Verilog HDL Verification

VerificationVerification
Gookyi Dennis A. N.Gookyi Dennis A. N.
October.07.2014

ContentsContents
 Static Timing Analysis
2

Static Timing AnalysisStatic Timing Analysis
 Problems with DTA:
Poses a bottleneck for large complex designs because
simulation requires a lot of time execute
Relies on the quality and coverage of testbenches
Difficult to figure out critical paths simulations
3

Fundamentals of STAFundamentals of STA
 STA determines if a circuit meets timing
constraints without having to simulate the design
in a cycle to cycle manner
 It computes the delay for each path of the design
and therefore the critical path can easily be found
 To perform STA, here are some assumptions made:
No combinational feedback loops are allowed
All register feedback paths must be broken
 STA is used to verify timing specification but not
the functionality of the design
4

Fundamentals of STAFundamentals of STA
 In STA, designs are broken into sets of signal
paths where each path has a start and an endpoint
 4 paths are defined in STA:
5
Path Description
Entry Starts at an input port and ends at the data input of a register
Stage Starts at the clock input to a register and ends at the data input to another register
Exit Starts at the clock input to a register and ends at an output port
Pad-to-pad Starts at an input port and ends at an output port
D Q D Q
Combinational
logic
Combinational
logic
Combinational
logic
Combinational
logic
Clock
Entry path
Pad-to-pad path
Stage path Exit path

Timing SpecificationsTiming Specifications
 The most used timing constraints are divided into:
Port related constraints:
 Input delay (offset-in)
 Output delay (offset-out)
 Input-output (pad to pad)
Clock related constraints:
 Clock period
 Setup time
 Hold time
6

Port-Related ConstraintsPort-Related Constraints
 The various port related constraints include:
7
Delay Description
Input Specifies the arrival time of the input signal relative to active edge of
the clock
Output Specifies the latest time that a signal from the output of a register may
reach may reach the output port
Input-Output Applies to the path from an input port to an output port without passing
through any register
D Q D Q
Combinational
logic
Combinational
logic
Combinational
logic
Combinational
logic
Clock
Input delay Offset in
Offset out Output delay

Clock Related ConstraintsClock Related Constraints
 Before defining the various constraints, the
factors that cause uncertainty of a clock signal
should be explored:
8
Factor Description
Clock jitter Clock period may shrink or expand on a cycle-by-cycle basis
Clock-to-Q delay Max propagation delay from clock edge (sampling edge) to new
value of Q output
Input capacitance Gate capacitance of both NMOS and PMOS transistors
Interconnect capacitance Parasitic capacitances of interconnect between any two nodes
Self-loading capacitance Output capacitance of both NMOS and PMOS transistors
Clock skew Misalignment of clock edges in a synchronous system circuit
temperature Affects currents of both NMOS and PMOS transistors
D Q D Q
Input capacitance
Self-loading capacitance
interconnect capacitance
Tq

Clock Related ConstraintsClock Related Constraints
 The clock related constraints include;
Clock period: applies to the path between regs and
specifies the max period of the clock of a
synchronous circuit
Setup time: amount of time that data must be stable
before the active clock transition
Hold time: amount of time data must remain stable
after the active clock transition
9

Timing AnalysisTiming Analysis
 After timing specification is given, STA can
proceed to measure the critical path
 Critical path is the path with the longest
propagation delay in the design
 Formally, critical path has a negative or smallest
slack time
slack time = required time – arrival time
 Critical path limits system performance
10

Timing ExceptionsTiming Exceptions
 Timing analysis tools usually treat all paths in
the design as single cycle by default before
performing STA
 In most designs, there are paths that exhibit
timing exceptions
 Two common timing exceptions include
False paths
Multi-cycle paths
11

False PathFalse Path
 A false path is identified as a timing path that
does not actually propagate a signal
 As shown in the diagram below, the path indicated
is never activated
12
1
0
1
0
50 ns
100 ns
100 ns
50 ns
False path

Multi-Cycle pathMulti-Cycle path
 In multi-cycle path, data may take more than one
clock cycle to reach their destination
 It is required to indicate which paths are multi-
cycle paths in other to avoid false results from
the STA
13

