systemverilog basics jean-michel chabloz. how we study systemverilog huge language: –last lrm has...

SystemVerilog basics

Jean-Michel Chabloz

How we study SystemVerilog

• Huge language:– last LRM has >1300 pages– Not possible to cover everything, we cover

maybe 5% of the constructs– You can succeed in the course using only the

subset of the language that is treated in these slides

– If you want you are free to use other constructs, research them by yourself

SystemVerilog Hello World

module M();

initial

$display(“Hello world”);

endmodule

SystemVerilog simple programmodule M(); logic a,b; logic [7:0] c; assign b = ~a;

initial begin a <= 0; #20ns; repeat(40) #5ns a <= ~a; #20ns $display(c); $finish(); end

initial c <= 0;

always @(posedge a) c <= c + 1;

endmodule

SystemVerilog syntax

• Case sensitive

• C-style comments: // or /*comment*/

• Similar to C, but code blocks are delimited by “begin” “end”. If a single-line, can be omitted

Modules

• Let’s start to consider systems without hierarchy (no submodules)

• A module contains objects declarations and concurrent processes that operate in parallel.– Initial blocks– Always blocks– Continuous assignments– (instantiations of submodules)

SystemVerilog basic data types

• Data types:– logic – 4 valued data type:

• 0, 1, X, Z• initialized to X• can also be called reg // deprecated verilog legacy name

– bit – 2 valued data type:• 0, 1• initialized to 0

– Defining an object of a certain type:• bit a;• logic b;• bit c, d;

Packed arrays of logic and bits

• bit [5:0] a;• logic [2:0] b;• logic [0:2047] [7:0] c; //array of 2048 bytes• integer: equivalent to logic [31:0]• int, byte: equivalents to bit [31:0] and bit [7:0]• Signed/unsigned affects operations such as “>”• arrays of logics default to unsigned, can be overriden with

the keyword signed• ex: logic signed [7:0] a;

• arrays of bits default to signed, can be overriden with the keyword unsigned

• ex: bit unsigned [7:0] a;

Literals

• Decimal literal:– a <= 54; // automatically extended to the length of a with 0

padding– a <= ‘d54; – a <= 12’d54; // specifies that a is 12-bits wide

• Unspecified length:– ‘1, ‘0, ‘x, ‘z // fills with all 1s, 0s, xs, zs

• binary literal– 12’b1000_1100_1110 // underscores can be put anywhere

except the beginning of a literal– ‘b11011 // automatically resized with zeroes if fed to something

bigger• hexadecimal literal:

– 12’hc; // “000000001100”– ‘hcd: // ”….0000011001101”

Packed array access

• Single element access:– bit [7:0] a

• a[5] <= a[6];

– bit [9:0][7:0] b:• b[5] <= 15;

• Packed arrays can be sliced:– bit [7:0] a;

• a[3:2] <= 2’b10;• a[3:0] <= a[7:4];

– bit [2047:0][7:0] a; bit [1:0][7:0] b;• a[2047:2046] <= b

packed structures

• equivalent to a packed array subdivided into named fields:– example: 48 bit packed array

– can be accessed as pack1[15:0] <= ‘b0;– can access pack1[9:4] <= 15;– can be accessed as pack1.d <= ‘b0;– the whole struct can be resetted with pack1 <= ‘b0;

• unpacked struct (no “packed” keyword) allow only acces through the named fields (pack1.d <=‘b0);

struct packed { int a; bit [7:0] c; bit [7:0] d;} pack1;

Other data types

• Enumerated data type:– enum bit [1:0] {idle, writing, reading} state– If skipping the type an int type is assumed– Can be typedeffed (like all other data types):

• typedef enum {red, green, blue, yellow, white, black} Colors;

• Colors [2:0] setOfColors; // array of 3 elements of type colors

Types in SV• SystemVerilog is a weakly-typed language

– advantages: simpler and shorter code– disadvantages: easy to do mistakes

• Many assignment-compatible types– a bit and a logic are assignment-compatible, we can assign one to

the other– a longer array can be assigned to a shorter one or viceversa

(truncation or extension will happen automatically)– arrays can be indexed by logic arrays, bit arrays– a packed struct has the same properties as an array

• struct packed {bit[3:0] a, b;} can be assigned a bit array, a logic array, etc.

