Upload Verilog Code to Block Design Vivado
In this mail service nosotros await at how we utilize Verilog to write a basic testbench. We get-go by looking at the compages of a Verilog testbench earlier considering some cardinal concepts in verilog testbench design. This includes modelling fourth dimension in verilog, the initial block, verilog-initial-block and the verilog system tasks. Finally, we get through a consummate verilog testbench example.
When using verilog to design digital circuits, nosotros normally besides create a testbench to stimulate the code and ensure that it functions equally expected.
We tin can write our testbench using a variety of languages, with VHDL, Verilog and System Verilog being the most popular.
Organisation Verilog is widely adopted in industry and is probably the almost common language to employ. If you are hoping to blueprint FPGAs professionally, so it will be important to learn this skill at some point.
Every bit it is meliorate to focus on one language as a fourth dimension, this blog post introduces the basic principles of testbench design in verilog. This allows us to test designs while working through the verilog tutorials on this site.
If you are interested in learning more nigh testbench pattern using either verilog or SystemVerilog, then there are several excellent courses paid course bachelor on sites such as udemy.
Compages of a Basic Testbench
Testbenches consist of not-synthesizable verilog code which generates inputs to the design and checks that the outputs are correct.
The diagram below shows the typical architecture of a simple testbench.
The stimulus block generates the inputs to our FPGA design and the output checker tests the outputs to ensure they have the correct values.
The stimulus and output checker will be in divide files for larger designs. It is as well possible to include all of these different elements in a single file.
The primary purpose of this post is to introduce the skills which volition permit us to exam our solutions to the exercises on this site.
Therefore, we don't discuss the output checking cake as it adds unnecessary complexity.
Instead, we tin use a simulation tool which allows for waveforms to exist viewed directly. The freely available software packages from Xilinx (Vivado) and Intel (Quartus) both offering this capability.
Alternatively, open source tools such every bit icarus verilog tin exist used in conjunction with GTKWave to run verilog simulations.
Nosotros tin can also make use of EDA playground which is a gratis online verilog simulation tool.
In this instance, we would need to use system tasks to monitor the outputs of our design. This gives us a textual output which we can use to check the state of our signals at given times in our simulation.
Instantiating the DUT
The start step in writing a testbench is creating a verilog module which acts as the meridian level of the test.
Unlike the verilog modules nosotros have discussed so far, we want to create a module which has no inputs or outputs in this instance. This is considering nosotros want the testbench module to be totally self independent.
The code snippet beneath shows the syntax for an empty module which we can employ as our testbench.
module <module_name> (); // Our testbench code goes here endmodule : <module_name> Afterwards we have created a testbench module, nosotros must and so instantiate the design which we are testing. This allows the states to connect signals to the blueprint in order to stimulate the code.
We have already discussed how we instantiate modules in the previous mail service on verilog modules. However, the code snippet below shows how this is washed using named instantiation.
<module_name> # ( // If the module uses parameters they are connected hither .<parameter_name> (<parameter_value>) ) <instance_name> ( // Connection to the module ports .<port_name> (<signal_name>), .<port_name> (signal_name>) ); Once we have done this, nosotros are fix to start writing our stimulus to the FPGA. This includes generating the clock and reset, as well creating test data to send to the FPGA.
In gild to this we need to use some verilog constructs which nosotros have not all the same encountered - initial blocks, forever loops and time consuming statements.
We will look at these in more than detail before nosotros go through a complete verilog testbench example.
Modelling Fourth dimension in Verilog
One of the key differences between testbench code and design code is that nosotros don't need to synthesize the testbench.
Every bit a result of this, nosotros can utilize special constructs which consume fourth dimension. In fact, this is crucial for creating test stimulus.
We take a construct available to us in Verilog which enables us to model delays. In verilog, we employ the # character followed by a number of time units to model delays.
As an example, the verilog code below shows an case of using the delay operator to await for 10 time units.
Ane important affair to note here is that there is no semi-colon at the stop of the lawmaking. When nosotros write code to model a delay in Verilog, this would actually outcome in compilation errors.
It is likewise common to write the delay in the same line of lawmaking as the assignment. This effectively acts as a scheduler, meaning that the modify in signal is scheduled to take place later the filibuster time.
The code snippet below shows an instance of this type of lawmaking.
// A is prepare to 1 afterwards ten time units #ten a = ane'b1; Timescale Compiler Directive
So far, we have talked almost delays which are x units of time. This is fairly meaningless until we really define what time units we should use.
In gild to specify the time units that we use during simulation, we use a verilog compiler directive which specifies the time unit and resolution. We but need to do this once in our testbench and it should be done outside of a module.
The code snippet below shows the compiler directive we apply to specify the time units in verilog.
`timescale <unit_time> / <resolution> We use the <unit_time> field to specify the primary time unit of our testbench and the <resolution> field to define the resolution of the time units in our simulation.
