Correct Methods For Adding Delays To Verilog Behavioral Models

Abstract

Design engineers frequently build Verilog models with behavioral delays. Most hardware description languages permit a wide variety of delay coding styles but very few of the permitted coding styles actually model realistic hardware delays. Some of the most common delay modeling styles are very poor representations of real hardware. This paper examines commonly used delay modeling styles and indicates which styles behave like real hardware, and which do not.

Full text

HDLCON 1999 1 Correct Methods For Adding Delays
Rev 1.1 To Verilog Behavioral Models
Correct Methods For Adding Delays To Verilog Behavioral Models
Clifford E. Cummings
Sunburst Design, Inc.
15870 SW Breccia Drive
Beaverton, OR 97007
cliffc@sunburst-design.com
Abstract
Design engineers frequently build Verilog models
with behavioral delays. Most hardware description
languages permit a wide variety of delay coding styles but
very few of the permitted coding styles actually model
realistic hardware delays. Some of the most common
delay modeling styles are very poor representations of
real hardware. This paper examines commonly used
delay modeling styles and indicates which styles behave
like real hardware, and which do not.
1.0 Introduction
One of the most common behavioral Verilog coding
styles used to model combinational logic is to place
delays to the left of blocking procedural assignments
inside of an always block. This coding style is flawed as it
can either easily produce the wrong output value or can
propagate inputs to an output in less time than permitted
by the model specifications.
This paper details delay-modeling styles using
continuous assignments with delays, and procedural
assignments using blocking and nonblocking assignments
with delays on either side of the assignment operator.
To help understand delay modeling, the next section
also includes a short description on inertial and transport
delays, and Verilog command line switches that are
commonly used to simulate a model that is neither a fully
inertial-delay model nor a fully transport-delay model.
2.0 Inertial and transport delay modeling
Inertial delay models only propagate signals to an
output after the input signals have remained unchanged
(been stable) for a time period equal to or greater than the
propagation delay of the model. If the time between two
input changes is shorter than a procedural assignment
delay, a continuous assignment delay, or gate delay, a
previously scheduled but unrealized output event is
replaced with a newly scheduled output event.
Transport delay models propagate all signals to an
output after any input signals change. Scheduled output
value changes are queued for transport delay models.
Reject & Error delay models propagate all signals
that are greater than the error setting, propagate unknown
values for signals that fall between the reject & error
settings, and do not propagate signals that fall below the
reject setting.
For most Verilog simulators, reject and error settings
are specified as a percentage of propagation delay in
multiples of 10%.
Pure inertial delay example using reject/error switches.
Add the Verilog command line options:
+pulse_r/100 +pulse_e/100
reject all pulses less than 100% of propagation delay.
Pure transport delay example using reject/error switches.
Add the Verilog command line options:
+pulse_r/0 +pulse_e/0
pass all pulses greater than 0% of propagation delay.
Semi-realistic delay example using reject/error switches.
Add the Verilog command line options:
+pulse_r/30 +pulse_e/70
reject pulses less than 30%, propagate unknowns for
pulses between 30-70% and pass all pulses greater
than 70% of propagation delay.

HDLCON 1999 2 Correct Methods For Adding Delays
Rev 1.1 To Verilog Behavioral Models
3.0 Blocking assignment delay models
Adding delays to the left-hand-side (LHS) or right-
hand-side (RHS) of blocking assignments (as shown in
Figure 1) to model combinational logic is very common
among new and even experienced Verilog users, but the
practice is flawed.
For the adder_t1 example shown in Figure 2, the
outputs should be updated 12ns after input changes. If the
a input changes at time 15 as shown in Figure 3, then if
the a, b and ci inputs all change during the next 9ns, the
outputs will be updated with the latest values of a, b and
ci. This modeling style has just permitted the ci input to
propagate a value to the sum and carry outputs after
only 3ns instead of the required 12ns propagation delay.
Adding delays to the left hand side (LHS) of any
sequence of blocking assignments to model
combinational logic is also flawed.
The adder_t7a example shown in Figure 4 places
the delay on the first blocking assignment and no delay on
the second assignment. This will have the same flawed
behavior as the adder_t1 example.
The adder_t7b example, also shown in Figure 4,
places the delay on the second blocking assignment and
no delay on the first. This model will sample the inputs on
the first input change and assign the outputs to a
temporary location until after completion of the specified
blocking delay. Then the outputs will be written with the
old temporary output values that are no longer valid.
Other input changes within the 12ns delay period will not
be evaluated, which means old erroneous values will
remain on the outputs until more input changes occur.
module adder_t1 (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
always @(a or b or ci)
#12 {co, sum} = a + b + ci;
endmodule
Figure 2 - LHS Blocking Assignment
Figure 3 - Waveforms for adder_t1.v Example
03612 15 17 19 21 24 27 29 31 33
A
2
F
3
1
a
b
ci
sum
co
0
0
3
1
0
0
0
X
X
Output changes only 3ns
after last input change
Trigger the
always block
always @(a)
y = ~a;
always @(a)
#5 y = ~a;
always @(a)
y = #5 ~a;
Figure 1 - Blocking Assignments with Delays
Procedural blocking
assignment - no delay
Procedural blocking
assignment - LHS delay
Procedural blocking
assignment - RHS delay

