit Shift Register; Verilog code for 8bit shift register; Verilog code for Generic N-bit Shift Register; verilog code for SIPO and Testbench; verilog code for SISO and testbench. In this project, Verilog code for counters with testbench will be presented including up counter, down counter, up-down counter, and r. Verilog code for D Flip Flop. D Flip-Flop is a fundamental component in digital logic circuits. Verilog code for D Flip Flop is presented in this project.
IntroductionRegisters are essential elements in digital circuits. We can store information, from 1 to N bits. They are used to implement processors, perform segmentation, storage of intermediate results, and more.The basic register captures the input data on the rising or falling edge of a clock, stores the data, and outputs it. The diagram is:In this chapter we will use a 4 bit register to make the 4 LEDs on the iCEstick board blink.blink4: Turning the LEDs on and offThe design we will use is the following:The 4-bit register is initialized to 0. For the output dout 4 bits are output to 0. These 4 bits are changed to 1 when passing through the invertor.
On the next rising ege of the clock, this new value of 4'b1111 is captured in the register. When the output dout is updated to 4b'1111, these results are again inverted and we've cycled back to the beginning. The result is the following sequence 0000, 1111, 0000, 1111. Is obtained with each change corresponding to the rising edge of the clock.
If the dout is connected to the LEDs, they were start turning on and off at 1/2 the clock frequency.Hardware DescriptionThe complete design to be implemented in the FPGA is shown in this figure:In order to actually be able to see the LEDs flicker, a prescaler is included. The hardware description in verilog is. //- blink4.v module blink4( input wire clk, //-clock output wire 3: 0 data //- output register); //- Bits for the prescaler parameter N = 22;//- main clock (prescalor) wire clkbase;//- register data reg 3: 0 dout = 0;//- wire to the register wire 3: 0 din;//- Instance of the prescaler prescaler #(.N(N))PRES (.clkin(clk).clkout(clkbase));//- Register always @( posedge(clkbase))dout. Module blink4(clk, data);input clk;output data;wire clk;wire 3: 0 data;The later form is used to define parametric components (see below)Synthesis of the FPGATo synthesize the design in the FPGA we run: $ make sintThe resources used are: ResourceutilizationPIOs3 / 96PLBs8 / 160BRAMs0 / 16To download to the FPGA we do: $ sudo iceprog blink4.binIn the video you can see the result:Initialization of registersThe synthesized registers always have an initial value of 0. The line of code defining the register is. Reg 3: 0 dout = 0;But the initialization in this line only works for simulation.
It can give any value and we will see it in the simulation, however in 'real hardware' we cannot do this. They will always be initialized to 0. To put another value we would have to load the register with this value.
In the example blink4, if we could load the record initially with the value 1'b1010 (instead of 1'b0000), inverting it woudl yield 1'b0101. So we would have a different sequence: 1010, 0101, 1010.
How coudl we implement this load? It is left as an exercise for the user to think about. In the following chapters we will show the necessary elements to accomplish it.SimulationThe test bench is very basic.
It simply instantiates the blink4 component, generates the clock signal, and starts the simulation. For the simulation to run for less cycles, the prescaler N parameter has been set to 1 bit.The code is.
The difference between Verilog reg and Verilog wire frequently confuses many programmers just starting with the language (certainly confused me!). As a beginner, I was told to follow these guidelines, which seemed to generally work:. Use Verilog reg for left hand side (LHS) of signals assigned inside in always blocks. Use Verilog wire for LHS of signals assigned outside always blocksThen when I adopted SystemVerilog for writing RTL designs, I was told everything can now be “type logic”.
That again generally worked, but every now and then I would run into a cryptic error message about variables, nets, and assignment.So I decided to find out exactly how these data types worked to write this article. I dug into the language reference manual, searched for the now-defunct Verilog-2005 standard document, and got into a bit of history lesson. Read on for my discovery of the differences between Verilog reg, Verilog wire, and SystemVerilog logic. Verilog data types, Verilog reg, Verilog wireVerilog data types are divided into two main groups: nets and variables. The distinction comes from how they are intended to represent different hardware structures.A net data type represents a physical connection between structural entities (think a plain wire), such as between gates or between modules. It does not store any value.
Its value is derived from what is being driven from its driver(s). Verilog wire is probably the most common net data type, although there are many other net data types such as tri, wand, supply0.A variable data type generally represents a piece of storage. It holds a value assigned to it until the next assignment. Verilog reg is probably the most common variable data type. Verilog reg is generally used to model hardware registers (although it can also represent combinatorial logic, like inside an always@(.) block). Other variable data types include integer, time, real, realtime.Almost all Verilog data types are 4-state, which means they can take on 4 values:.
0 represents a logic zero, or a false condition. 1 represents a logic one, or a true condition. X represents an unknown logic value. Z represents a high-impedance state. Verilog rule of thumb 1: use Verilog reg when you want to represent a piece of storage, and use Verilog wire when you want to represent a physical connection.Assigning values to Verilog reg, Verilog wireVerilog net data types can only be assigned values by continuous assignments. This means using constructs like continuous assignment statement ( assign statement), or drive it from an output port. A continuous assignment drives a net similar to how a gate drives a net.
The expression on the right hand side can be thought of as a combinatorial circuit that drives the net continuously.Verilog variable data types can only be assigned values using procedural assignments. This means inside an always block, an initial block, a task, a function.