Multi-Cycle pathMulti-Cycle path
 Example of multi-cycle path (code and testbench)
Variable qout_a is updated every clk cycle
Variable qout_b is updated every two clk cycles
Variable qout_c is updated every three clk cycles
14

Multi-Cycle PathMulti-Cycle Path
 Testbench and waveform:
15

Multi-Cycle PathMulti-Cycle Path
 RTL schematic
16

Value Change Dump (VCD) FilesValue Change Dump (VCD) Files
 A VCD file is a text file that contains information
about value changes on selected variables in a
design
 The main purpose is to provide information for
debug tools
 Two types of VCD files include:
Four-state VCD file: selected variable changes in
{0,1,x,z} without any strength information
Extended VCD file: selected variable changes in all
states and strength information
17

Four-State VCD FileFour-State VCD File
 “adder_nbit” is the module instantiated to generate
the VCD file
18

Four-State VCD fileFour-State VCD file
 Generating VCD file for adder_nbit (testbench)
19
Specify the dump file
Select which variables to dump
into the specified file
Used to control simulation period
$dumpoff is used to pause the simulation
$dumpon is used to resume the simulation
Used to set the maximum
size of the specified VCD
file

Four-State VCD fileFour-State VCD file
 VCD file format:
20
$date
Fri Sep 05 17:32:13 2014
$end
$version
ModelSim Version 10.1c
$end
$timescale
1ps
$end
$scope module adder_nbit_vcd_tb $end
$scope module UUT $end
$var wire 1 ! sum [3] $end
$var wire 1 " sum [2] $end
$var wire 1 # sum [1] $end
$var wire 1 $ sum [0] $end
$var wire 1 % c_out $end
$var wire 1 & x [3] $end
$var wire 1 ' x [2] $end
$var wire 1 ( x [1] $end
$var wire 1 ) x [0] $end
$var wire 1 * y [3] $end
$var wire 1 + y [2] $end
$var wire 1 , y [1] $end
$var wire 1 - y [0] $end
$var wire 1 . c_in $end
$upscope $end
$upscope $end
$enddefinitions $end
#0
$dumpvars
0$
0#
0"
0!
0%
0.
0)
0(
0'
0&
0-
0,
0+
0*
$end
#5000
1-
1$
#10000
0-
1,
0$
1#
#15000
1-
1$
#20000
$dumpoff
x$
x#
x"
x!
x%
x.
x)
x(
x'
x&
x-
x,
x+
x*
$end

ISE Design FlowISE Design Flow
 ISE design flow can be grouped into
Design entry
Synthesis
Implementation
Configure FPGA
21

Design EntryDesign Entry
 Design entry is based on HDL
 After a design is entered into the design flow, the
functional verification is followed through
simulation or formal proof
22

SynthesisSynthesis
 Synthesis extract logic from HDL and translates it
into an EDIF file (*.edf)
 The synthesis phase is divided into:
23
Phase Description
Parsing Syntax errors are identified
Synthesis Extract FSM and identify resources that can be shared
Optimization Timing optimization, LUT mapping and register replication

ImplementationImplementation
 The implementation step includes three main steps:
 The map step finishes the logic synthesis
operations
 A resource used report is obtained after the map
step is finalized
24
Step Description
Translation Merges multiple design files into a single gate-level netlist
Map Groups logical symbols from the gate-level netlist into physical
components including CLBs and IOBs
PAR Places synthesized logic components into FPGA fabrics

 Translation report of “adder_nbit”:
25

 Mapping: The following gives the resource used in
module adder_nbit
 From this report, the feasibility of the design is
seen 26

 In the PAR step, logic elements are placed and
connected onto the device
 Implementation tools extract timing data after PAR
step
 Post-PAR STA of adder_nbit is given below:
27
Critical path

Configure FPGAConfigure FPGA
 This generates a programming file (.bit)
 This is used to configure the specified FPGA so
that it would work as expected
 The bit file is can be uploaded directly to the
FPGA
28

Verilog HDL Verification

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