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Lect 10 - Stimulus & Response

Lect 10 - Stimulus & Response. Applying input stimulus to a design Creating clock signals Other waveforms Synchronizing inputs Using text I/O. Stimulus. The purpose of writing testbenches is to apply stimulus signals to a design and observe the response of the design to those signals

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Lect 10 - Stimulus & Response

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  1. Lect 10 - Stimulus & Response • Applying input stimulus to a design • Creating clock signals • Other waveforms • Synchronizing inputs • Using text I/O EE694v-Verification-Lect10

  2. Stimulus • The purpose of writing testbenches is to apply stimulus signals to a design and observe the response of the design to those signals • The response is then compared to both the value and timing expected • Greatest challenge with stimulus is making sure they have values and timing that represents what the physical design will see in the environment EE694v-Verification-Lect10

  3. Applying Stimulus • Generating stimulus is the process of providing input signals, both the value and the timing, to a design • Inputs vary from very simple to complex • Inputs have relationships that establish specific states in the design under test EE694v-Verification-Lect10

  4. Clock Signals • Clock signals have 2 key parameters • Duty cycle • Period • Clock generation in VHDL • For a 50% duty cycle: clk <= not clk after 5 ns; • Or using a process PROCESS --Clock Process BEGIN CLK <= ‘1’; WAIT FOR 5 NS; CLK <= ‘0’; WAIT FOR 5 NS; END PROCESS; EE694v-Verification-Lect10

  5. Random Period/Duty Cycle Clocks • Determine when clock is low and for how long • Determine when clock is high and for how long • This method avoids problems of integer division PROCESS BEGIN CLK <= ‘1’; WAIT FOR “high-time” NS; CLK <= ‘0’; WAIT FOR “low-time” NS; END PROCESS; EE694v-Verification-Lect10

  6. Other Easy Waveforms • Waveforms with deterministic edge-to-edge relationships with a defined period are also easy to define. EE694v-Verification-Lect10

  7. Generating Complex Waveforms • Checking for the tolerances of the design to variations in edge-to-edge timing relationships. • Random numbers are not part of the VHDL language but can be generated. EE694v-Verification-Lect10

  8. Stressing the Design • To generate waveforms likely to stress the design under test, need to insure enough instances of minimum and maximum stimulus values are used • May need to modify how random number is generated to skew the distribution. EE694v-Verification-Lect10

  9. Generating Synchronized Waveforms • Stimulus is never composed of a single signal (even if the design is very small) • Must align edges of inputs (or don’t align) such that it mimics the real environment and the operation of the device under test. EE694v-Verification-Lect10

  10. Synchronizing Waveforms Clk <= not Clk after 50 ns; Process Begin rst <= ‘0’; wait until Clk |= ‘x’; wait until falling_edge(Clk); rst <= ‘1’; wait until falling_edge(Clk); wait until falling_edge(Clk); end process; • Waveforms can be synchronized in both VHDL and Verilog EE694v-Verification-Lect10

  11. Delta Cycles • Derived waveforms are never applied to the model at the same time as the signal they are derived from unless special coding is used. • Otherwise, derived signal are applied at least one delta cycle after the signal they are derived from changes. Signal edges can be aligned (even in delta time) if needed EE694v-Verification-Lect10

  12. Synchronizing Inputs • The interface specification for the application of inputs will never specify signal-to-signal synchronization as exactly zero. • Delays in real interfaces are going to be varied • Clocks may synchronize inputs or multiple inputs to a design • If inputs are synchronized and if the portion of the design that synchronizes them is not part of the design under test, the inputs need to be synchronized by the testbench. EE694v-Verification-Lect10

  13. Using Files for Input Data • An excellent approach to a verification effort is transaction based verification where each set of vectors is a transaction. • In VHDL the text I/O features of the language can be used such that the value of input vectors are stored in a file • Stimulus signal values are read from this file • Values are read using text I/O and assigned to signals • Also possible to include the correct response in the file along with the inputs EE694v-Verification-Lect10

  14. Using Files (cont.) • Allows for easy modifications to both the stimulus and the expected response as no recompile if needed. Also, it becomes very easy to add or modify the stimulus vectors/expected result. • So it is easy to add more test transactions. • Text I/O is very different in the 1987 and the 1993 versions of the language. Both are line I/O orientated philosophy. EE694v-Verification-Lect10

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