XST Synthesis FPGA Design Workshop

Presentation Name 2 Objectives After completing this module, you will be able to…  List the synthesis options for XST  Describe how to insert code from the Language Template  Specify various methods for entering constraints

Presentation Name 3 HDL codeSchematic Netlist Implement Synthesize BIT File Xilinx Design Process Step1 : Design – Two design entry methods: HDL (Verilog or VHDL) or schematic drawings Step 2 : Synthesize to create Netlist – Translates V, VHD, SCH files into an industry standard format EDIF file Step 3 : Implement design (netlist) – Translate, Map, Place & Route Step 4 : Configure FPGA – Download BIT file into FPGA

Presentation Name 4 XST Features RAM inferencing Schematic viewer Error navigation Timing-driven synthesis Fanout control Note: Mixed language designs are not supported

Presentation Name 5 XST Synthesis in Project Navigator  Module/entity selected in Sources window treated as “top”  XST-specific processes – Synthesize View Synthesis Report Analyze Hierarchy Check Syntax  XST-specific properties – Synthesis Options – HDL Options – Xilinx Specific Options AM2910 as top-level

Presentation Name 6 Device Support  FPGAs – Virtex – Virtex-E – Virtex-II – Virtex-II Pro – Spartan-II – Spartan-IIE  CPLDs – XC XC9500XL - XC9500XV - CoolRunner - CoolRunner-II XST provides technology specific optimization for:

Presentation Name 7 XST Flow To Implementation Tools Synthesis Report File Synthesis Technology Specific Optimization Constraints VHDLVerilog.LOG.NGC

Presentation Name 8 HDL HDL Parsing Identification of language syntax errors HDL Synthesis Macro recognition, FSM extraction, resource sharing Low Level Optimization Macro implementation, timing optimization, LUT mapping, register replication.NGC.LOG Main Synthesis Steps

Presentation Name 9 Macro Inference Macro inference consists of two main steps: ¶ Recognition (HDL synthesis level): XST tries to recognize as many macros as possible · Implementation (low level optimization): XST makes technology dependent choices – Improve design performance and decrease area Preserve the macro as a macro? Merge the macro with surrounded logic? Choices depends on the macro type and size – If XST decides to preserve the macro, you must decide the way the macro is to be implemented

Presentation Name 10 Black boxes Primitives Macro Inference Registers Latches Counters Accumulators Shift registers RAMs ROMS FSMs Reg. multipliers Arithmetic Adders Subtractors Adder/subtractors Comparators Multipliers Dividers Multiplexers Decoders Priority encoders Logical shifters Three states Supported Macros SequentialCombinatorialSpecial Cases

Presentation Name 11 FSM Recognition  XST is able to recognize state machines independent of the modeling style used – For example, you may have several processes (one, two, or three) in your description, depending on how you consider and decompose  Notes – XST can handle/recognize synchronous state machines – Currently, XST requires FSM with initialization signals, which can be asynchronous or synchronous

Presentation Name 12 FSM Optimization  Optimization is based on: – State assignment – Flip-flop (FF) type selection  State assignment - XST supports: – One-Hot- speed optimization – Compact- area optimization (Xilinx proprietary) – Gray - minimizes hazards and glitches – Also Johnson, Sequential, User Defined User Encoding - XST will use the original encoding specified in the HDL file If you use enumerated type for your state registers, you can use the “enum_encoding” constraint to assign a specific binary value to each state

Presentation Name 13 ISE GUI  Synthesis options – Global synthesis and optimization goal and effort  HDL options – Family-specific inference and optimization options  Xilinx Specific options – Specific low-level implementation and optimization algorithms

Presentation Name 14 XST: Synthesis Options Set global synthesis, optimization goal, and effort – Optimization Goal (speed/area) – Optimization Effort (normal/high) – Synthesis Constraints File Any text file – Use Synthesis Constraints File – Global Optimization Goal – Generate RTL Schematic – Write Timing Constraints – Verilog 2001

