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ECE 551: Digital System Design & Synthesis

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  1. ECE 551: Digital System Design & Synthesis Lecture Set 12 12.1: Introduction to VHDL (Separate File) 12.2: VHDL versus Verilog

  2. ECE 551 - Digital System Design & SynthesisLecture 12.2 - VHDL versus Verilog Overview • Background • Comparison Space • Capability • Compilation • Data Types • Design Reuse • Ease of Learning • Error- Checking • Forward and Backward Annotation

  3. ECE551 - Digital System Design & SynthesisLecture 12.2 - VHDL versus Verilog Overview • Comparison Space (Continued) • High-Level Constructs • Language Extensions • Libraries • Low-Level Constructs • Managing Large Designs • Operators • Parameterized Models

  4. ECE 551 - Digital System Design & SynthesisLecture 12.2 - VHDL versus Verilog Overview Comparison Space (Continued) • Procedures and Tasks • Readability • Structural Replication • Testbenches • Verboseness • Comparison Summary • Algorithmic and RTL Level Examples

  5. Background VHDL (VHSIC Hardware Description Language) VHSIC (Very High Speed Integrated Circuit) • Developed under US military contract • Based on Ada • IEEE Standard 1076 (1987) • IEEE Standard 1076 (1993)

  6. Background Verilog • Developed by Gateway Design Automation in 1983 • Based on C and Ada • Originally an interpretive language • Gateway bought by Cadence in 1989 • Put in public domain in 1990 • IEEE Standard 1364-1995 • IEEE Standard 1364-2001

  7. VHDL VITAL Models needed at gate level Better at abstract modeling Verilog May require use of PLI for modeling abstract hardware Has built-in gate models Capability - Tie

  8. VHDL Separate compilation of multiple design units in the same file available Should probably keep each design unit in separate file (tedious in some cases!) Verilog Compilation can be dependent on order of code in files and on compilation order of multiple files Compilation - VHDL +

  9. VHDL Many language and user-defined data types Strong type checking Dedicated conversion functions needed to integrate types in design Helps to prevent errors Creates major headache for hardware modeling Verilog Data types simple and focused on hardware modeling Weak type checking No user-defined data types Register type confusing - became variable type in 2001 standard Overall Simple Data Types - Verilog +

  10. VHDL Packages available for sharing functions, procedures, types, and components Verilog Functions and procedures must be within same module, or In separate system file with ‘include compiler directive for use. Design Reuse - VHDL +

  11. VHDL Less intuitive for novice due to strong typing Many ways to model the same circuit Verilog Easiest to grasp and understand for novice Compiler directive language and PLI add complexity Ease of Learning - Verilog +

  12. VHDL Strong typing and bit width consistency permits more checks Verilog Weak typing and mixed bit width can be more error-prone and not caught by checks Error-Checking - VHDL +

  13. VHDL SDF can be used with VHDL in many commercial simulation products Verilog Origin of Standard Delay Format (SDF) for forward- and back-annotating delays Forward and Backward Annotation - Tie

  14. VHDL Packages for reuse Configuration statements for choosing entities and architectures Generate statements for replicating structure Generics for parameterizing models Verilog Parameterization of models via overload Configurations added in 2001 Generate statements for replicating structure added in 2001 High-Level Constructs - VHDL +

  15. VHDL Attribute ‘foreign allows architectures and subprograms in another language Verilog PLI between Verilog and Verilog software tools Language Extensions - Verilog + Comment: • Use not recommended • But, in captive, non-vendor environment, can be very useful

  16. VHDL Libraries to store compiled entities, architectures, packages, and configurations Verilog Libraries and configurations added in 2001 Libraries - Tie

  17. No gate level primitives Cell primitives defined using VITAL language Gate level primitives UDPs and specify blocks for modeling cell primitives for ASICs and PLDs Low-Level Constructs - Verilog ++

  18. Packages Configuration Generate Generic Configurations added in 2001 Generate added in 2001 Managing Large Designs - VHDL +

  19. VHDL Similar to Verilog Lacking in unary reduction operators (can use loop) Does not allow concatenate on LHS Verilog Similar to VHDL Allows concatenate on LHS Operators - Verilog +

  20. VHDL Specific width, delay, etc. model can be instantiated from generic n-bit model using generic statements Synthesizes only if value of generic given Verilog Can be instantiated from a generic fixed bit number model using overload parameter values Synthesizes without overload uses default values Parameterized Models - Tie

  21. VHDL Allows concurrent procedure calls Verilog Allows concurrent task and function calls in 2001 Procedures and Tasks

  22. VHDL Verbose (Concise?!) More sentence-like Verilog Concise More cryptic Bit width mixing confusing and error- prone Preferred by C programmers, but C + Ada mix may be confusing Readability - VHDL +

  23. VHDL Generate Verilog Arrays of instances using concatenation Generate in 2001 Structural Replication - Tie

  24. VHDL Generics useful Configuration statements useful More transparent file handling Verilog File handling is hardware like Configuration statements useful Testbenches - VHDL +

  25. VHDL Requires defined and matching data types 1987 required components Overall - verbose and often longer code Verilog Less explicit type modeling Overall - less verbose and often much shorter Verboseness - Verilog ++

  26. By the numbers (no weighting): VHDL ++ 0 + 7 Tie 3 Verilog 2 4 3 Comparison Summary • Verilog 2001 has leveled the playing field!

  27. Comparative Examples • Algorithmic Level • See D. J. Smith paper • RTL Level • See D. J. Smith paper