1. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Topics Modeling with hardware description languages (HDLs).

2. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Hardware description languages Textual languages for describing hardware: –structure; –function. Most people today use textual languages rather than schematics for most digital design. –Schematics make poor use of screen space.

3. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Major HDLs Two major HDLs designed for simulation: –VHDL; –Verilog. –Similar capabilities but somewhat different language philosophies. EDIF is a standard netlist format.

4. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Simulation vs. programming Simulation tags computations with times. –Must know when signals change to properly simulate hardware. Simulation is parallel. –Many statements can execute at the same (simulation) time. –Just like hardware.

5. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Types of simulation Compiled code simulation. –Generate program that evaluates a hardware block. –Operational details within the hardware block are lost. Event-driven simulation. –Propagate events through simulation. –Don’t simulate a block until its inputs change.

6. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Event-driven simulation An event is a change in a net’s value. An event has two components: –value; –time. timet=35 ns net1=0 @ 35 ns net event net1

7. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Propagate events only when nets change value. If an input change doesn’t cause an output change, no event is propagated. 0 1 1 0 no event 1 0

8. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Timewheel The timewheel is a data structure in the simulator that efficiently determines the order of events processed. Events are placed on the timewheel in time order. Events are taken out of the head of the timewheel to process them in order.

9. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Timewheel operation a b c 1 0 1 a=1 @ 0 ns netlist timewheel b=1 @ 1 ns 1 c=0 @ 2 ns 0 time

10. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Order of evaluation Order of evaluation is important. –Causality must be obeyed. Evaluating events in the wrong order can cause inaccurate results.

11. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Order of evaluation example a b c 0 0 0 netlist timewheel b=1 @ 1 ns 1 d=1 @ 2 ns 1 d e 1 e=0 @ 4 ns 0 1 time

12. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Modeling Structural modeling describes the connections between components. –Netlists are structural models. Behavioral models describes the functional relationship between inputs and outputs. –Similar to programming but values are events.

13. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf HDLs language constructs Must be able to define component types. –A model may be behavioral or structural. May be able to define abstract data types. –A wire may carry an enumerated value. –Multi-valued simulation may be defined using abstract data types. May be able to define modules to control the scope of names.

14. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Testbenches A testbench is a model used to exercise a simulation. –Provides stimulus. –Checks outputs. Testbenches help automate design verification. –Rerun edited module against testbench. –Run models at behavioral, RTL levels against the same testbench.

15. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Synthesis subsets VHDL and Verilog were designed for simulation. A synthesis subset is: –synthesizable; –produces consistent simulation results. Different tools may use different synthesis subsets.

16. Modern VLSI Design 4e: Chapter 8 Copyright  2008 Wayne Wolf Register-transfer synthesis Most common type of synthesis. Synthesizes gates from abstract RT model. –Registers are explicit. –Some tools will infer storage elements---be careful. Optimized for performance, area, power.