The assignment occurs on some kind of trigger (like the posedge of a clock), after which the variable retains its value until the next assignment (at the next trigger). This makes variables ideal for modeling storage elements like flip-flops. Verilog rule of thmb 2: drive a Verilog wire with assign statement or port output, and drive a Verilog reg from an always block. If you want to drive a physical connection with combinatorial logic inside an always@(.) block, then you have to declare the physical connection as Verilog reg.SystemVerilog logic, data types, and data objectsSystemVerilog introduces a new 2-state data type—where only logic 0 and logic 1 are allowed, not X or Z—for testbench modeling. To distinguish the old Verilog 4-state behaviour, a new SystemVerilog logic data type is added to describe a generic 4-state data type.What used to be data types in Verilog, like wire, reg, wand, are now called data objects in SystemVerilog.
Wire, reg, wand (and almost all previous Verilog data types) are 4-state data objects. Bit, byte, shortint, int, longint are the new SystemVerilog 2-state data objects.There are still the two main groups of data objects: nets and variables. All the Verilog data types (now data objects) that we are familiar with, since they are 4-state, should now properly also contain the SystemVerilog logic keyword. Var reg r; // variable regDon’t worry too much about the var keyword. It was added for language preciseness (it’s what happens as a language evolves and language gurus strive to maintain backward-compatibility), and you’ll likely not see it in an RTL design.
I’m confused Just tell me how I should use SystemVerilog logic!After all that technical specification gobbledygook, I have good news if you’re using SystemVerilog for RTL design. For everyday usage in RTL design, you can pretty much forget all of that!The SystemVerilog logic keyword standalone will declare a variable, but the rules have been rewritten such that you can pretty much use a variable everywhere in RTL design.
Hence, you see in my example code from other articles, I use SystemVerilog logic to declare variables and ports. EndmoduleWhen you use SystemVerilog logic standalone this way, there is another advantage of improved checking for unintended multiple drivers. Multiple assignments, or mixing continuous and procedural ( always block) assignments, to a SystemVerilog variable is an error, which means you will most likely see a compile time error. Mixing and multiple assignments is allowed for a net.
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So if you really want a multiply-driven net you will need to declare it a wire.In Verilog it was legal to have an assignment to a module output port (declared as Verilog wire or Verilog reg) from outside the module, or to have an assignment inside the module to a net declared as an input port. Both of these are frequently unintended wiring mistakes, causing contention.
With SystemVerilog, an output port declared as SystemVerilog logic variable prohibits multiple drivers, and an assignment to an input port declared as SystemVerilog logic variable is also illegal. So if you make this kind of wiring mistake, you will likely again get a compile time error. SystemVerilog rule of thumb 1: if using SystemVerilog for RTL design, use SystemVerilog logic to declare:. All point-to-point nets. If you specifically need a multi-driver net, then use one of the traditional net types like wire. All variables (logic driven by always blocks). All input ports.
All output portsIf you follow this rule, you can pretty much forget about the differences between Verilog reg and Verilog wire! (well, most of the time) ConclusionWhen I first wondered why it was possible to always write RTL using SystemVerilog logic keyword, I never expected it to become a major undertaking that involved reading and interpreting two different specifications, understanding complex language rules, and figuring out their nuances. At least I can say that the recommendations are easy to remember.I hope this article gives you a good summary of Verilog reg, Verilog wire, SystemVerilog logic, their history, and a useful set of recommendations for RTL coding. I do not claim to be a Verilog or SystemVerilog language expert, so please do correct me if you felt I misinterpreted anything in the specifications. References. Sample Source CodeThe accompanying source code for this article is a SystemVerilog design and testbench toy example that demonstrates the difference between using Verilog reg, Verilog wire, and SystemVerilog logic to code design modules. Download the code to see how it works!
Good article. However, there is one significant gotcha that users need to be aware of. When converting RTL from wire/reg to logic, if you were using a net declaration assignment for a wire, that will not work with logic!
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As a variable type, assigning a value to a logic variable as part of the declaration merely initializes it to that value. No continuous assignment is inferred.This is the only case where logic has not been a drop-in replacement for me in RTL.Example:wire mysignal0 = A & B; // continuous assignment, AND gatelogic mysignal1 = A &B; // not synthesizable, initializes mysignal1 to the value of A & B at time 0 and then makes no further changes to it.logic mysignal2;assign mysignal2 = A & B; // Continuous assignment, AND gate. Thanks for pointing that out Evan!
I looked through the assignment section of the LRM and you’re correct. Like you said, the particular form of assignment in the first row of your example code is called net declaration assignment (section 10.3.1 of SV-2012 LRM), and as the name suggests it only works on nets. The second line in your example is a variable declaration assignment (section 10.5), and would only initialize the variable and not continuously drive it. That is indeed a gotcha if one just replaced all instances of wire with logic. Great comment!. A always@.
block will be sensitive to all input signals to the block (with one exception). Therefore, if you use a reg type inside a always@. block, it will become combinational logic and not infer flip-flop or latches.The one exception is if the always@. calls a function or task, and that function or task contains signals that are external to the function, task, and always@. block. SystemVerilog alwayscomb avoids this issue, so if you code in SystemVerilog, you should use alwayscomb instead of always@.
See my other article on.