• ifs, whiles, etc. can take as condition bits, logic, arrays, etc.– non-zero values count as TRUE, all-zero values count as false– if a is a bit or logic, then we can write if (a==1) or if (a), they do the

same thing

Processes

• Modules contain processes (initial, always) – code inside processes is called “procedural code”– Initial: executed only once, at the beginning of the

simulation

initial begin

#10ns;

a <= 1’b1;

#20ns;

a <= 1’b0;

Processes

– Always with no sensitivity list: triggers as soon as it finishes executing

always begin

#10ns;

a <= 1’b1;

#20ns;

a <= 1’b0;

Processes

– Always with sensitivity list: triggers when it has finished executing and one of the events in the sensitivity list happens

always @(posedge b, negedge c) begin

#10ns;

a <= 1’b1;

#20ns;

a <= 1’b0;

•posedge: positive edge

•negedge: negative edge

•signal name: any toggle

Always block, combinational process

– To make combinational processes, the sensitivity list must contain all elements in the right-hand side of statements (also in conditions, loop headers, …) – in general, anything that influences the output

always @(a,b) begin

c <= a+b;

– SystemVerilog allows using always_comb instead• the sensitivity list is automatically compiled to make a combinational block• always_comb begin

c <= a+b;

• the compiler also checks for latches, as in the following example• always_comb begin // latch will be detected

if (c)

d <= a+b;

Always block, flip-flop

– D flip-flop with async reset:

– Possible to specify always_ff to declare intent• we declare to the compiler we want to do a FF (in the sense

of edge-triggered logic, can also be an FSM), if it is not an FF we get an error

always @(posedge clk, negedge rst) begin if (rst==0) q <= 0; else // posedge clk, rst==1 q <= d; end

always_ff @(posedge clk, negedge rst) begin if (rst==0) q <= 0; else // posedge clk, rst==1 q <= d; end

Procedural code

if (a==1) begin //codeend

while (a==1) begin //codeend

forever begin // loops forever //codeend

for (i=0; i<3; i++) begin // loops three times //codeend

if (a) begin // 1 counts as true //codeend

repeat (3) begin //codeend

if trees

if (condition) begin

else begin

if treesif (condition1) begin …endelse if (condition2) begin …endelse if (condition3) begin …endelse begin …end

• No elsif construct, but this is equivalent

Bitwise logic operators

• Bitwise logic operators – return a number of bits equal to the length of the inputs:– &: and– | : or– ^ : xor– ~ : not

• Negate one bit/logic array:– a <= ~a

• Do a bit-wise OR between two bit/logic arrays:– c <= a | b

logic operators

• logic operators – return one bit only, treat as one everything that is non-zero:– &&: and– | |: or– ! : not

• for one-bit elements “if (!a)” is equal to “if (~a)”• for a 4-bit elements, if a=1100

– if(!a) will not execute (!a returns 0)– if(~a) will execute (~a returns 0011 which is not all-

zeros)

comparisons

• equality: ==

• diseguality: !=

• greather than: >

• lower than: <

• greater or equal than: >=

• lower or equal than: <=

arithmetic

• + and – can be used with logic arrays, bit arrays, automatically wrap around:– up counter:

• ……….• 11101• 11110• 11111• 00000• 00001• 00010• ……….

Timing Control in Processes• #10ns: waits for 10 ns• #10: wait for 10 time units – time unit specified during elaboration or

with a `timescale directive in the code• #(a): wait for a number of time units equal to the value of variable a• #(a*1ps): wait for a number of picoseconds equal to the value of a

• @(posedge a): waits for the positive edge of a• @(b): wait until b toggles

• wait(expr): waits until expr is true• wait(b): wait until b is one

• Timing checks can be bundled with the next instr: #10ns a<=!a

fork… join• Spawn concurrent processes from a single process: A is printed at

30ns; B at 20ns; join waits until both subprocesses have finished, “both finished” is displayed at 40ns

initial begin #10ns; fork begin #20ns; $display( “A\n" ); end begin #10ns; $display( “B\n" ); #20ns; end join $display(“both finished”);end

fork… join_any• Spawn concurrent processes from a single process: A is printed at

30ns; B at 20ns; join_any waits until any subprocess has finished, “at least one finished” is displayed at 30ns

initial begin #10ns; fork begin #20ns; $display( “A\n" ); end begin #10ns; $display( “B\n" ); #20ns; end join_any $display(“at least one finished”);end

fork… join_none• Spawn concurrent processes from a single process: A is printed at

30ns; B at 20ns; join_none executes as soon as the fork is encountered, “fork entered” is displayed at 10ns.

initial begin #10ns; fork begin #20ns; $display( “A\n" ); end begin #10ns; $display( “B\n" ); #20ns; end join_none $display(“fork entered”);end

The number of forked processes (begin...end blocks) in any fork can be any (minimum one).