Presentation Name 15 XST: HDL Options Set family-specific inference and optimization options – FSM Encoding Algorithm – RAM/ROM/Multiplexer Extraction – RAM/Multiplexer Style – Decoder/Priority Encoder Extraction – Shift Register/Logical Shifter Extraction – XOR Collapsing – Resource Sharing – Complex Clock Enable Extraction

Presentation Name 16 XST: Xilinx-Specific Options Set specific low-level implementation and optimization algorithms – Add I/O Buffers – Maximum Fanout – Equivalent Register Removal – Register Balancing – Move First/Last Flip-Flop Stage – Slice Packing – Pack I/O Registers into IOBs

Presentation Name 17 Language Templates  Two methods to open templates: – Language Icon – Edit -> Language Templates  Language Templates provide common templates for designs: – Component instantiation – Language templates – Synthesis templates

Presentation Name 18 Language Templates  To use template, be sure that an HDL source file is already opened  Place cursor at the location for the code to be entered  In the Language Template GUI, right- click on the template you wish to use  Select “Use in…”  Be sure the appropriate file name is listed

Presentation Name 19 HDL Libraries  Create VHDL Library using New Source Wizard  HDL Libraries Displayed in Libraries Tab of Sources Window

Presentation Name 20 What are Constraints?  Writing constraints is a method of communicating your design and performance objectives to the synthesis tools and implementation tools

Presentation Name 21 Xilinx Design Process Step1: Design – Two design entry methods: HDL(Verilog or VHDL) or schematic drawings Step 2: Synthesize to create Netlist – Translates V, VHD, SCH files into an industry standard format EDIF file Step 3: Implement design (netlist) – Translate, Map, Place & Route Step 4: Configure FPGA – Download BIT file into FPGA HDL codeSchematic Netlist Implement Synthesize BIT File Synthesis CONSTRAINTS Implementation CONSTRAINTS

Presentation Name 22 XST Constraints XST will accept synthesis constraints through the Xilinx Constraints File (XCF) – Do not confuse this with the User Constraints File (UCF), which contains implementation constraints for the Xilinx tools When using an XCF file, specify the file in the Synthesis Options tab

Presentation Name 23 XST Constraints To quickly enable or disable the use of a constraint file by XST, you can check or uncheck the Use Synthesis Constraint File menu -uc -iuc

Presentation Name 24 Constraint Types Constraints in the XCF file can be divided into two groups: – Timing – Non-timing For all non-timing constraints, the MODEL or BEGIN MODEL...END constructs must be used – This is true for pure XST constraints, such as FSM_EXTRACT or RAM_STYLE, as well as for implementation constraints, such as RLOC or KEEP For timing constraints, such as PERIOD, OFFSET, TNM_NET, TIMEGRP, TIG, FROM-TO, etc., XST supports native UCF syntax, including the use of wildcards and hierarchical names – Do not use these constraints inside the BEGIN MODEL... END construct because XST will issue an error

Presentation Name 25 XCF - MODEL To apply a constraint to the entire entity or module, use the following syntax: MODEL entity_name constraint_name = constraint_value; Note: If a constraint is applied to an entity or module, the constraint will be applied to each instance of the entity/module To apply constraints to specific instances or signals within an entity or module, use the INST or NET keywords: BEGIN MODEL entity_name INST instance_name constraint_name = constraint_value; NET signal_name constraint_name = constraint_value; END;

Presentation Name 26 Sample FPGA Constraints/Attributes  allclocknets  period  max_delay  mux_style  ram_style  maxfanout  register_duplication  keep_hierarchy  For more information on attribute syntax, consult the XST User’s Guide

Presentation Name 27 Design Constraints  If XST decides to push flip-flops to IOBs, then the following cases are taken into account Flip -flops controlling OBUFTs will be replicated FF3FF1FOEFF3F0E3FF1F0E1

Presentation Name 28 Design Constraints  Flip-flops having feedback will be replicated reg RES A CLK reg A CLK reg RES IOB=TRUE