The <resolution> field is important as we can utilise non-integer numbers to specify the delay in our verilog code. For example, if we want to have a delay of 10.5ns, we could merely write #10.5 as the delay.
Therefore, the <resolution> field in the compiler directive determines the smallest fourth dimension pace we can really model in our Verilog lawmaking.
Both of the fields in this compiler directive have a fourth dimension type such as 1ps or 1ns.
Verilog initial block
In the postal service on always blocks in verilog, we saw how we tin can utilise procedural blocks to execute code sequentially.
Some other blazon of procedural block which nosotros can use in verilog is known as the initial block.
Any code which we write inside an initial block is executed one time, and but one time, at the beginning of a simulation.
The verilog code below shows the syntax we use for an initial block.
initial begin // Our code goes here cease Unlike the ever block, verilog code written within initial block is non synthesizable. As a result of this, we use them nearly exclusively for simulation purposes.
However, nosotros can also use initial blocks in our verilog RTL to initialise signals.
When we write stimulus code in our verilog testbench we nigh always use the initial block.
To give a better agreement of how we use the initial block to write stimulus in verilog, let's consider a basic example.
For this instance imagine that nosotros want to exam a basic two input and gate.
To do this, we would need code which generates each of the four possible input combinations.
In addition, nosotros would too need to use the delay operator in order to wait for some time between generating the inputs.
This is important equally it allows fourth dimension for the signals to propagate through our design.
The verilog code beneath shows the method nosotros would utilize to write this test within an initial block.
initial begin // Generate each input to an AND gate // Waiting ten time units between each and_in = 2b'00; #10 and_in = 2b'01; #10 and_in = 2b'ten; #10 and_in = 2b'11; cease Verilog forever loop
Although we haven't withal discussed loops, they tin can be used to perform important functions in Verilog. In fact, we volition hash out verilog loops in detail in a afterwards postal service in this serial
Yet, there is one important type of loop which nosotros can utilise in a verilog testbench - the forever loop.
When we use this construct we are really creating an infinite loop. This ways we create a department of code which runs contimnuously during our simulation.
The verilog code below shows the syntax nosotros utilise to write forever loops.
forever begin // our lawmaking goes here end When writing code in other programming languages, we would likely consider an infinite loop as a serious issues which should be avoided.
However, we must remember that verilog is non like other programming languages. When nosotros write verilog lawmaking we are describing hardware and not writing software.
Therefore, nosotros have at least one example where we tin can use an infinite loop - to generate a clock signal in our verilog testbench.
To practise this, we need a manner of continually inverting the signal at regular intervals. The forever loop provides us with an easy method to implement this.
The verilog lawmaking below shows how we tin use the forever loop to generate a clock in our testbench. Information technology is important to note that any loops we write must be independent with in a procedural block or generate block.
initial begin clk = 1'b0; forever brainstorm #1 clk = ~clk; end stop Verilog System Tasks
When nosotros write testbenches in verilog, we have some inbuilt tasks and functions which we can use to assistance united states.
Collectively, these are known as system tasks or system functions and we tin can identify them hands equally they always begin wtih a dollar symbol.
There are actually several of these tasks available. However, we will merely expect at iii of the near ordinarily used verilog organisation tasks - $display, $monitor and $time.
$display
The $brandish function is ane of the nearly commonly used system tasks in verilog. We apply this to output a message which is displayed on the panel during simulation.
We apply the $display macro in a very like style to the printf part in C.
This means we can easily create text statements in our testbench and apply them to display data about the status of our simulation.
We tin also use a special character (%) in the string to display signals in our blueprint. When we do this we must also include a format alphabetic character which tells the chore what format to brandish the variable in.
The most commonly used format codes are b (binary), d (decimal) and h (hex). We tin also include a number in front of this format code to decide the number of digits to display.
The verilog code below shows the full general syntax for the $display arrangement task. This code snippet besides includes an example use case.
// General syntax $display(<string_to_display>, <variables_to_display); // Example - display value of ten as a binary, hex and decimal number $brandish("x (bin) = %b, x (hex) = %h, x (decimal) = %d", x, x, x); The full list of dissimilar formats nosotros can utilise with the $display system task are shown in the tabular array below.
| Format Code | Description |
|---|---|
| %b or %B | Brandish as binary |
| %d or %D | Brandish every bit decimal |
| %h or %H | Display equally hexidecimal |
| %o or %O | Display as octal format |
| %c or %C | Display as ASCII character |
| %m or %M | Display the hierarchical name of our module |
| %southward or %Due south | Display as a string |
| %t or %T | Display as time |
$monitor
The $monitor office is very similar to the $display function, except that it has slightly more intelligent behaviour.
We use this function to monitor the value of signals in our testbench and display a bulletin whenever ane of these signals changes state.
