CSE241A VLSI Digital Circuits Winter 2003 Recitation 3: Synthesis

1. CSE241AVLSI Digital CircuitsWinter 2003Recitation 3: Synthesis

2. Logic Synthesis: explained • Logic synthesis: • Process of transforming Hardware Description Language (HDL) code into a logic circuit • HDL • VDHL • Verilog (we’ll only use Verilog RTL) • The circuitry: • Structural-level HDL netlist • Components from a technology specific library

3. Logic Synthesis • Logic synthesis converts a software code into a connected set a standard cells • Real cell properties must be accounted in order to insure that the actual circuit will perform correctly: • Propagation Delay through cells • Connection Delay between cells • Load Capacitance • Drive Resistance • Slew rate (10%- 90% v) • Area of Cells • Clock rates • Setup & Hold times • Power Consumption

4. Ideal is converted into Real case (Sel) 2'b00 : status <= A; 2'b01 : status <= B; 2'b10 : status <= 2'b10; 2'b11 : status <= 2'b11; endcase Sel A 00 status B 01 10 10 11 11 Mux4x1 1x Drive Strength Area: 12microns Max Transition 1.0 ns Input Pin capacitance: .0015pf Output Pin Max Load: .45 pf Cell Delay (A -> Z) with .455pf load and .5ns slew is .3ns

5. Synthesis Related File Types • Script file .scr • Verilog file .v • Synthesized Verilog file .sv • VHDL file .vhd • Synthesized VHDL files .svhd • EDIF file .edif • Synopsys database file .db • Reports .rpt • Log file(standard for most tools).log

6. Synthesis Related File Types • Library File .lib • TCL script file .tcl • Ambit Library Format file .alf • Ambit Database file .adb • Magma Database file .volcano • Command file .cmd • Standard Delay Format file .sdf • Standard Parasitics Exchange Format .spef

7. Synthesis Data Flow HDL Code Tech Library Constraints 2 1 3 Synthesis Tool Gate Level Netlist

8. Step 1: Synthesis Setup • Setup accomplished through setup files and global shell variables • Synopsys (.synopsys_dc.setup & script) • Setup directories (example: Project is called Lab2) • Script (/ee260b/ee260b/lab2/script) • Reports (/ee260b/ee260b/lab2/reports) • Libaries (/ee260b/ee260b/lab2/libraries) (local) • Verilog (/ee260b/ee260b/lab2/src)

9. Example of .synopsys_dc.setup Company = “UCSD” Designer = “Ben Cichy” Technology = “0.13 micron” Search_path = search_path + {“.” “./libraries/”} Target_library = {lsi_10k.db} Link_library = {“*” lsi_10k.db} Symbol_library = {lsi_10k.sdb} //work library for intermediate files Define_design_lib work -path work

10. Reading a Library • Synopsys: read_db msi_10k.db • If the .db library file doesn’t exist then it must be created from the .lib library file (vendor supplied) • .lib is readable by the user , .db is internal format • Use library compiler to create the.db. [ libcompile msi_10k.lib -> msi_10k.db ] • Library Compiler • Checks the .lib for errors • Translates to .db

11. Purpose of the Library • The Library contains the cells of the technology (.18, .13u) • Cells are “Building Blocks” for the circuit • Must use technology library for physical properties • Synthesis tools considers properties and function of cells • The key properties: • Cell delay • Rise/fall transitions • Capacitive load • Drive strength • Area and power • DelayTotal = DelayCell + DelayWire

12. Contents of a Library • Units (V, A, pW, KOhm, nS, etc) • Default parameters • Max transition • Input pin cap • Wireload mode • Operating condition • Max fanout • Nominal Parameters (PVT) • Operating Conditions • Worst Case /Best Case • Scaling factors • K Factors • Wireload Models • Estimate for fan-in, fan-out • Look-up table templates • Cells: all properties & attributes, Delay Tables, Rise/Fall Transition Tables, Power Tables

13. Wireload model • wire_load("45Kto75K") { • capacitance : 0.000070; • resistance : 0.000042; • area : 0.28; • slope : 40.258665; • fanout_length(1, 40.258865); • fanout_length(2, 80.517750); • fanout_length(3, 120.776600); • fanout_length(4, 161.045450); • fanout_length(5, 241.543200); • fanout_length(6, 322.070900); • fanout_length(7, 402.587600); • }

14. Reading RTL • Synthesis tool reads the RTL files. • The synthesis tool checks for • Syntax errors • Enters design into synopsys .db format Command: Synopsys: analyze –format verilog –lib work ./src/example.v

15. Generic Tech Generation Synopsys: elaborate example –arch “BEHAVIORAL” –lib work • Generates Control Data Flow Graphs (CDFGs) • Performs high level logic optimizations • Performs resource allocation • Adders • Multipliers • Produces a table of the resources that are used • Generates a hierarchical netlist • Generic” components (gtech.db) • No timing or electrical properties in the design • Genlib cells

16. Design Error Checking • Check: • Warnings and Errors in the log file • Examples of Warnings and Errors: • Signals missing in sensitivity lists • Shorted inputs or outputs • Unused & Undriven inputs and outputs • Blackboxes • Dangling, undeclared and undriven wires • Unsynthesizeable or ignored HDL constructs (# delay, etc.) • Might need linting tool • Latches

17. Constrain the Design • Constraints define the parameters of the block which the synthesis tool must meet. • Constraints define the relationships between the block and the rest of the chip. • These items are defined: • Clocks • Arrival times • Loading on the output pins • Drive resistance on the input pins • Timing exceptions • Operating Conditions • Wireload model mode

