Efficient Coding Techniques for High-Performance Digital Systems

1. ECE 551: Digital System Design & Synthesis Lecture Set 7 7.1: Coding for if and case 7.2: Coding logic building blocks (In separate file) 7.3: High-Performance Coding (In separate file)

2. ECE 551 - Digital System Design & SynthesisLecture 7.1 Coding for Synthesis of Combinational Logic Overview • Premise • Basic coding for if and case • Synopsis case directives • Late signal arrival coding for if and case • Data, Control • Coding Logic Building Blocks • Decoder, Priority Encoder, Reduction XOR, Multiplexer • High-Performance Methods • Datapath Duplication, Operator in if condition • General Coding Issues • Resource Sharing • Arithmetic Expression Optimization

3. Premise • HDL coding influences the hardware implementations produced by synthesis • Affects figure of merit and tradeoff • FoM = delay x area • Delay/area tradeoff • Think hardware when coding! • Results are Synopsys specific!

4. References • Synopsys, Guide to HDL Coding Styles for Synthesis, version 2001.08, GHCS. • HDL Compiler for Verilog Reference Manual, version 2001.08 - Chapter 8, HCVR. • Design Compiler User Guide, version 2001.08 - Chapter 3, DCUG. • Can be referenced on CAE Unix: /afs/engr.wisc.edu/apps/eda/synopsys.2001.08/doc/online/

5. Basic Coding Using if and case • Example 1-1 GHCS • Multiplexer with multiple if’s • Figure 1-1 Resulting structure using SELECT_OP in cascade structure with effective priority encoding of selection. • Example 1-3 GHCS • Multiplexer with single if • Figure 1-2 Resulting structure using SELECT_OP and gates is “flattened,” but control that results is still priority encoded selection. • Example 1-5 GHCS • Uses casex to permit use of don’t caresin conditions • Control signals not priority encoded.

6. Example 1-1 Verilog: Multiple Ifs

7. Example 1-1 Circuit: Cascade • Note last statement has highest priority! • Long worst case delay for both data and control

8. Example 1-3 Verilog: Single If

9. Example 1-3 Circuit: Single Select – Priority Decode • Note first condition has highest priority! • Short delays

10. Example 1-5 Verilog: Casex • Note use of casex and x’s to obtain optimized logic

11. Example 1-5 Circuit: Single Select • Priority encoded – first condition has highest priority • Short delays

12. Synopsys Case Directives • case (condition) // synopsys parallel_case • Eliminates the priority decoder • Selection lines go directly into Select Op • case (condition) // synopsys full_case • Declares that all “cases” are present • Prevents formation of latches

13. Late Signal Arrival Coding for if and case - Data • Objective - force smaller delay to output from “late arriving” data signal(s). • Example 2-1 GHCS • Multiplexer with multiple if’s • Figure 2-1 Resulting structure using SELECT_OP in cascade structure with late arriving data b forced to output - priority encoding behavior preserved.

14. Late-Arriving Data Verilog: Fixed • Original is Example 1-1: Late data b is far from output • Lateness of b and long logic delay add

15. Late-Arriving Data Circuit: Fixed

16. Late signal arrival coding for if and case - Control • Objective - force smaller delay to output from “late arriving” control signal(s). • Example 2-3 GHCS • Multiplexer with single if • Figure 1-2 Resulting structure similar using SELECT_OP with “flattened” priority control logic • Example 2-5 GHCS • Multiplexer with single if • Figure 2-2 Resulting structure is decomposition with late arrival control closest to output. Note that delays are reduced due to smaller SELECT_OP due to lower gate fan-in.

17. Late-Arriving Control Verilog: Original • Original is Example 1-1: Late data b is far from output • Lateness of b and long logic delay add

18. Late-Arriving Control Circuit: Fixed

19. Late-Arriving Control Verilog: Fixed - 1

20. Late-Arriving Control Verilog: Fixed - 2

21. Late-Arriving Control Circuit: Fixed

22. Late signal arrival coding for if with nested case - Data • Objective - force smaller delay to output from “late arriving” data signal(s). • Example 2-7 GHCS • Complex selector with single if with nested case • Figure 2-3 Resulting structure is cascaded SELECT_OP • Example 2-9 GHCS • Decomposition placing selection of late data near output • Figure 2-4 Resulting structure has late arriving data passing through single 2-way SELECT_OP

23. Late-Arriving Data – If with Nested Case Verilog: Original - 1

24. Late-Arriving Data – If with Nested Case Verilog: Original - 2

25. Late-Arriving Data – If with Nested Case Circuit: Original - 2 • Late-arriving data passes through two Select_Op’s

26. Late-Arriving Data – If with Nested Case Verilog: Fixed - 1

27. Late-Arriving Data – If with Nested Case Verilog: Fixed - 2

28. Late-Arriving Data – If with Nested Case Circuit: Fixed • Late-arriving data enters last Select_Op

29. Late signal arrival coding for case with nested if – Control • Objective - force smaller delay to output from “late arriving” control signal(s). • Example 2-11 GHCS • Complex selector with single case with nested if • Figure 2-5 Resulting structure is cascaded SELECT_OP • Example 2-13 GHCS • Decomposition placing selection of late data near output • Table 2-1 Resulting design has lower delay and lower area(better FoM)

30. Late-Arriving Control – Case with Nested If Verilog: Original

31. Late-Arriving Control – Case with Nested If Circuit: Original

32. Late-Arriving Control – Case with Nested If Verilog: Fixed - 1

33. Late-Arriving Control – Case with Nested If Verilog: Fixed - 2

34. Late-Arriving Control – Case with Nested If Synth Data: Compare • Delay improved by 16% • Small area improvement

35. Summary • Area and timing are coding dependent • Learn to visualize hardware that results from code • Priority decoding • Natural due to the sequential processing of statements • Often not needed, so in such cases: • Learn how to avoid it to save area and time • Delay control can be done through coding