All system tasks are actually ignored by the synthesizer then we could fifty-fifty include $monitor statements in our verilog RTL code, although this is not mutual.
The general syntax for this system task is shown in the lawmaking snippet below. This lawmaking snippet also includes an example use example.
// General syntax $monitor(<message_to_display>, <variables_to_display>); // Example - monitor the values of the in_a and in_b signals $monitor("in_a=%b, in_b=%b\northward", in_a, in_b); $time
The final system job which nosotros normally utilise in testbenches is the $time function. Nosotros use this system chore to become the current simulation time.
In our verilog testbenches, we commonly use the $time part together with either the $brandish or $monitor tasks to display the fourth dimension in our messages.
The verilog lawmaking below shows how we use the $time and $display tasks together to create a bulletin.
$display("Electric current simulation time = %t", $fourth dimension); Verilog Testbench Example
Now that we have discussed the almost important topics for testbench blueprint, let's consider a compete example.
We will use a very unproblematic circuit for this and build a testbench which generates every possible input combination.
The excursion shown below is the one we volition use for this example. This consists of a simple two input and gate as well as a flip flip.
1. Create a Testbench Module
The get-go thing nosotros do in the testbench is declare an empty module to write our testbench code in.
The code snippet below shows the declaration of the module for this testbench.
Note that it is good practise to keep the proper noun of the design being tested and the testbench like. Usually this is washed by simply appending _tb or _test to the terminate of the blueprint name.
module example_tb (); // Our testbench lawmaking goes here endmodule : example_tb two. Instantiate the DUT
Now that we have a bare testbench module to work with, nosotros need to instantiate the design we are going to test.
As named instantiation is generally easy to maintain than positional instantiation, as well as existence easier to sympathise, this is the method we use.
The code snippet below shows how we would instantiate the DUT, assuming that the signals clk, in_1, in_b and out_q are declared previously.
example_design dut ( .clock (clk), .reset (reset), .a (in_a), .b (in_b), .q (out_q) ); 3. Generate the Clock and Reset
The next matter we do is generate a clock and reset point in our verilog testbench.
In both cases, we tin can write the code for this within an initial block. We then use the verilog delay operator to schedule the changes of state.
In the instance of the clock signal, we use the forever keyword to continually run the clock signal during our tests.
Using this construct, we schedule an inversion every 1 ns, giving a clock frequency of 1GHz.
This frequency is called purely to requite a fast simulation fourth dimension. In reality, 1GHz clock rates in FPGAs are not achievable and the testbench clock frequency should friction match the frequency of the hardware clock.
The verilog code below shows how the clock and the reset signals are generated in our testbench.
// generate the clock initial begin clk = ane'b0; forever #1 clk = ~clk; end // Generate the reset initial begin reset = i'b1; #10 reset = 1'b0; cease 4. Write the Stimulus
The last role of the testbench that we need to write is the test stimulus.
In club to exam the circuit nosotros need to generate each of the four possible input combinations in turn. Nosotros and then need to wait for a short time while the signals propagate through our code cake.
To do this, we assign the inputs a value and so use the verilog filibuster operator to allow for propagation through the FPGA.
We besides want to monitor the values of the inputs and outputs, which nosotros can practise with the $monitor verilog system task.
The code snippet below shows the lawmaking for this.
initial begin // Use the monitor task to display the FPGA IO $monitor("time=%3d, in_a=%b, in_b=%b, q=%2b \due north", $time, in_a, in_b, q); // Generate each input with a 20 ns filibuster betwixt them in_a = 1'b0; in_b = i'b0; #xx in_a = 1'b1; #20 in_a = ane'b0; in_b = one'b1; #twenty in_a = 1'b1; terminate Full Case Code
The verilog code below shows the testbench example in its entirety.
`timescale 1ns / 1ps module example_tb (); // Clock and reset signals reg clk; reg reset; // Design Inputs and outputs reg in_a; reg in_b; wire out_q; // DUT instantiation example_design dut ( .clock (clk), .reset (reset), .a (in_a), .b (in_b), .q (out_q) ); // generate the clock initial begin clk = i'b0; forever #1 clk = ~clk; finish // Generate the reset initial begin reset = one'b1; #ten reset = 1'b0; finish // Test stimulus initial brainstorm // Use the monitor task to display the FPGA IO $monitor("time=%3d, in_a=%b, in_b=%b, q=%2b \northward", $time, in_a, in_b, q); // Generate each input with a 20 ns delay between them in_a = i'b0; in_b = ane'b0; #20 in_a = 1'b1; #xx in_a = ane'b0; in_b = 1'b1; #twenty in_a = ane'b1; finish endmodule : example_tb Exercises
shipmanthrainater.blogspot.com
Source: https://fpgatutorial.com/how-to-write-a-basic-verilog-testbench/
Post a Comment for "Upload Verilog Code to Block Design Vivado"