12004 MAPLDVHDL Design Review VHDL Design Review And Presentation

22004 MAPLDVHDL Design Review Designer’s Responsibilities (Besides Doing the Design): Make the design reviewable and transferable –Documentation Theory of operation Proof that spec and WCA are met –Organization Partitioning into logical components –Presentation Readability of schematics, VHDL, etc. Inadequate documentation biggest design review problem –How would you, the designer, explain your design to someone else? Reviewer’s question: If the designer can’t present the design, does the designer understand the design?

32004 MAPLDVHDL Design Review How to Review VHDL Designs How does one perform a design review, in general? –Most design review tasks are independent of how the design is presented What does VHDL add to the task?

42004 MAPLDVHDL Design Review What VHDL Adds to the Review Process Probably, an awful lot more work!! VHDL introduces serious problems: –It hides design details –It is not WYSIWYG: What you see (as your design concept in VHDL) may not be what you get (as an output of the synthesizer) –Coupled with FPGAs, it encourages bad design practices Understanding design by reading code extremely difficult

52004 MAPLDVHDL Design Review VHDL Hides Design Details Connectivity hard to follow in VHDL files Behavior of sequential circuits can be hard to follow through processes Interactions between modules can be difficult to understand Spelling errors → undetected circuit errors

62004 MAPLDVHDL Design Review Following connectivity Signal name changes make task difficult My method as a reviewer: –Import modules into Libero –Create symbols for modules –Create a schematic with modules and print it out on E-size paper –Connect modules with colored pencils Designer should do this as part of design presentation

72004 MAPLDVHDL Design Review Following Connectivity Simple Input and Output Example MA1: COUNT23 port map (RST_N => SIM_CLR_N, SYNC_CLR => EQUALS_internal, CLOCK => C5MHZ, Q => TIME_NOW_internal ); MA3: MUX_23_4 port map (A => R1HZ, B => R6HZ, C => R8HZ, D => R10HZ, T => THZ, T_SEL => TEST_SEQ, S1 => RATE_SEL(1), S0 => RATE_SEL(0), Y => Y ); MA2: COMP23 port map (DATAA => TIME_NOW_internal, DATAB=> Y, AEB => EQUALS_internal ); MA7: GATE_RANGE port map (RST_N => RST_N, CLOCK => C5MHZ, OPEN_VALUE => OPEN_VALUE,CLOSE_VALUE => CLOSE_VALUE, TIME_NOW => TIME_NOW_internal, GATE => GATE ); MA8: BIN2GRAY23 port map (A => TIME_NOW_internal, Y => GRAYTIME ); EQUALS <= EQUALS_internal; TIME_NOW <= TIME_NOW_internal; end RTL_ARCH; Note: Had to print out the entity just to make this slide. Inputs Outputs

82004 MAPLDVHDL Design Review Following Connectivity Signals In a Simple Module MA1: COUNT23 port map (RST_N => SIM_CLR_N, SYNC_CLR => EQUALS_internal, CLOCK => C5MHZ, Q => TIME_NOW_internal ); MA3: MUX_23_4 port map (A => R1HZ, B => R6HZ, C => R8HZ, D => R10HZ, T => THZ, T_SEL => TEST_SEQ, S1 => RATE_SEL(1), S0 => RATE_SEL(0), Y => Y ); MA2: COMP23 port map (DATAA => TIME_NOW_internal, DATAB=> Y, AEB => EQUALS_internal ); MA7: GATE_RANGE port map (RST_N => RST_N, CLOCK => C5MHZ, OPEN_VALUE => OPEN_VALUE,CLOSE_VALUE => CLOSE_VALUE, TIME_NOW => TIME_NOW_internal, GATE => GATE ); MA8: BIN2GRAY23 port map (A => TIME_NOW_internal, Y => GRAYTIME ); EQUALS <= EQUALS_internal; TIME_NOW <= TIME_NOW_internal; end RTL_ARCH; Making this chart was a lot of work and was error prone.

