nonblocking assignment continuous assignment scheduling semantics code coverage

1. Introduction to Verilog HDL
Jyun-Kai HuSeptember 11, 2020

2. Outline
 Verilog Semantic Introduction
 Continuous assignment
 Nonblocking assignment
 Simulation by VCS
 Code Coverage
2

3. Language Relationship
Verilog
SystemVerilog
UVM
class librarySynthesizable
SystemVerilog
Synthesizable
Verilog
3

4. Basic Syntax
4
1 module Top;
2 initial $display(“hello, world");
3 endmodule
this will be executed at program startup
like C’s printf, but automatically append newline character
uninstantiated module as top-level module

5. Value Set
5
 0 – logic zero
 1 – logic one
 x – unknown value
 both true & false, don’t care (in some condition)
 z – high impedance state
 neither true or false, open circuit

6. Event Control
6
 blocked until event occurs
 @a $display(a);
 call display by any value change in a
 @(posedge clk) $display(a);
 call display by positive edge on clk
to
from 0 1 x z
0 no pos pos pos
1 neg no neg neg
x neg pos no no
z neg pos no no

7. Always Construct
7
Basically, a, b, c have the same behavior
But assignment at time zero is unspecified
1 initial while (1) @u a = u;
2
3 initial forever @u b = u;
4
5 always @u c = u;
6
7 always d = u;
unintentional looping

8. Non-Constant Data Type
8
 Variable
 reg, integer, real, time…
 assigned only in procedure block or declaration
 initial, always, task, function block
 Net
 wire, tri, supply1…
 continuous assigned only
 net declaration assignment, port connection…
 have drive strength

9. Multiple Assignment
9
0 1 x z
0 0 x x 0
1 x 1 x 1
x x x x x
z 0 1 x z
1 reg a;
2 wire b;
3
4 wire #1 u = 0;
5 wire #1 v = 1;
6
7 always @u a = u;
8 always @v a = v;
9
10 assign b = u;
11 assign b = v;
1 unit delay, which is specified by timescale
result in 0 or 1 after 1 unit delay
(race condition)
result in unknown x after 1 unit delay
(multiple driven)
multiple driven on wire

10. Registers
10
 Typically composed of D flip-flops
 capture signal at the rising edge of the clock
 Not confused with the data type “reg“

11. 1 module DFFR (
2 input CLK,
3 input D,
4 input R,
5 output reg Q
6 );
7 always @ (posedge CLK, negedge R)
8 if (!R)
9 Q = 0;
10 else
11 Q = D;
12 endmodule
D Flip-Flop
11
same as “or”
does that describe D flip-flop correctly?
implicit type wire

12. Shift Registers
12
1 module DFFR (
2 input CLK,
3 input D,
4 input R,
5 output reg Q
6 );
7 always @ (posedge CLK, negedge R)
8 if (!R)
9 Q = 0;
10 else
11 Q = D;
12 endmodule
1 module Testbench;
2 reg clk = 0,
3 d1 = 0;
4
5 DFFR dffr1 (
6 .CLK(clk),
7 .R(1'b1),
8 .D(d1),
9 .Q(q1)
10 );
11
12 DFFR dffr2 (
13 .CLK(clk),
14 .R(1'b1),
15 .D(q1),
16 .Q(q2)
17 );
18
19 always #1 clk = !clk;
20 initial #10 $finish;
21 endmodule
1 module DFFR (
2 input CLK,
3 input D,
4 input R,
5 output reg Q
6 );
7 always @ (posedge CLK, negedge R)
8 if (!R)
9 Q = 0;
10 else
11 Q = D;
12 endmodule
race condition!