HDLCON 1999 3 Correct Methods For Adding Delays
Rev 1.1 To Verilog Behavioral Models
These adders do not model any known hardware.
Modeling Guideline: do not place delays on the LHS of
blocking assignments to model combinational logic. This
is a bad coding style.
Testbench Guideline: placing delays on the LHS of
blocking assignments in a testbench is reasonable since
the delay is just being used to time-space sequential input
stimulus events.
3.1 RHS blocking delays
Adding delays to the right hand side (RHS) of
blocking assignments to model combinational logic is
also flawed.
For the adder_t6 example shown in Figure 5, the
outputs should be updated 12ns after input changes. If the
a input changes at time 15, the RHS input values will be
sampled and the outputs will be updated with the sampled
value, while all other a, b and ci input changes during
the next 12ns will not be evaluated. This means old
erroneous values will remain on the outputs until more
input changes occur.
The same problem exists with multiple blocking
assignments when delays are placed on the RHS of the
assignment statements. The adder_t11a and
adder_t11b examples shown in Figure 6 demonstrate
the same flawed behavior as the adder_t6 example .
None of the adder examples with delays on the RHS
of blocking assignments behave like any known
hardware.
Modeling Guideline: do not place delays on the RHS of
blocking assignments to model combinational logic. This
is a bad coding style.
Testbench Guideline: do not place delays on the RHS of
blocking assignments in a testbench.
General Guideline: placing a delay on the RHS of any
blocking assignment is both confusing and a poor coding
style. This Verilog coding practice should be avoided.
module adder_t11a (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
reg [4:0] tmp;
always @(a or b or ci) begin
tmp = #12 a + b + ci;
{co, sum} = tmp;
end
endmodule
module adder_t11b (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
reg [4:0] tmp;
always @(a or b or ci) begin
tmp = a + b + ci;
{co, sum} = #12 tmp;
end
endmodule
Figure 6 - Multiple RHS Blocking Assignments
module adder_t7a (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
reg [4:0] tmp;
always @(a or b or ci) begin
#12 tmp = a + b + ci;
{co, sum} = tmp;
end
endmodule
module adder_t7b (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
reg [4:0] tmp;
always @(a or b or ci) begin
tmp = a + b + ci;
#12 {co, sum} = tmp;
end
endmodule
Figure 4 - Multiple LHS Blocking Assignments
module adder_t6 (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
always @(a or b or ci)
{co, sum} = #12 a + b + ci;
endmodule
Figure 5 - RHS Blocking Assignment