Note 1: having one forked process only makes sense with join_none

Note 2: if processes have a single line, no begin...end is necessary

Procedural assignments

• Non-blocking assignment– “<=“– Write-back takes place after a delta delay

• Blocking assignment– “=“– takes place immediately

• The two can be mixed – but probably not a good idea

• blocking assignments behaves like the VHDL variable assignment “:=“

• non-blocking assignments behaves like the VHDL signal assignment “<=“

• BUT:– In VHDL := is reserved for variables, <= for signals– In Verilog, both can be used for variables– Possible to mix them - but probably not a good idea– A better idea is to use some objects as VHDL

variables and only assign them with “=“, others as VHDL signals and only assign them with “<=“

always @(posedge clk) begin a <= b; b <= a;end

always @(posedge clk) begin a = b; b = a;end

always @(posedge clk) begin a <= b; b <= a;end

always @(posedge clk) begin a = b; b = a;end

Swaps A and B

copies B to A(B is unchanged)

initial begin a = 1; $display(a);end

initial begin a <= 1; $display(a);end

initial begin a <= 1; #10ns; $display(a);end

initial begin a = 1; $display(a);end

initial begin a <= 1; $display(a);end

initial begin a <= 1; #10ns; $display(a);end

1 is displayed

The old value of A is displayed

1 is displayed

Default assignments

• default values to avoid latches and to avoid writing long if else trees

• works like in VHDL (the last write is kept)

always_comb begin a <= 0; // default value of a … if (c) if (b==100) a <= 1;end

Console/control commands

• introduced with the $ keyword– $display – used to display information to the

console– ex:

• $display(“hello”); // displays “hello”• $display(a); // displays the value of a, depending

on its data type

– $stop(), $finish(): stop (break) and finish (terminate) the simulation

Direct generation of random numbers

• $urandom returns a 32-bit random unsigned number every time it is called

• Can be automatically assigned to shorter values, automatic clipping will take place:

bit a; a <= $urandom;

• To generate a random number between 0 and 59 we can use: $urandom%60 (modulus)

• Note: if not seeded, every time the testbench is run we get the same values– this behavior is required for being able to repeat tests

Direct generation of random numbers

• $urandom_range(10,0) returns a random number between 10 and 0

• Note: if not seeded, every time the testbench is run we get the same values– this behavior is required for being able to

repeat tests

• Seed can be given during elaboration

Event-driven simulation• The simulator executes in a random order any of the

operations scheduled for a given timestep.• It continues until the event queue for the timestep is

empty, then advances time to the next non-empty timestamp

• This might create race conditions:– What happens is not defined by the rules of SystemVerilog

– No error is signaled

– The behavior of the system might be simulator-dependent or even change from run to run

Race condition

initial begin #10ns; a = 1;end

initial begin #10 ns; a = 0;end

initial begin #20 ns; $display(a);end

Race condition

initial begin #10ns; a = 1;end

Value displayed is random, because two assignments to A with different values take place in different processes at timestep 10ns

Race condition

initial begin #10ns; a <= 1;end

initial begin #10 ns; a <= 0;end

Race condition

initial begin #10 ns; a <= 0;end

Value displayed is random, because two assignments to A with different values take place in different processes at timestep 10ns+delta

Race condition

Value displayed is 1, because A is assigned 0 at timestep 10ns and 1 at timestep 10ns+delta

Race condition

initial begin #10ns; a = 1; a = 0;end

Race condition

initial begin #10ns; a = 1; a = 0;end

Value displayed is 0, because, although there are two assignments to A with different values taking place at timestep 10ns, the assignments are in the same process and are therefore executed in order

Race condition

initial begin #10ns; a <= 1; a <= 0;end

Race condition

initial begin #10ns; a <= 1; a <= 0;end

Value displayed is 0, because, although there are two assignments to A with different values taking place at timestep 10ns+delta, the assignments are in the same process and are therefore executed in order