13. Nonblocking Assignment
13
1 module DFFR (
2 input CLK,
3 input D,
4 input R,
5 output reg Q
6 );
7 always @ (posedge CLK, negedge R)
8 if (!R)
9 Q <= 0;
10 else
11 Q <= D;
12 endmodule
1 module Testbench;
2 reg clk = 0,
3 d1 = 0;
4
5 DFFR dffr1 (
6 .CLK(clk),
7 .R(1'b1),
8 .D(d1),
9 .Q(q1)
10 );
11
12 DFFR dffr2 (
13 .CLK(clk),
14 .R(1'b1),
15 .D(q1),
16 .Q(q2)
17 );
18
19 always #1 clk = !clk;
20 initial #10 $finish;
21 endmodule
1 module DFFR (
2 input CLK,
3 input D,
4 input R,
5 output reg Q
6 );
7 always @ (posedge CLK, negedge R)
8 if (!R)
9 Q <= 0;
10 else
11 Q <= D;
12 endmodule
evaluate both hand side immediately
but assign at the end of the time step

14. Stratified Event Queue
 4 event queues at the current simulation time
 Active
 Inactive
 #0, tf_synchronize, vpi_register_cb
 Nonblocking assign update
 <=
 Monitor
 $monitor, $strobe
14

15. Scheduling Algorithm
15

16. Simulation by VCS

17. Synopsys VCS Tool Chain
17

18. Separate Compilation
18
compile
time
elaboration
time
run
time
1 module Module;
2 reg in;
3 wire out;
4
5 SubModule subModule (
6 .in(in),
7 .out(out)
8 );
9
10 initial in = some_function();
11 endmodule
module, task, function will be resolved in elaboration time
i.e. the prototype can be unavailable during compile time
this makes separate compilation possible
incremental compilation isn’t based on timestamp in VCS

19. Interact with Your Simulation
 Compile time
 `define FNAME “vec1”
 vcs +define+FNAME=“vec2” …
 Elaboration time
 parameter IDX_FNAME = 1
 vcs –pvalue+<hier>.FNAME=2 …
 Run time
 $value$plusargs(“FNAME=%s”, FNAME)
 ./simv +FNAME=vec2
 $fscanf(“%s”, FNAME)
19

20. VCS Code Coverage
 Line coverage
 100% coverage is required
 Toggle coverage
 Branch coverage
 Condition coverage
 FSM coverage
20

21. Toggle Coverage
 Monitor transition on each bit
 0 -> 1, 1 -> 0
 x & z are ignored
 0 -> x -> 1 equals 0 -> 1
21

22. Verilog Branch
22
1 reg v;
2 reg [2:0] m = 3'b010,
3 n = 3'b110,
4 a, b, c;
5
6
7 if (v)
8 a = m;
9 else
10 a = n;
11
12
13 case (v)
14 1'b0:
15 b = m;
16 1'b1:
17 b = n;
18 endcase
19
20
21 c = v ? m : n;
v a b c
0 3’b110
1 3’b010
x or z 3’b110 old value 3’bx10

23. Branch Coverage
 Monitor scenarios
 if, case, casex, casez, ?:
23
a b c
0 1 -
0 0/x/z -
1 - 0
1 - 1
default - -
only 5 cases need to be covered
1 reg a, b, c;
2
3 case (a)
4 0:
5 if (b)
6 // do something
7 1:
8 c = c ? a : b;
9 endcase

24. Condition Coverage
 Monitor boolean formula
 ==, !=
 &&, ||
 &, |, ^, ~^  scalar-only
 vector condition
24
a b C
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
a b C
0 1 1
1 0 1
1 1 0
1 1 1
a
0
1
b
0
1
c
0
1
basic std allvectors
a && b && c

25. FSM coverage
 Monitor finite-state machine
 transition
 S0->S0, S0->S1, S1->S0
25
1 module FSM #(
2 parameter S0 = 0,
3 S1 = 1
4 ) (
5 input clk,
6 rstN,
7 in,
8 output reg state
9 );
10 always @ (posedge clk, negedge rstN)
11 if (!rstN)
12 state <= S0;
13 else case (state)
14 S0:
15 if (in)
16 state <= S1;
17 S1:
18 state <= S0;
19 endcase
20 endmodule

26. Coverage Analysis
26
we need to add test case to cover the
condition SEN = 0 & SRST = 1 in the
instance tb.scmCEDS