This is another example of a SystemVerilog testbench using OOP concepts like inheritance, polymorphism to build a functional testbench for a simple design.
Design
module switch
# (parameter ADDR_WIDTH = 8,
parameter DATA_WIDTH = 16,
parameter ADDR_DIV = 8'h3F
)
( input clk,
input rstn,
input vld,
input [ADDR_WIDTH-1:0] addr,
input [DATA_WIDTH-1:0] data,
output reg [ADDR_WIDTH-1:0] addr_a,
output reg [DATA_WIDTH-1:0] data_a,
output reg [ADDR_WIDTH-1:0] addr_b,
output reg [DATA_WIDTH-1:0] data_b
);
always @ (posedge clk) begin
if (!rstn) begin
addr_a <= 0;
data_a <= 0;
addr_b <= 0;
data_b <= 0; end else begin if (vld) begin if (addr >= 0 & addr <= ADDR_DIV) begin
addr_a <= addr;
data_a <= data;
addr_b <= 0;
data_b <= 0;
end else begin
addr_a <= 0;
data_a <= 0;
addr_b <= addr;
data_b <= data;
end
end
end
end
endmodule
Transaction Object
// This is the base transaction object that will be used
// in the environment to initiate new transactions and
// capture transactions at DUT interface
class switch_item;
rand bit [7:0] addr;
rand bit [15:0] data;
bit [7:0] addr_a;
bit [15:0] data_a;
bit [7:0] addr_b;
bit [15:0] data_b;
// This function allows us to print contents of the data
// packet so that it is easier to track in a logfile
function void print (string tag="");
$display ("T=%0t %s addr=0x%0h data=0x%0h addr_a=0x%0h data_a=0x%0h addr_b=0x%0h data_b=0x%0h",
$time, tag, addr, data, addr_a, data_a, addr_b, data_b);
endfunction
endclass
Generator
// The generator class is used to generate a random
// number of transactions with random addresses and data
// that can be driven to the design
class generator;
mailbox drv_mbx;
event drv_done;
int num = 20;
task run();
for (int i = 0; i < num; i++) begin
switch_item item = new;
item.randomize();
$display ("T=%0t [Generator] Loop:%0d/%0d create next item", $time, i+1, num);
drv_mbx.put(item);
@(drv_done);
end
$display ("T=%0t [Generator] Done generation of %0d items", $time, num);
endtask
endclass
Driver
// The driver is responsible for driving transactions to the DUT
// All it does is to get a transaction from the mailbox if it is
// available and drive it out into the DUT interface.
class driver;
virtual switch_if vif;
event drv_done;
mailbox drv_mbx;
task run();
$display ("T=%0t [Driver] starting ...", $time);
@ (posedge vif.clk);
// Try to get a new transaction every time and then assign
// packet contents to the interface. But do this only if the
// design is ready to accept new transactions
forever begin
switch_item item;
$display ("T=%0t [Driver] waiting for item ...", $time);
drv_mbx.get(item);
item.print("Driver");
vif.vld <= 1;
vif.addr <= item.addr;
vif.data <= item.data;
// When transfer is over, raise the done event
@ (posedge vif.clk);
vif.vld <= 0; ->drv_done;
end
endtask
endclass
Monitor
// The monitor has a virtual interface handle with which
// it can monitor the events happening on the interface.
// It sees new transactions and then captures information
// into a packet and sends it to the scoreboard
// using another mailbox.
class monitor;
virtual switch_if vif;
mailbox scb_mbx;
semaphore sema4;
function new ();
sema4 = new(1);
endfunction
task run();
$display ("T=%0t [Monitor] starting ...", $time);
// To get a pipeline effect of transfers, fork two threads
// where each thread uses a semaphore for the address phase
fork
sample_port("Thread0");
sample_port("Thread1");
join
endtask
task sample_port(string tag="");
// This task monitors the interface for a complete
// transaction and pushes into the mailbox when the
// transaction is complete
forever begin
@(posedge vif.clk);
if (vif.rstn & vif.vld) begin
switch_item item = new;
sema4.get();
item.addr = vif.addr;
item.data = vif.data;
$display("T=%0t [Monitor] %s First part over",
$time, tag);
@(posedge vif.clk);
sema4.put();
item.addr_a = vif.addr_a;
item.data_a = vif.data_a;
item.addr_b = vif.addr_b;
item.data_b = vif.data_b;
$display("T=%0t [Monitor] %s Second part over",
$time, tag);
scb_mbx.put(item);
item.print({"Monitor_", tag});
end
end
endtask
endclass
Scoreboard
// The scoreboard is responsible to check data integrity. Since
// the design routes packets based on an address range, the
// scoreboard checks that the packet's address is within valid
// range.