HDLCON 1999 4 Correct Methods For Adding Delays
Rev 1.1 To Verilog Behavioral Models
4.0 Nonblocking assignment delay models
Adding delays to the left-hand-side (LHS) of
nonblocking assignments (as shown in Figure 7) to model
combinational logic is flawed.
The same problem exists in the adder_t2 example
shown in Figure 8 (nonblocking assignments) that existed
in the adder_t1 example shown in Figure 2 (blocking
assignments). If the a input changes at time 15, then if the
a, b and ci inputs all change during the next 9ns, the
outputs will be updated with the latest values of a, b and
ci. This modeling style permitted the ci input to
propagate a value to the sum and carry outputs after
only 3ns instead of the required 12ns propagation delay.
It can similarly be shown that adding delays to the
left hand side (LHS) of any sequence of nonblocking
assignments to model combinational logic is also flawed.
Adders modeled with LHS nonblocking assignments
do not model any known hardware.
Modeling Guideline: do not place delays on the LHS of
nonblocking assignments to model combinational logic.
This is a bad coding style.
Testbench Guideline: nonblocking assignments are less
efficient to simulate than blocking assignments; therefore,
in general, placing delays on the LHS of nonblocking
assignments for either modeling or testbench generation is
discouraged.
4.1 RHS nonblocking delays
Adding delays to the right hand side (RHS) of
nonblocking assignments (as shown in Figure 9) will
accurately model combinational logic with transport
delays.
In the adder_t3 example shown in Figure 9, if the
a input changes at time 15 as shown in Figure 10 (next
page), then all inputs will be evaluated and new output
values will be queued for assignment 12ns later.
Immediately after the outputs have been queued
(scheduled for future assignment) but not yet assigned,
the always block will again be setup to trigger on the next
input event. This means that all input events will queue
new values to be placed on the outputs after a 12ns delay.
This coding style models combinational logic with
transport delays.
Recommended Application: Use this coding style to
model behavioral delay-line logic.
Modeling Guideline: place delays on the RHS of
nonblocking assignments only when trying to model
transport output-propagation behavior. This coding style
will accurately model delay lines and combinational logic
with pure transport delays; however, this coding style
generally causes slower simulations.
Testbench Guideline: This coding style is often used in
testbenches when stimulus must be scheduled on future
clock edges or after a set delay, while not blocking the
assignment of subsequent stimulus events in the same
procedural block.
always @(a)
y <= ~a;
always @(a)
#5 y <= ~a;
always @(a)
y <= #5 ~a;
Figure 7 - Nonblocking Assignments with Delays
Procedural nonblocking
assignment - no delay
Procedural nonblocking
assignment - LHS delay
Procedural nonblocking
assignment - RHS delay
module adder_t2 (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
always @(a or b or ci)
#12 {co, sum} <= a + b + ci;
endmodule
Figure 8 - LHS Nonblocking Assignment
module adder_t3 (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
always @(a or b or ci)
{co, sum} <= #12 a + b + ci;
endmodule
Figure 9 - RHS Nonblocking Assignment

HDLCON 1999 5 Correct Methods For Adding Delays
Rev 1.1 To Verilog Behavioral Models
4.2 Multiple RHS nonblocking delays
Adding delays to the right hand side (RHS) of
multiple sequential nonblocking assignments to model
combinational logic is flawed, unless all of the RHS input
identifiers are listed in the sensitivity list, including
intermediate temporary values that are only assigned and
used inside the always block, as shown in Figure 11.
For the adder_t9c and adder_t9d examples
shown in Figure 11, the nonblocking assignments are
executed in parallel and after tmp is updated, since tmp
is in the sensitivity list, the always block will again be
triggered, evaluate the RHS equations and update the
LHS equations with the correct values (on the second pass
through the always block).
Modeling Guideline: in general, do not place delays on
the RHS of nonblocking assignments to model
combinational logic. This coding style can be confusing
and is not very simulation efficient. It is a common and
sometimes useful practice to place delays on the RHS of
nonblocking assignments to model clock-to-output
behavior on sequential logic.
Testbench Guideline: there are some multi-clock design
verification suites that benefit from using multiple
nonblocking assignments with RHS delays; however, this
coding style can be confusing, therefore placing delays on
the RHS of nonblocking assignments in testbenches is not
generally recommended.
module adder_t9c (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
reg [4:0] tmp;
always @(a or b or ci or tmp) begin
tmp <= #12 a + b + ci;
{co, sum} <= tmp;
end
endmodule
module adder_t9d (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg co;
reg [3:0] sum;
reg [4:0] tmp;
always @(a or b or ci or tmp) begin
tmp <= a + b + ci;
{co, sum} <= #12 tmp;
end
endmodule
Figure 11 - Multiple Nonblocking Assignments
with Delays
Figure 10 - Waveforms for adder_t3 Example
03612 15 17 19 21 24 27 29 31 33
A
2
F
3
0
a
b
ci
0
0
X
X
sum
co
0
0
2
1
5
D
A
Trigger the
always block

HDLCON 1999 6 Correct Methods For Adding Delays
Rev 1.1 To Verilog Behavioral Models
5.0 Continuous assignment delay models
Adding delays to continuous assignments (as shown
in Figure 12) accurately models combinational logic with
inertial delays and is a recommended coding style.
For the adder_t4 example shown in Figure 13, the
outputs do not change until 12ns after the last input
change (12ns after all inputs have been stable). Any
sequence of input changes that occur less than 12ns apart
will cause any future scheduled output-event (output
value with corresponding assignment time) to be replaced
with a new output-event.
Figure 14 shows the output waveforms for a
simulation run on the adder_t4 code shown in Figure
13. The first a-input change occurs at time 15, which
causes an output event to be scheduled for time 27, but a
change on the b-input and two more changes on the a-
input at times 17, 19 and 21 respectively, cause three new
output events to be scheduled. Only the last output event
actually completes and the outputs are assigned at time
33. Continuous assignments do not "queue up" output
assignments, they only keep track of the next output value
and when it will occur; therefore, continuous assignments
model combinational logic with inertial delays.
module adder_t4 (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
assign #12 {co, sum} = a + b + ci;
endmodule
Figure 13 - Continuous Assignment with Delay
assign y = ~a;
assign #5 y = ~a;
assign y = #5 ~a;
assign y <= ~a;
Figure 12 - Continuous Assignments with Delays
Continuous
assignment - no delay
Continuous
assignment - LHS delay
Illegal continuous
assignment - RHS delay
Illegal continuous
nonblocking assignment
Figure 14 - Waveforms for adder_t4.v Example
03612 15 17 19 21 24 27 29 31 33
A
2
F
3
0
a
b
ci
0
0
X
X
sum
co
0
0
2
1
Trigger the
assign statement