Race conditions

• Happen when two different processes try to write the same signal during the same time step

• Ways to avoid:– don’t write the same signal in different

processes, unless you really know what you do (you know that the two processes will never write the signal in the same time step)

Continuous assignments

• continuously performs an assignment

• outside procedural code

• ex: assign a = b+c;

• Note: module input/output ports “count” as continuous assignments

• can be done on variables or nets

• nets can be driven by multiple continuous assignments, variables no

Variables vs Nets

• Variables:– Are defined as: var type name– Example: var logic a (logic is default, can be

omitted)– The keyword “var” is the default, it can be

omitted– So when we define something like

• logic a;• bit [7:0] b;

we are actually defining variables

Variables vs Nets

• Variables can be assigned:– in a procedural assignment (blocking or non-

blocking assignment inside an initial or always process)

– By a single continuous assignment

How variables work

initial

#10ns a <= 1;

initial

#20ns a <= 0;

• a variable keeps the newest value that is written to it. Variable A will have value 1 between time 10 and 20, then will have value 0.

• VARIABLES HAVE NOTHING TO DO WITH VHDL VARIABLES

Variables vs Nets

• Nets:– Different types: wire, wand, wor, etc. We consider

only wire– Are defined as: wire type name– Examples: wire logic a, wire logic [2:0] c– logic is the default and can be omitted– A wire cannot be a 2-valued data type (like bit)

• A net can be assigned only by one or more continuous assignments, cannot be assigned into procedural code

Variables vs Nets

• So there is only one thing in SV that nets can do and that variables cannot: be driven by multiple continuous assignments

• Nets should be used when modeling tri-state buffers and buses

• The value is determined by a resolution function

0 1 X Z

0 0 X X 0

1 X 1 X 1

X X X X X

Z 0 1 X Z

Objects scope

• objects declared inside modules/programs:– local to that module/program

• objects declared inside blocks (ifs, loops, etc.) between a begin and an end:– local to that block of code

Subroutines

• Functions: return a value, cannot consume time

• Tasks: no return, can consume time

Functions• input/ouput/inout ports (inouts are read at the beginning and written at the end)• the keyword input is the default, no need to specify it.• cannot consume time• a function can return void (no return value)• It is allowed to have non-blocking assignments, writes to clocking drivers and other

constructs that schedule assignments for the future but don’t delay the function execution

function logic myfunc3(input int a, output int b, c);

b = a + 1;

c <= a + 2; // this schedules a write-back to C for a

// delta delay later, but does not consume time

// (the function returns in the same timestep it

// was called)

return (a==1000);

endfunction

task light (output color, input [31:0] tics);

repeat (tics)

@ (posedge clock);

color = off; // turn light off.

endtask: light

Tasks can consume time, they do not return values

Packages• type definitions, functions, etc. can be defined in packages

package ComplexPkg; typedef struct { shortreal i, r; } Complex; function Complex add(Complex a, b); add.r = a.r + b.r; add.i = a.i + b.i; endfunction function Complex mul(Complex a, b); mul.r = (a.r * b.r) - (a.i * b.i); mul.i = (a.r * b.i) + (a.i * b.r); endfunctionendpackage

Packages

• Stuff that is in package can be called as:– c <= PackageName::FunctionName(a,b)

• or• the package can be imported, then we can just

write:– c <= FunctionName(a,b);

• Importing a package is done through:– import PackageName::*;

Unpacked arrays

• bit a [5:0];• Arrays can have multiple unpacked dimensions or

can even mix packed and unpacked dimensions:• logic [7:0] c [0:2047]; // array of 2048 bytes

• Unpacked dimensions cannot be sliced, only single elements can be accessed

• They do not reside in memory in contiguous locations – they can be bigger than a packed array because of this reason

dynamic arrays

• arrays with an unpacked dimension that is not specified in the code. Defined as:– logic [7:0] b [];

• Can only be used after having been initialized with the keyword “new”– b = new[100];

Can be resized with: b = new[200](b);• After having been initialized, it can be used

like any other array with unpacked dimension

associative arrays

• associative array of bytes:– declared as logic [7:0] a [*];