class scoreboard;
mailbox scb_mbx;
task run();
forever begin
switch_item item;
scb_mbx.get(item);
if (item.addr inside {[0:'h3f]}) begin
if (item.addr_a != item.addr | item.data_a != item.data)
$display ("T=%0t [Scoreboard] ERROR! Mismatch addr=0x%0h data=0x%0h addr_a=0x%0h data_a=0x%0h", $time, item.addr, item.data, item.addr_a, item.data_a);
else
$display ("T=%0t [Scoreboard] PASS! Mismatch addr=0x%0h data=0x%0h addr_a=0x%0h data_a=0x%0h", $time, item.addr, item.data, item.addr_a, item.data_a);
end else begin
if (item.addr_b != item.addr | item.data_b != item.data)
$display ("T=%0t [Scoreboard] ERROR! Mismatch addr=0x%0h data=0x%0h addr_b=0x%0h data_b=0x%0h", $time, item.addr, item.data, item.addr_b, item.data_b);
else
$display ("T=%0t [Scoreboard] PASS! Mismatch addr=0x%0h data=0x%0h addr_b=0x%0h data_b=0x%0h", $time, item.addr, item.data, item.addr_b, item.data_b);
end
end
endtask
endclass
Environment
// The environment is a container object simply to hold
// all verification components together. This environment can
// then be reused later and all components in it would be
// automatically connected and available for use
class env;
driver d0; // Driver handle
monitor m0; // Monitor handle
generator g0; // Generator Handle
scoreboard s0; // Scoreboard handle
mailbox drv_mbx; // Connect GEN -> DRV
mailbox scb_mbx; // Connect MON -> SCB
event drv_done; // Indicates when driver is done
virtual switch_if vif; // Virtual interface handle
function new();
d0 = new;
m0 = new;
g0 = new;
s0 = new;
drv_mbx = new();
scb_mbx = new();
d0.drv_mbx = drv_mbx;
g0.drv_mbx = drv_mbx;
m0.scb_mbx = scb_mbx;
s0.scb_mbx = scb_mbx;
d0.drv_done = drv_done;
g0.drv_done = drv_done;
endfunction
virtual task run();
d0.vif = vif;
m0.vif = vif;
fork
d0.run();
m0.run();
g0.run();
s0.run();
join_any
endtask
endclass
Test
// Test class instantiates the environment and starts it.
class test;
env e0;
function new();
e0 = new;
endfunction
task run();
e0.run();
endtask
endclass
Interface
// Design interface used to monitor activity and capture/drive
// transactions
interface switch_if (input bit clk);
logic rstn;
logic vld;
logic [7:0] addr;
logic [15:0] data;
logic [7:0] addr_a;
logic [15:0] data_a;
logic [7:0] addr_b;
logic [15:0] data_b;
endinterface
Testbench Top
// Top level testbench module to instantiate design, interface
// start clocks and run the test
module tb;
reg clk;
always #10 clk =~ clk;
switch_if _if (clk);
switch u0 ( .clk(clk),
.rstn(_if.rstn),
.addr(_if.addr),
.data(_if.data),
.vld (_if.vld),
.addr_a(_if.addr_a),
.data_a(_if.data_a),
.addr_b(_if.addr_b),
.data_b(_if.data_b));
test t0;
initial begin
{clk, _if.rstn} <= 0;
// Apply reset and start stimulus
#20 _if.rstn <= 1;
t0 = new;
t0.e0.vif = _if;
t0.run();
// Because multiple components and clock are running
// in the background, we need to call $finish explicitly
#50 $finish;
end
// System tasks to dump VCD waveform file
initial begin
$dumpvars;
$dumpfile ("dump.vcd");
end
endmodule