HDLCON 1999 7 Correct Methods For Adding Delays
Rev 1.1 To Verilog Behavioral Models
5.1 Multiple continuous assignments
It can similarly be shown that modeling logic
functionality by adding delays to continuous assignments,
whose outputs are used to drive the inputs of other
continuous assignments with delays, as shown in Figure
15, also accurately models combinational logic with
inertial delays.
5.2 Mixed no-delay always blocks and
continuous assignments
Modeling logic functionality in an always block with
no delays, then passing the always block intermediate
values to a continuous assignment with delays as, shown
in Figure 16, will accurately model combinational logic
with inertial delays.
For the adder_t5 example shown in Figure 16, the
tmp variable is updated after any and all input events.
The continuous assignment outputs do not change until
12ns after the last change on the tmp variable. Any
sequence of always block input changes will cause tmp to
change, which will cause a new output event on to be
scheduled on the continuous assignment outputs. The
continuous assignment outputs will not be updated until
tmp remains unchanged for 12ns. This coding style
models combinational logic with inertial delays.
Modeling Guideline: Use continuous assignments with
delays to model simple combinational logic. This coding
style will accurately model combinational logic with
inertial delays.
Modeling Guideline: Use always blocks with no delays
to model complex combinational logic that are more
easily rendered using Verilog behavioral constructs such
as "case-casez-casex", "if-else", etc. The outputs from the
no-delay always blocks can be driven into continuous
assignments to apply behavioral delays to the models.
This coding style will accurately model complex
combinational logic with inertial delays.
Testbench Guideline: Continuous assignments can be
used anywhere in a testbench to drive stimulus values
onto input ports and bi-directional ports of instantiated
models.
6.0 Conclusions
Any delay added to statements inside of an always
block does not accurately model the behavior of real
hardware and should not be done. The one exception is to
carefully add delays to the right hand side of nonblocking
assignments, which will accurately model transport
delays, generally at the cost of simulator performance.
Adding delays to any sequence of continuous
assignments, or modeling complex logic with no delays
inside of an always block and driving the always block
outputs through continuous assignments with delays, both
accurately model inertial delays and are recommended
coding styles for modeling combinational logic.
Author & Contact Information
Cliff Cummings, President of Sunburst Design, Inc., is an
independent EDA consultant and trainer with 19 years of
ASIC, FPGA and system design experience and nine
years of Verilog, synthesis and methodology training
experience.
module adder_t10a (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
wire [4:0] tmp;
assign tmp = a + b + ci;
assign #12 {co, sum} = tmp;
endmodule
module adder_t10b (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
wire [4:0] tmp;
assign #12 tmp = a + b + ci;
assign {co, sum} = tmp;
endmodule
Figure 15 - Multiple Continuous Assignments
module adder_t5 (co, sum, a, b, ci);
output co;
output [3:0] sum;
input [3:0] a, b;
input ci;
reg [4:0] tmp;
always @(a or b or ci) begin
tmp = a + b + ci;
end
assign #12 {co, sum} = tmp;
endmodule
Figure 16 - No-Delay Always Block & Continuous
Assignment

HDLCON 1999 8 Correct Methods For Adding Delays
Rev 1.1 To Verilog Behavioral Models
Mr. Cummings, a member of the IEEE 1364 Verilog
Standards Group (VSG) since 1994, chaired the VSG
Behavioral Task Force, which was charged with
proposing enhancements to the Verilog language. Mr.
Cummings is also a member of the IEEE Verilog
Synthesis Interoperability Working Group.
Mr. Cummings holds a BSEE from Brigham Young
University and an MSEE from Oregon State University.
E-mail Address: cliffc@sunburst-design.com
This paper can be downloaded from the web site:
www.sunburst-design.com/papers
(Data accurate as of March 7th, 2001)