VHDL

1. VHDL Main topics: • Circuit design based on VHDL • VHDL basics • Advanced VHDL language structures • Circuit examples

2. Shortly About the VHDL • VHDL is an acronym of VHSIC Hardware Description Language • VHSIC is an acronym of Very High Speed Integrated Circuits • A Formal Language for Specifying the Behavior and Structure of a Digital Circuit • Allows Top-Down Design

3. Behavior Structural Processors, memories Sequential programs Registers, FUs, MUXs Register transfers Gates, flip-flops Logic equations/FSM Transistors Transfer functions Cell Layout Modules Chips Boards Physical Gajski’s Y-chart Each axis represents type of description • Behavioral • Defines outputs as function of inputs • Algorithms but no implementation • Structural • Implements behavior by connecting components with known behavior • Physical • Gives size/locations of components and wires on chip/board • Design process is illustrated by travel route

4. Test Vector Generator Executable Specification Results, Errors = A Series of Refined Models Test Vectors Final Chip Model VHDL for Simulation & Synthesis

5. VHDL for Simulation & Synthesis VHDL requirements for Simulation • Creation of testbenches => • File I/O • Detection of errors (function & timing) • Multiple simultaneous models • Combination of low & high level models (for efficiency)

6. VHDL for Simulation & Synthesis VHDL requirements for HW Description • Behavioral models => • Combinatorial & Sequential Logic • RTL models • Structural models • Timing models

7. VHDL for Simulation & Synthesis Requirements for VHDL Synthesis Tools • Pre- & post synthesis behavior should be identical • Synthesis should be efficient => • Requires interaction with place & route tools • Logic Synthesis • FSM Synthesis • Area & Timing Optimization

8. New possibilities • VHDL frees the designer from having to use von Neumann structures • (Neumann János = John von Neumann) • It allows him to work with real concurrency instead of sequential machines • This opens up completely new possibilities for the designer

9. Reasons for using VHDL • Shorter development times for electronic design • Simpler maintenance • Traditional way: schematic design

10. Origin of the VHDL • VHDL originated in the early 1980s • The American Department of Defense initiated the development of VHDL in the early 1980s • because the US military needed a standardized method of describing electronic systems • VHDL was standardized in 1987 by the IEEE • It is now accepted as one of the most important standard languages for • specifying • verifying • designing of electronics

11. Standardization 1 • IEEE standard specification language (IEEE 1076-1993) for describing digital hardware used by industry worldwide • VHDL enables hardware modeling from the gate level to the system level • All the major tool manufacturers now support the VHDL standard • VHDL is now a standardized language, with the advantage that it is easy to move VHDL code between different commercial platforms (tools) => VHDL code is interchangeable among the different tools

12. Standardization 2 • VHDL is an acronym of VHSIC Hardware Description Language • VHSIC is an acronym of Very High Speed Integrated Circuits • All the major tool manufacturers now support the VHDL standard • VHDL is now a standardized language, with the advantage that it it easy to move VHDL code between different commercial platforms (tools) => VHDL code is interchangeable among the different tools

13. Standardization 3 • It was the American Department of Defense which initiated the development of VHDL in the early 1980s because the US military needed a standardized method of describing electronic systems • VHDL was standardized in 1987 by the IEEE • IEEE Std-1076-1987 • ANSI Standard in 1988 • Added Support for RTL Design • VITAL: VHDL Initiative Towards ASIC Library • Revised version in 1993 • IEEE Std-1076-1993

14. Standardization 4 • 1995: • numeric_std/bit: IEEE-1076.3 • VITAL: IEEE-1076.4 • 1999: IEEE-1076.1 (VHDL-AMS ) • 2000: • IEEE-1076-2000 • IEEE-1076.1-2000 (VITAL-2000, SDF 4.0) • Added mixed-signal supportto VHDL in 2001 -> • VHDL-AMS • IEEE Std-1076.1-2001 • 2002: IEEE-1076-2002

15. Tools • Good VHDL tools, and VHDL simulators in particular, have also been developed for PCs • Prices have fallen dramatically, enabling smaller companies to use VHDL, too • There are also PC synthesis tools, primarily for FPGAs and EPLDs

16. Usage • High-tech companies • Texas Instruments, Intel use VHDL • mostEuropean companies use VHDL • Universities • VHDL groups to support new users

17. IEEE • IEEE is the Institute of Electrical and Electronics Engineers • The reference manual is called IEEE VHDL Language Reference Manual Draft Standard version 1076/B • It was ratified in December 1987 as IEEE 1076-1987 • Important: • the VHDL is standardized for system specification • but not for design