• Acts exactly as a vector of 2^32 bytes:– a[4102345432] <= 8’b10000110 is legal

• Memory space is allocated only when used• Accessing the elements is slow in terms of

simulation time• If we would try to write to all locations, we would

crash everything or generate an error• Ideal to model big memories used only sparsely

queues

• logic [7:0] q[$];– Supports all operations that can be done on unpacked

arrays, plus other functions:– q.push_front(a); // pushes element to the front– q.push_back(a); // pushes element to the back– b=q.pop_back(); // pops element from back– b=q.pop_front(); // pops element from front– q.insert(3,a) // inserts element at position 3– q.delete(3) // deletes the third element– q.delete() // delete all the queue– q.size() // returns size of queue

queues

• Can also be accessed using slicing and $:• q = { q, 6 }; // q.push_back(6)• q = { e, q }; // q.push_front(e)• q = q[1:$]; // q.pop_front() or q.delete(0)• q = q[0:$-1]; // q.pop_back() or q.delete(q.size-1)• q = { q[0:pos-1], e, q[pos:$] }; // q.insert(pos, e)• q = { q[0:pos], e, q[pos+1:$] }; // q.insert(pos+1, e)• q = {}; // q.delete()

Structure

• Hierarchy is encapsulated and hidden in modules module dut (

output bit c,

input bit [7:0] a,

input bit [7:0] b);

// module code (processes, continuous assignments, instantiations of submodules)

endmodule

• There exists also legacy Verilog port declaration methods

Structure

• Verilog legacy port declarations, not recommended.

module test(a,b,c);

input logic [7:0] a;

input b; //unspecified type: logic

output bit [7:0] c;

endmodule

Structure

• Module declaration with ports

module simple_fifo ( input bit clk, input bit rst, input bit [7:0] a, output bit [7:0] b);

// module code (processes, continuous assignments, instantiations of submodules)

endmodule

Structure

• Module instantiation in a top-level testbench module tb (); // top-level testbench has no inputs/outputs bit clk, reset; bit [7:0] av, bv;

simple_fifo dut(.clk(clk), // module instantiation .rst(reset), .b(bv), .a(av));

always #5ns clk <= !clk;

initial #30ns reset <= 1;

initial begin forever #10ns av <= $random(); end

endmodule

Module instantiation

• Module instantiation in a top-level testbench• Ports can be named out of order module tb (); bit clk, reset; bit [7:0] av, bv;

simple_fifo dut(.clk(clk), // module instantiation .rst(reset), .a(av), .b(bv));

endmodule

Module Instantiation

• If signals have the same names in the including and the included modules we can use the syntax (.*) for port connection.

module tb (); bit clk, rst; bit [7:0] a, b;

simple_fifo dut(.*); // a->a; b->b; clk->clk; rst-> rst

initial #30ns rst <= 1;

initial begin forever #10ns a <= $random(); end

endmodule

Module Instantiation• positional connection, each signal is connected to

the port in the same position – easy to make errors

module tb (); // top-level testbench has no inputs/outputs bit clk, reset; bit [7:0] av, bv;

simple_fifo dut(clk,reset,av,bv);

initial #30ns rst <= 1;

initial begin forever #10ns a <= $random(); end

endmodule

Module instantiation module tb (); bit clk, reset; bit [7:0] av, bv;

simple_fifo dut(.*, // ports are connected to signals with the same name .a(av), // except the ones named later .b()); // b is left open - unconnected

endmodule

Parameters

• used to express configurabilitymodule tb (); // top-level testbench has no inputs/outputs bit clk, rst; bit [7:0] a, b;

simple_fifo #( .DEPTH(64), .WIDTH(8)) dut ( .clk(clk), .rst(rst), .a(a), .b(b));

endmodule

module #( parameter DEPTH=64, parameter WIDTH=8 ) simple_fifo ( input logic clk, input logic rst, input logic [WIDTH-1:0] a, output logic [WIDTH-1:0] b );

localparam internal_param_name;

endmodule

Parameters

• used to express configurabilitymodule tb (); // top-level testbench has no inputs/outputs bit clk, rst; bit [7:0] a, b;

endmodule

Parameters

• Positional instantiationmodule tb (); // top-level testbench has no inputs/outputs bit clk, rst; bit [7:0] a, b;

simple_fifo #(64,8) dut ( .clk(clk), .rst(rst), .a(a), .b(b));

endmodule

Parameters

• Other notationmodule tb (); // top-level testbench has no inputs/outputs bit clk, rst; bit [7:0] a, b;

endmodule

module simple_fifo ( input logic clk, input logic rst, input logic [WIDTH-1:0] a, output logic [WIDTH-1:0] b );parameter DEPTH=64parameter WIDTH=8localparam internal_param_name;…endmodule

