stimulus and response. simple stimulus verifying the output self-checking testbenches complex...
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Stimulus and Response
Stimulus and Response Simple Stimulus Verifying the Output Self-Checking Testbenches Complex Stimulus Complex Response Predicting the Output
Simple Stimulus Generating Stimulus is the process
of providing input signals to the DUV
Every input to the DUV is an output from a stimulus model
Any deterministic waveform is easy to generate
Verilog Example 1‘timescale 1ns/1nsModule testbench;…Reg clk;Parameter cycle = 15;AlwaysBegin
#(cycle/2);clk=1’b0;#(cycle/2);clk=1b1;
Endendmodule
Verilog Example 2‘timescale 1ns/1nsModule testbench;…Reg clk;Parameter cycle = 15;AlwaysBegin
#(cycle/2.0);clk=1’b0;#(cycle/2.0);clk=1b1;
Endendmodule
Verilog Example 3‘timescale 1ns/100psModule testbench;…Reg clk;Parameter cycle = 15;AlwaysBegin
#(cycle/2);clk=1’b0;#(cycle/2);clk=1b1;
Endendmodule
Simple Stimulus (Cont) – Complex Waveforms Complex waveforms
Care must be taken not to over constrain the waveform generation or limit it to a subset of its possible variations.
Need to make sure that there are many instances of absolute min and max values
Controlled Randomization
Simple Stimulus (Cont) – Synchronized Waveforms Synchronized Waveforms
Stimulus for a DUV is never composed of 1 signal. You must synchronize all inputs to the DUV properly
In a synchronous design, most signals should be aligned with the clock
Sample of Synchronized Waveforms Sample 5-9AlwaysBegin
#50 clk = 1’b0;#50 clk = 1’b1;
EndInitialBegin
Rst = 1’b0;#150 rst = 1’b1;#200 rst = 1’b0;
End
Sample 5-10AlwaysBegin
#50 clk <= 1’b0;#50 clk <= 1’b1;
EndInitialBegin
Rst = 1’b0;#150 rst <= 1’b1;#200 rst <= 1’b0;
End
Simple Stimulus (Cont) – Generating Waveforms We talked about Delta Cycles – Can be equivalent
to real delays. If, due to delta cycle problems, you miss a value at one
clock edge, then you will get that value on the next! Must have everything aligned!
Generating Data Waveforms If not done properly, could produce race conditions
Can’t ensure that total number of delta cycles between clock and data is maintained, or at least in favor of the data signal.
Interfaces specs never specify 0-delay values, thus when generating synchronous data, always provide a real delay between active edge and transition on the data signal.
Simple Stimulus (Cont) – Encapsulating and Abstraction Waveform Generation Encapsulating Waveform Generation
Generation of waves may need to be repeated during simulation
Place then generation in a subprogram and call that subprogram with the vector to be applied as the input to the subprogram
Abstracting Waveform Generation Using synchronous test vectors (as above) is
cumbersome and hard to interpret (maintainability)
Easier if operations accomplished by the vectors were abstracted!
Try to apply worst possible combinations of inputs
Simple Stimulus (Cont)– Abstraction Waveform Generation Example
2-1 input sync reset D flip flop Inputs: rst, d0, d1, sel, clk Output d_out
Subprograms needed: Reset Load input d0 Load input d1
Abstraction Waveform Generation Example (Cont)
Reset Worst possible condition:
D0=1 D1=1 Sel = randomly set
Load d0 Worst possible condition:
D1 is complement of d0
Abstraction Waveform Generation Example (Cont)
Task sync_reset;
Begin
rst<= 1’b1;
d0 <= 1’b1;
d1 <= 1’b1;
@(posedge clk);
#(Thold);
{rst,d0,d1,sel} <=4’bxxxx;
#(cycle-Thold-Tsetup);
End
endtask
Task load_d0;input data;
Beginrst <= 1’b0;d0 <= data;d1 <= ~data;sel <= 1’b0;@(posedge clk);#(Thold);{rst,d0,d1,sel} <=4’bxxxx;#(cycle-Thold-Tsetup);
Endendtask
Abstraction Waveform Generation Example (Cont)
initialBegin
sync_reset;load_d0(1’b1);sync_reset;load_d1(1’b1);load_d0(1’b0);load_d1(1’b1);sync_reset;…..