18. Technology independence • The design of VHDL components can be technology-independent or more-or-less technology independent for a technical family • The components can be stored in a library for reuse in several different designs • VHDL models of commercial IC standard components can now be bought, which is a great advantage when it comes to verifying entire circuit boards

19. Analog world • VHDL has not yet been standardized for analog electronics • Standardization is in progress on VHDL with an analog extension (AHDL) to allow analog systems to be described as well • This new standard will be based wholly on the VHDL standard and will have a number of additions for describing analog functions

20. VHDL-Related Newsgroups • comp.arch.fpga • comp.lang.vhdl • comp.cad.synthesis

21. Other HDL languages • There are several other language extensions built to either aid in RTL construction or assist in modeling: • ParaCore - http://www.dilloneng.com/paracore.shtml • RubyHDL - http://www.aracnet.com/~ptkwt/ruby_stuff/RHDL/index.shtml • MyHDL - http://jandecaluwe.com/Tools/MyHDL/Overview.shtml • JHDL - http://www.jhdl.org/ • Lava - http://www.xilinx.com/labs/lava/ • HDLmaker - http://www.polybus.com/hdlmaker/users_guide/ • SystemC • AHDL – http://www.altera.com • It is goodfor Altera-made chips only, which limits its usefulness • But it is easy to pick up anduse successfully • The main purpose of a language -- programming, hdl, or otherwise -- is to ease the expression of design

22. Verilog • Verifying Logic • Phil Moorby from Gateway Design Automation in 1984 to 1987 • Absorbed by Cadence • Cadence's ownership of Verilog => others support VHDL • Verilog-XL simulator from GDA in 1986 • Synopsis Synthesis Tool in 1988 • In 1990 became open language • OVI: Open Verilog International • IEEE Standard in 1995 • IEEE Std-1364-1995 • Last revision in 2001 • IEEE Std-1364-2001 • Ongoing work for adding • Mixed-signal constructs: Verilog-AMS • System-level constructs: SystemVerilog

23. VHDL vs. Verilog

24. VHDL vs. Verilog (Cont.) • It does seem that Verilog is easier for designing at the gate-level, but that people who do higher level simulations express a preference for VHDL • VHDL places constraints on evaluation order that limit theoptimizations that can be performed • Verilog allows the simulatorgreater freedom • For example, multiple levels of zero-delay gatescan be collapsed into a single super-gate evaluation in Verilog • VHDL requires preserving the original number of delta cycles of delayin propagating through those levels

25. VHDL vs. Verilog: Process block • VHDL: process (siga, sigb) begin …... end; • Verilog: always @ (siga or sigb) begin …. end

26. VHDL vs. Verilog:Concurrent Signal Assignment • VHDL: c <= a and b; • Verilog: assign c = a & b ;

27. VHDL vs. Verilog: Signal Delays • VHDL: a <= transport b after 1 ns; • Verilog: #1 assign a = b; • ‘a’ output is delayed by 1 time unit • The ‘# ‘ operator is the delay operator • # N will delay forN simulation units • Delays can assigned to both inputsand outputs #1 assign a = #1 b; • ‘b’ is delayed by 1 unit, then assigned to ‘a’, which is thendelayed by 1 time unit

28. VHDL vs. Verilog: Clock Generator • VHDL: signal clk : std_logic := ‘0’; process begin clk <= not (clk) after clkperiod/2; wait on clk; end; • Verilog: initial clk = 0; always #(clkperiod/2) clk = ~ clk;

29. Verilog Weakness • Not well suited for complex, high level modeling • No user defined type definition • No concept of libraries, packages, configurations • No ‘generate’ statement - can’t build parameterizedstructural models • No complex types above a two-dimensional array

30. VHDL vs. Verilog:Managing Large designs • VHDL: • Configuration, generate, generic and package statements all help manage large design structures • Verilog: • There are no statements in Verilog that help manage large designs

31. VHDL vs. Verilog:Procedures and Tasks • VHDL: • allows concurrent procedure calls • Verilog: • does not allow concurrent task calls

32. VHDL vs. Verilog:Structural Replication • VHDL: • The generate statement replicates a number of instances of the same design-unit or some sub part of a design, and connects it appropriately • Verilog: • There is no equivalent to the generate statement in Verilog.

33. Languages “under development” • SystemVerilog • Extending Verilog to higher levels of abstraction for architectural and algorithm design and advanced verification • VHDL 200x • Goal of VHDL Analysis and Standards Group (VASG): • Enhance/update VHDL for to improve performance, modeling capability, ease of use, simulation control, and the type system • e.g.: Data types and abstractions: • variant records • interfaces