1; display in: b, b select. a, b, s, out 1; This is non-synthesizable simulator code. As you would guess, the display command is the verilog equivalent of "printf and simply outputs values to the standard output. The "b" flag is used to output values as strings of binary digits, which we often want to to when working with digital hardware. Commands starting with a dollar sign are always commands to the synthesizer or simulator, and cannot be synthesized. It looks like a display is all we need - what are those funny hashes littering the code around it? The 1' command is a delay, and will wait for 1 unit of time.
Verilog Synthesis Tutorial Part-iii
Note that the 'initial' statement is non-synthesizable, which means that while we can use it in files meant for simulation (such as valet test-benches) there is no way to realize this statement in hardware. If you try to synthesize a file with an 'initial' statement, it will either be ignored or give you an error. A 0; b 1; s 0; Let's give our registers some values. In Verilog, there are two types of assignment. The first type of assignment, which we use here, is known as non-blocking assignment, and uses the " " symbol. It's the non-blocking I/o version of writing to a variable: the value will be written to that register, but we're not sure when. The reason we have this in Verilog is because things in circuits are inherently parallel. Everything we're doing is just defining circuit elements and the wires connecting them, and voltages propagate through a circuit in all paths of the circuit at once. The non-blocking assignment lets us take advantage of that, and although it can make things much more difficult to reason about, it makes our circuits much, much faster. If you require your assignments to be sequential, verilog also has blocking assignment via the " symbol: if you have several things using blocking assignment, you are guaranteed that the blocking assignments that come earlier in your code will actually happen before the blocking assignments. Both blocking and non-blocking assignments are synthesizable, which means they can be turned into real digital circuits.
Think of it as a c include" or php "include statement, which creates a circuit that we can use.) Note that we take registers (a, b, s) as input, and our output is a wire (out). Reading from registers makes sense - it's just connecting the output of the flip-flops to this circuit. Similarly, using the 'out' wire as an output makes sense - we're just labeling the output wire of our multiplexer as "out". Since we've already connected our multiplexer to "out we cannot connect anything else to "out" - that would mean that "out" is assigned to two things, which makes no sense. Similarly, we cannot have a register as an output; if 'out' was declared as a register, then we would get an error, because registers must be explicitly written to, and cannot just be the destination of some signal from a combinational logic circuit. Initial begin / Simulation code end, this part is simple. 'initial' simply defines something that happens at the beginning of a simulation. The 'begin end' combination is just Verilog's scoping method, equivalent to braces in c or java or indentation levels friend in Python. The code that goes inside the 'initial' statement can be thought of as quasi-imperative commands.
It's just a node in our circuit diagram, and giving it a name lets us connect things to it in order to use it as an input or an output to various logic circuits. Since registers are actually stored in memory, that means we can actually assign to them multiple times and give listing them different values, while wires cannot be assigned to in the same manner. Mux my_mux(a, b, s, out This line declares a subcomponent of our circuit. The subcomponent is taken from the 'mux' module we defined earlier, and is called 'my_mux'. It has inputs and outputs defined in the same order as in the 'mux' module definition. This is the verilog equivalent of a function call! (But don't think of it as a function call.
In order to do a simulation, we design a separate test-bench file: module mux_test; reg a, b, s; wire out; mux my_mux(a, b, s, out initial begin a 0; b 1; s 0; 1; display in: b, b select. a, b, s, out 1; s 1; 1; display in: b, b select. a, b, s, out 1; a 1; b 0; 1; display in: b, b select. a, b, s, out end endmodule, let's go through this step-by-step. Reg a, b, s; wire out; we use the 'reg' and 'wire' keywords to declare our "variables". 'a 'b and 's' are registers. This means that they are actually stored data, in some form of memory - most likely something built out of D-flip flops, as discussed in the previous post. On the other hand, 'out' is just a wire.
Verilog Full Adder example - reference designer
If 'select' is on, then connect 'a' to 'out, otherwise, connect 'b' to out. It says nothing about how to do this, and that is left up to the synthesizer to decide - the synthesizer can often do a better job than a person at implementing logic with an optimal number of gates, transistors, or whatever else you may. In this writing short piece of code, there is a small pitfall which you might be tempted to fall in to, especially if you are a programmer. Remember - verilog is a hardware description language. This 'assign' statement does not perform an imperative assign. You cannot think of this as assigning "select?
A : b" to the variable 'out'. Instead, this is a combinational logic circuit - the wire named "out" is connected to the wires 'select 'a and 'b' through a circuit which performs some operation on them. If a comparison between Verilog and programming needs to be made, it would be more appropriate to compare the 'assign' statement to the 'let' statement in strict functional languages such as ocaml and Haskell. Once something is 'assigned it cannot be assigned again. Designing hardware is a difficult thing, so it's important to test it rigorously. For wanted part of that, we can use a verilog simulator - you can find one either in freely available fgpa software, such as Altera's quartus suite, or use the open source 'iverilog' simulator.
But we've still got a lot to cover! As our first Verilog example, we're going to introduce an incredibly fundamental digital logic element known as the multiplexer, or, more affectionately, the mux. A mux has two k-bit input signals and a control signal and outputs a single k-bit output signal. The control signal, also known as the select signal, tells the mux which of the input signals to forward to the ouput. For instance, if we have a mux with input signals a and b and a select signal s, and s 0, the output signal will be equal to the input signal. Conversely, if s 1, the output will be equal to the input signal.