Presentation Name 29 SLICE Design Constraints If the output flip-flop belongs to a shift register and represents its last stage, then it will be pushed to an IOB – Note : XST will not reduce the number of stages in SRL and infer additional flip- flops in order to improve the clock-to-out of the slice – Example: If the user has described a 16-bit shift register, then: reg SRL 15 bit RES SI CLK IOB=TRUE reg SRL 14 bit RES SI CLK IOB=TRUE reg RESULT Generated by XST

Presentation Name 30 How XST Identifies Critical Paths During Timing Optimization Notes – Other synthesis tools apply frequency specification to all four regions – ALLCLOCKNETS (the default constraint for timing optimization) in XST represents only clock-to-clock regions – MAX_DELAY is the constraint incorporating all four regions. FF logic IPAD OPAD Offset_in_BeforeOffset_out_After Allclocknets Period Inpad_to_Outpad

Presentation Name 31 How XST Identifies Critical Paths During Timing Optimization The identification of a critical path depends on the timing constraints and is based on the slack calculation The value of the slack depends on the way the constraints are applied As soon as all of the slacks are identified, XST will choose the smallest (most negative) one in order to identify the Critical Region Let us consider the following example

Presentation Name 32 How XST Identifies Critical Paths During Timing Optimization Suppose we have two clocks (clk1, clk2) in the design. Before timing optimization their periods are estimated as: clk1 : 30 ns clk2 : 25 ns If no value is supplied with the ALLCLOCKNETS constraint, XST will calculate the slack assuming the goal delay is 0 ns: clk1 : -30 ns clk2 : -25 ns As a consequence, clk1 will be considered the critical one and XST will try to optimize this clock first

Presentation Name 33 How XST identifies Critical Paths During Timing Optimization Suppose a PERIOD constraint defines individual timing requirement for each clock: 25 ns for clk1, 15 ns for clk2. In this case the slack will be: – clk1 : -5 ns – clk2 : -10 ns As a result, clk2 will be considered the critical one, and XST will try to optimize this clock first The ultimate goal, in both cases, is to increase the slack of all paths within the Critical Region. However, the final results of optimization are directly affected by the types and values of the constraints applied

Presentation Name 34 Understanding the Log File The log file can be divided into three main parts: Table of synthesis options Messages generated during synthesis Final report General Statistic Table Timing Report HDL Analysis HDL Synthesis Low Level Synthesis HDL Compilation

Presentation Name 35 Log File Organization TABLE OF CONTENTS 1) Synthesis Options Summary 2) HDL Compilation 3) HDL Analysis 4) HDL Synthesis 4.1) HDL Synthesis Report 5) Low Level Synthesis 6) Final Report 6.1) Device utilization summary 6.2) TIMING REPORT... ========================================================================= * HDL Compilation * ========================================================================= Compiling vhdl file constant.vhd in Library my_lib.... ========================================================================= * HDL Synthesis * ========================================================================= Synthesizing Unit....

Presentation Name 36 Messages During Synthesis The structure of this part directly reflects the main steps of the synthesis Messages generated during synthesis... HDL Analysis HDL Synthesis Low Level Synthesis... Syntax check Warnings and Errors Information on extracted macros and FSMs for each hierarchical block Summary Table on extracted macros for the entire design Encoding style chosen for each FSM Information on register replication and removal HDL Compilation Lists files used during synthesis

Presentation Name Final Report General Statistic Table Timing Report (FPGA only) Final Report Messages generated during synthesis

Presentation Name 38 Timing Report XST is trying to keep its post-synthesis timing report close to the report generated by TRACE Moreover, we have added a new table at the beginning of the report summarizing clock information of the design – List of all clocks in the design – How each clock is bufferized – How many loads each clock has

Presentation Name 39 Summary  XST is provided with v5.2i ISE software  XST provides various options for synthesizing designs  Language Templates allow you to re-use commonly used code  Several methods for entering constraints