End
Verifying the Output Generating Stimulus is only about
30% of job, 70% is in verifying output
Most obvious method is visually ASCII output Waveforms
Producing Simulation Results Which signals are significant change
with time In order to determine what is correct,
must model this knowledge Producing the proper simulation results
involves modeling the behavior of the signal sampling Sample at regular intervals (clk) Sample on interested signals (only when
they change)
Minimizing Sampling Minimizing the sampling improves
the simulator performance In VHDL only put interesting signals
on sensitivity list or use:‘wait until <interesting condition>’
In Verilog use $monitor(“…”, <signal list>) $monitoroff $monitoron
Visual Inspection of Waveforms Results are better (to understand)
when plotted over time Advantage is that it plots the signal
continuously overtime, not at specified points as in text view (the samples)
Tool dependent on how to turn on Performance impact, want to minimize
the total number of signals to view Mostly used for debug
Self-Checking Testbenches Use self-checking – different
techniques Specify input and expected output for
each clock cycle Problems
Difficult to maintain Difficult to specify Difficult to debug Require perfectly synchronous interfaces
Self-Checking Testbenches (Cont) Golden Vectors – Set of reference
simulation results DUV vectors are captured and then
compared against the golden set. If results are stored in ASCII format, use
diff command Some tools allow for waveform
comparisons Significant maintenance Separate clock domain references
Self-Checking Testbenches (Cont) Run-Time Result Verification
Results compared in parallel with the stimulus generation
Use a reference model The DUV and reference model are subjected
to same stimulus Outputs of both, DUV and reference model,
are constantly monitored and compared.
Self-Checking Testbenches (Cont) Focus on operations instead of input
and output vectors Include the verification of the
operations that were put into the subprograms.
Instead of simply applying stimulus, include the checking, now just run the operations, individually or in sequence.
Must verify that the operations are being performed.
Complex Stimulus Talked about simple stimulus Complex stimulus includes
feedback from DUV to the stimulator
Most desirable is a bus-functional model that is configurable.
Complex Stimulus (Cont) Feedback between stimulus and design
Generator can wait for feedback before continuing Include timing and functional verification in the
feedback monitoring Using feedback can cause deadlock during
testing. DUV may not provide feedback and the model
may not provide any more stimulus until there is feedback
Eliminate the possibility of deadlock! I.e. a timeout (with error!) and test continues Testcase fails and stops immediately
Complex Stimulus (Cont) Asynchronous interfaces
Most vectors are inherently synchronous Many interfaces are specified in an
asynchronous fashion (even though synchronous – FSM’s, flip-flop’s, etc)
If a clock is not specified in the specification, then it should not be part of the verification nor part of the stimulus
Behavioral model do not need a clock Need to think of all failure modes
Complex Stimulus (Cont) – Example CPU operations encapsulated using
procedures Encapsulating complex stimulus is
known as BFM’s (Bus Functional Models)
If you had a specification for a 386sx read cycle, then using vector stimulus would be inefficient – why?
Complex Stimulus (Cont) - Example Extend the CPU operations to
include writes. Using test vectors, can’t do read-modify-write operations. To do this, you’ll want to use the
value returned on a read for a future write (after modifying it).
Complex Stimulus (Cont) Configurable operations
If you have interfaces that have certain signals that are configurable
Don’t want to create nearly identical models – maintenance issues
Simple configurable elements become complex when grouped.
Solution - Create one model with configurable operations. Now you can use the model however you need to.
Complex Response We identified that visual inspection
is not the way to go. And that was with simple responses, what about complex responses. Must automate this, one way to
perform this is with BFM’s What is a complex response?
Complex Response – (Cont) Example – UART transmit path
Waiting for output before applying next input would prevent the ability to stress (or cause interesting conditions)
Filling up the FIFO is one Stress the DUV under max conditions,
must decouple generation from checking.
Complex Response – (Cont) How do you deal with unknown or
variable latency? This latency is usually a by-product of
the architecture or implementation. You may not care what it is.
If it is a by-product of the implementation and not a design requirement, why enforce one in verification?
Complex Response – (Cont) How to verify output
independently? Put output checkers and stimulus
generators in separate execution threads.