Muxes look like this in circuit diagrams: 2-signal multiplexer. In this diagram, if 'select' is zero, the output is a, and if 'select' is a one, the output. Let's make our multiplexer in Verilog. Circuit elements are defined as modules in Verilog, with stated inputs and outputs. This module definition gives us the geometry of the multiplexer, as in the circuit diagram shown previously. Module mux(input a, input b, input select, output out assign out select? A : b; endmodule, the code (saved in mux. V) describes exactly what this hardware does.
Verilog Formal Syntax Specification
But we're still miles and miles away from, say, a modern Intel processor. Besides a lot of computer engineering knowledge, we're still missing something major: Verilog. Verilog, verilog (like it's distant cousin, vhdl) is an incredibly powerful tool. It is essentially a programming language for hardware. It allows you to write a description of a piece of hardware, which can then be fed either to a simulator or a synthesizer. The simulator allows hardware designers to test their digital circuits before building them into circuits, and the synthesizer figures out how the hardware description can be implemented in logic gates and transistors. In programming language terms, the simulator is a verilog interpreter, while yardage the synthesizer is a verilog compiler. Verilog is what is known as a hardware description language (HDL). We're going to use verilog in order to introduce a lot of basic digital circuits, and eventually design our own processor.
Always if(a b) gt 1'b1; / no eq assignment in branch else if(a b) eq 1'b1; / no gt assignment in branch / final else branch omiitted Here we break both incomplete output assinment rules and branch According to verilog definition gt and eq keep. Here is how we could correct this: always if(a b) begin gt 1'b1; eq 1'b0; end else if (a b) begin gt 1'b0; eq 1'b1; end else begin gt 1'b0; eq 1'b0; end Or easier still assign default values to variables at the beginning. Case(a) 2'b00: y 1'b1; 2'b10: y 1'b0; 2'b11: y 1'b1; default : y 1'b1; endcase Adder with carry module adder parameter N4) ( input wire n-1:0 a, b, output wire n-1:0 sum, output wire cout constant Declaration localparam N1 n-1; signal Declaration wire N:0 sum_ext;. Cout(c4 endmodule localParams localparam n 4). This is a continuation of, computing with Transistors, a series of blog posts describing how computers work from the ground. The first post and namesake of the series is available as Computing with Transistors. In this post, i build off the concepts of logic gates and memory, which were introduced in the previous post, The digital State. At the lowest level, digital circuits and processors are built out of transistors - logic gates, memory arrays, flip-flops, and. Various combinations of transistors, wires, capacitors, and so on can be assembled into functional units - individual components (gates, memory, etc) which do something useful.
In casez, the? Is used to indicate either x or Z state. Tasks and Functions are used when a set of operations are commonly repeated and used, this save you writing the same thing out over and over a gain. Functions can only be used for modelling combinational logic cannot drive more than one output and cannot contain delays, unlike tasks which can. Tasks also can not return a value unlike functions which can. Below shows a function for calculating parity, something that may be used frequently in digital logic design. Function parity; input 31:0 data; integer i; begin parity 0; for (i 0; i 32; i i 1) begin parity parity datai; end end endfunction Common Errors Variable assigned in multiple always blocks Incomplete sensitivity list Incomplete branch and incomplete output assignment Multiple assignment always. Below is how this should have been writtten: always if(en) y 1'b0; else y a b; Incomplete sensitivity list Incomplete sensitivity list (missing b). B could change but the y output would not, causing unexpected behavior againg this is not synthesiseable. Always a) y a b; fixed versions always a, b) y a b; or simple cure all always y a b; incomplete branch or output assignment Incomplete branch or output assignment, do not infer state in combinational circuits.
The reg data type is like the wire data type but used with a procedural output. The integer data type represents a fixed-size (usually 32 bits) signed number in 2's-complement format. Since its size is fixed, we usually don't use it in synthesis. The other data types are for modeling and simulation and cannot be synthesized. Registers A year register is simple memory wire to hold state, normally implemented as d-types output reg Conditional Examples binary encoder en a1 a2 y module pri_encoder ( input wire 3:0 r, output wire en output wire 1:0 y ) always if(r3) en, y 3'b111; else. When synthesized, a parallel case statement usually infers a multiplexing routing network and a non-parallel case statement usually infers a priority routing network. Unlike c where conditional constructs are executed serially using branches and jumps, with hdl these are realised by routing netowrks.
Verilog examples useful for fpga & asic synthesis
Wires wire input a, b; wire output y; wire input a21:0,b21:0; wire output y21:0; wire output y32:0; An assign statement is used for modeling only combinational logic and it is executed continuously. So the assign statement is called 'continuous assignment statement' as there is no sensitive list (see always blocks later). Assign y a; Logic bitwise primitives, negation assign y a; and gate assign y a b; or gate assign y a b; Exclusive or gate assign y a b; Reduction assign y a2; is equivalent to: assign y a21 a20; Concatenation and Replication assign. Rotation, rotate right 1 bit assign y3r y30,y32:1; Rotate right 2 bit assign y3r y31:0,y32; Operator precedence conditionals, tertiary assign max (a b)? A : b; If/Else if(a b) assign min a; else assign min b; if(boolean) / if code else if (boolean) / if else 1 code else / else code if(boolean) begin / begin code end else begin / else code end Synthesis of z and. A : 1'bz; This gps sort of construuct is useful for biderectional ports or buses. The synthesis of x is don't care, the value may be either 0 or 1, this can improve the efficiency or optimisation of combinational circuits assign y (a 2'b00)? 1'b1: 1'bx; / i 2'b11 Behavioural Blocks Procedural blocks using always block, these black box sections describe behaviour using procedural statements, always behavioural blocks are defined with an event control expression or sensitivity list always sensitivity list) begin Optional label optional local variable declarations; procedural.