18. Clocks Input Delay Output Delay Output Load Input Drive Resistance False Paths Multicycle Paths Operating Conditions Wireload Model Constraints: Commands Synopsys create_clock set_input_delay set_output_delay set_load set_driving_cell set_false_path set_multicycle_path set_operating_conditions set_wire_load

19. Constraints: set performance targets • Synthesized design: • Bounded by constraints • Input/Output, clk; all bound design • The RTL defines the functionality of design • The library contains the building blocks for building the design • The constraints tell the synthesis tool the timing and electrical relationships between the block and the chip

20. Optimization Optimization: • The generic netlist • Mapped to the cells in the tech library a • Timing is computed (with a clocked design) • Iteratively • Tools restructures design until timing is met • Targets: • Power • Area • Timing (cycle time) Note: See Lecture 5 for more details Command: Synopsys: compile

21. Timing Reports • Timing reports: • Are created in synopsys script • Generate longest paths • Timing: set-up/hold-time violations • Timing not met: • Reconstrain design • Error in netlist • Too long (depth) logic path

22. Other Reports • Log file output: • Command window shows errors per excuted command (gui) • Log file (dc_shell) shows execution trace • Area Report • Just make sure design is not too large • Don’t try to overoptimize in DC • Hierarchy Report • Library Report • State Machine (FSM) report

23. Final Netlist Generation • Synopsys: • Write verilog netlist write –format verilog –hierarchy –output example.mapped.sv • Write synopsys database file write –hierarchy –output example.mapped.db

24. Compilation Strategies • Top-Down Compilation Strategy Method: Constraints are applied at the top level and the entire chip is synthesized. • Time-Budgeting Compilation Strategy (Bottom-Up Approach) Method: Constrain and synthesize each sub-block. Then import results and synthesize top level. • Compile-Characterize-Write Script-Recompile Strategy (CCWSR) Method: Apply timing constraints to top level. Let tool propagate timing constraints to lower level blocks (i.e. let the tool do a timing budget). Have tool generate a constraint script on the characterized sub-blocks. Synthesize sub-blocks then synthesize top level block.

25. Top-Down Compilation Strategy • Advantages • Only top level constraints are needed. • Better results are achieved because optimization is performed across the entire design. • Disadvantages • Long compile times • A small change in the design requires that the entire design be re-synthesized • If the design contains multiple clocks or generated clocks the tool doesn’t perform as well

26. Time-Budgeting Compilation Strategy • Advantages • Easier to manage because each sub-block has it’s own constraint and synthesis scripts. • A change in a sub-block does not necessary require that other sub-blocks be resynthesized. • Multiple and generated clocks are more easily handled. • Disadvantages • Each block is individually synthesized • May not produce optimal design. Prone to error due to manually specified time budgets. • Very tedious and time-consuming to update and maintain multiple scripts. • Some critical paths don’t become apparent until top level compile is performed. (unobservability of critical paths) • A final incremental compile of entire design may be necessary in order to remove DRC errors.

27. The Automated Chip Synthesis Approach • Advantages • Less CPU memory is needed to do synthesis • Automated Chip Synthesis (ACS) can create subblock constraint budgets based on the top-level constraints because budgeting is an integrated part of the ACS flow. It can see across the hierarchy like the top-down methodology. • Individual scripts are created for each sub-block • Disadvantages • Tool generated constraint scripts are not very readable • A change in a lower level sub-block requires that the entire design be re-synthesized • May be difficult to achieve convergence between sub-blocks because of a “Ping-Pong” effect while synthesizing sub-blocks. Synopsys ACS White Paper

28. Synthesizeable Verilog Constructs • (For Verilog 1997) always endcase module repeat and endmodule nand supply0 assign endfunction negedge supply1 begin endtask task buf for nor tri bufif0 function not wait bufif1 if notif0 wand case inout notif1 while casex input or wire casez integer output wor default large parameter xnor else macromodule posedge xor end medium reg sometimes

29. Ignored Keywords • <delay specifications> • scalared, vectored • small, large, medium • specify • time (some tools treat these as integers) • weak1, weak0, highz0, highz1, pull0, pull1 • $keyword (some tools use these to set synthesis constraints) • wait (some tools support wait with a bounded condition)

30. Unsupported Constructs • <global variables> • = = =, != = • cmos, nmos, rcmos, rnmos, pmos, rpmos • deassign • defparam • event • force • fork, join • forever, while • initial • pullup, pulldown • release • repeat • rtran, tran, tranif0, tranif1, rtranif0, rtranif1 • table, endtable, primitive, endprimitive

31. Common Synthesis Problems • Latches • Unassagined statements • Setup Time Violations/ Hold Time Violations • RTL, flip-flop designs • Multiple Clock domains correctly • Poor Constraints • What are the specifications? • Inaccuracy of Wireload models • Only real #s through place and route • Slow run-times • Flat design • Achieving Timing Closure • Many problems: constraints, RTL design, library, routing

32. Timing Closure • Timing problem: • Can’t reach closure • to rapidly achieve timing closure on a design is a HUGE PROBLEM for semiconductor industry • A timing closure problem exists when all timing violations in a design cannot be eliminated despite multiple synthesis and place & route iterations • The timing closure problem occurs for a few reasons • Poor design practices • Inaccurate wireload models • No placement information used during synthesis; therefore inaccurate connection delays