Processes in VHDL Always/initial and fork/join in Verilog
Must synchronize to know when to start checking, etc.
Complex Response – (Cont) Earlier we encapsulated input operations
– can do the same for outputs For stimulus, the subprograms took the
arguments as stimulus. For output operations, take the arguments as
the expect results (results that the DUV should output)
Implementation should be as configurable as the stimulus.
Remember – consider all possible failure modes.
Complex Response – (Cont) This procedure ‘recv’ is very limited. Only can be used in the current scope.
You pass in the expect and it compares the actual to this expect (predefined).
What if output is to be ignored until a predetermined sequence of events? Or data?
What if the output needs to be fed back to the stimulus model?
What if….? What if….?
Solution is to create a more generic output monitor.
Complex Response – (Cont) Generic Output monitor:
Return the data that the DUV output back to the caller!
The ‘higher authority’ now makes the call to what is correct and what is not. It is also controlling the stimulus model, therefore it knows more of the state of the environment and what is to be tested.
The other things (protocols, etc) are still be verified.
But do not arbitrarily constrain the input.
Complex Response – (Cont) Monitoring multiple possible operations
You may have a situation where more than one type of output may be OK. (branch prediction, out of order processing, etc)
Can’t predict unless you model the details of implementation.
If you verify for a particular order, over constraining environment (starting directed tests).
Complex Response – (Cont) How do you write an encapsulated
output monitor for this? 1st write a monitor that identifies the
next cycle. Verifies the preamble to all operations on
the output interface until it becomes unique
It then returns any information collected thus far to the testbench.
Testbench is left up to call the appropriate subprograms to complete the verification.
Complex Response – (Cont) We defined a stimulator as one
who has outputs. If a monitor must provide output back to the DUV, is it not a stimulator? Stimulator (or generator) is a model
that initiates a transaction Monitor is a model that may/many not
respond to an operation initiated by the DUV.
Complex Response – (Cont) Monitoring bi-di interfaces
Example: bridge chip Cycles initiated on the on-chip bus are
translated to PCI transactions (if addresses match)
Allows master devices (on-chip) to transparently access slave devices on the PCI bus
Complex Response – (Cont) What do you need to verify this?
On-chip bus cycle generator PCI bus cycle monitor Could you use a memory as the slave
device instead of monitor? Have the generator write to PCI space
then read it back, and continue doing this in a random fashion?
Complex Response – (Cont) Using the PCI monitor reduces the
risk Have the monitor detect the PCI cycle
and have it notify to the testbench along with the address being read or written.
The testbench would decide if it is correct.
Complex Response – (Cont) Let’s slice up the PCI cycle up into multiple
monitors One to handle the preamble and type of cycle One to handle each data transfer
Input/Output of monitor for data read/written Output to indicate whether to continue with more data xfers
or terminate One to handle cycle termination
Now that the cycle is broken up, we have the ability to provide the master and slave to throttle the transfer rate. It can assert irdy_n and trdy_n (these can be parameters for randomization).
Complex Response – (Cont) By using the generic PCI bus monitor,
the testcase becomes shorter. The monitor provides access to all bus values.
Also provides an easy mechanism to catch exceptions
Also enable the use of few addresses to provide a sufficient test suite.
Predicting the Output Unstated assumption with self-
checking testbenches is that you have detailed knowledge of the output to be expected This is the most crucial factor
Knowing exactly which output to expect and when determines the functional correctness.
Predicting the Output – (Cont) Data formatters
Expect output = data input (reformatted) Simplest output prediction process May want to forward data to the monitor one value
at a time. Due to pipelines and latency, this may constrain the
generator (can’t change data until it is checked) Instead of one value at a time, use a FIFO structure May want to use a global array that both generator
and monitor use. Another aspect (similar to global array) is to read in
values from a file. This way can be more dynamic.
Predicting the Output – (Cont) Packet Processors
Portion of packet is transformed somehow
Other portion is untouched Use untouched field to encode the
expected transformation Simplifies testbench
All controls for stimulus and expect generation are in one location.
Include all necessary information in payload to determine correctness.
Predicting the Output – (Cont) Complex Transformations
Expected output can only be determined by reproducing same transformation.
use alternative means (different algorithm)
Example was the DSP (using reals)