Parameters

• If not overwritten, they keep their default value• Can have a type:

– parameter logic [7:0] DEPTH = 64;

• localparam: like parameters, but cannot be modified hierarchically during the instantiation

• Used to indicate a parameter that there is not any sense for it to be modified by some higher block in the hierarchy

Hierarchical access

• From anywhere, it is possible to access any object in the hierarchy by accessing through its full path starting from the top module name:

• a <= top.dut.subDutUnit.intObjectName;

• Using this, the testbench can monitor any internal DUT signal (white/grey box verification) without having the signal forwarded through ports

SystemVerilog Programs

• Programs behave and look like modules, but:– They cannot contain always blocks– They cannot include modules– Simulation finishes automatically when all initial have

been completed

• Always blocks are the basic building block of RTL code, but testbenches don’t need them. Using always blocks in tyestbenches leads to bad coding style

• So, programs can do everything that is needed for verification

Clocking blockdefault clocking ck1 @(posedge clk); default input #1ns output #1ns; // reading and writing skew input a; // a, b, … objects visible from this scope output b; // input: can read; output: can write output negedge rst; // overwrite the default skew to the negedge

// of the clock inout c; // inout: can both read and write input d = top.dut.internal_dut_signal_name;endclocking

• Inside a module/program, we access signals for read/write inside processes in this way:

– ck1.a <= 1’b1;– c = ck1.b; // or c <= ck1.b;– ck1.d <= e;

• A write will take place 1ns after the clock edge, a read will read the value that the signal had 1ns before the clock edge

Clocking block

• The clocking block makes the timing relation (positive-edge of the clock or other) explicit

• Since we only verify synchronous systems with a single clock, we need a single clocking block– We add only one clocking block and specify it as

“default”

• It specifies input and output skews to read/write to the signals

• The input/output skew can also be omitted

Clocking blocks and programs• The rules of SV say that if we access signals from a program through

a clocking block, there will be no race condition between testbench and DUT– Even if no timing skew is specified

• When there is a default clocking block in a program/module, we can use the ##n timing construct: wait n cycles as specified by the default clocking block– examples:

• ##3; // wait 3 cycles• ##(2*a); //wait a number of cycles equal to the double of the value of variable ainitial begin

##3 reset <= 0;forever begin

##1 a <= ~a;end

end• ##1 executed at a time instant in which there is no clock edge will

delay by a fraction of the clock cycle (wait until the first clock edge only)

generate• Used to generate processes and continuous assignments• No need of generate… endgenerate statements• Define a variable of type genvar• We can then do a generate using a loop or if with with the genvar

• Example:genvar i;

initial ##20 b <= 0;

for(i=0;i<3;i++) begin initial begin ##(i) a <= $urandom; end if (i==1) begin always @(posedge clk) begin … end endend

named blocks

• after every begin there can be an optional name• Allows hierarchical access to local variables and

to find the variables in the simulator window

– ex:initial begin : inputController

if (a==10) begin : terminationHandler

Good Testbench Structure – required for the labs

• Write all your testbenches in this way

• The testbench program must access all DUT signals for read and write through the default clocking block

• Only timing construct allowed in the testbench program: ##

top modulegeneration of clock

testbenchprogram

driveDUT inputs,

checkDUT outputs

Good Testbench Structure

• Alternative, also good for the labs: several programs are allowed• all use clocking block in links to the DUT

testbenchprogram

drive inputs

testbenchprogram

check outputs

Good testbench structure• You are free to divide programs as you like

– The one below is more UVM-like

testbenchprogram

drive some inputs

testbenchprogram

collect outputstranslate them into

a higher levelmodel

testbenchprogram

drive other inputs

programcheck outputs

Good testbench structure – required for the labs - summary

• You can use multiple programs, but given the size of the testbenches used in this course, one program that does everything is a good choice

• All inputs/outputs to the DUT through the default clocking block

• Only timing construct allowed: ## (no need for other levels of timing granularity)

• Try to keep separated the different functions (input generation, output checking) using several initial blocks

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