92004 MAPLDVHDL Design Review E.g., A Spelling Error? A VHDL module contained two signals: –CSEN appeared only on the left side of a replacement statement: CSEN <= … –CS_EN sourced several signals, i.e., appeared on the right side X <= CS_EN… Were they intended to be the same signal?

MAPLDVHDL Design Review E.g., Meaning of Names -- ADDRESS DECODE LOGIC VALUES IF (ADDRCOUNT >= " ") THEN ADCGE8_I <= '1'; [note name ends in “8” and comparison value is 8] ELSE ADCGE8_I <= '0'; END IF; IF (ADDRCOUNT >= " ") THEN ADCGE6_I <= '1'; [note name ends in “6” and comparison value is 6] ELSE ADCGE6_I <= '0'; END IF; IF (ADDRCOUNT = " " OR LOADAC = '1') THEN ADCGE36_D <= '1'; [note name ends in “36” but comparison value is 35] Lesson: Be careful with your names!

MAPLDVHDL Design Review VHDL is Not WYSIWYG Signals intended to be combinational can end up being sequential, and vice versa Sequential circuits can have unexpected, undesirable SEU behavior –Paper: “Logic Design Pathology and Space Flight Electronics”, R. Katz, R. Barto, K. Erickson, 2000 MAPLD International Conference The designer gives up some control over the design to unvalidated software

MAPLDVHDL Design Review VHDL and Bad Design Practices VHDL and FPGAs combine to allow designers to treat design as software –Especially for FPGAs for which there is no reprogramming penalty, e.g., Xilinx Rather than designing by analysis, designers simply “try” design concepts

MAPLDVHDL Design Review E.g., part of a 16 page process -- V1.02 & V DATA WILL STOP TANSFERING IFF BOTH HOLD AND OUTPUT ENABEL ARE -- ACTIVE FOR THE SAME PORT -- HOLD2 <= ((((HLD2TX_N_Q AND O_EN_Q(2)) OR -- (HLDTX_N_Q AND O_EN_Q(1)) OR -- (ROFRDY_N_Q AND O_EN_Q(0))) AND -- NOT(BYPASS_EN_Q AND (HLDTX_N_Q AND O_EN_Q(1))))); HOLD1_I <= ((HLDTX_N_Q AND O_EN_Q(1)) OR (ROFRDY_N_Q AND O_EN_Q(0)));-- V2.2 HOLD2 <= (((((HLD2TX_N_Q AND O_EN_Q(2)) OR (HLDTX_N_Q AND O_EN_Q(1)) OR (ROFRDY_N_Q AND O_EN_Q(0))) AND NOT(BYPASS_EN_Q AND (HLDTX_N_Q AND O_EN_Q(1))))) OR (((HLD2TX_N_Q AND O_EN_Q(2)) OR (HLDTX_N_Q AND O_EN_Q(1))) AND (BYPASS_EN_Q AND HLDTX_N_Q AND O_EN_Q(1))));

MAPLDVHDL Design Review Simplifying Let: a=HDL2TX_N_Q and O_EN_Q(2) b=HLDTX_N_Q and O_EN_Q(1) c=ROFRDY_N_Q and O_EN_Q(0) d=BYPASS_EN_Q Then HOLD2=(a+b+c)·(d·b)’ + (a+b)·(d·b) = a+b+c. What happened to d=BYPASS_EN_Q ??

MAPLDVHDL Design Review Lessons Don’t just try things, think about what you’re doing –Either BYPASS_EN_Q is needed or it’s not – what’s the requirement of the system? Make modules small enough to test via VHDL simulation, and test them fully. –If this logic was tested by itself, the error would have been found. It’s on orbit, now

MAPLDVHDL Design Review Combined Effects of VHDL Hidden design details + Non-WYSIWYG nature + Bad design practices  Designer can lose control of design i.e., the designer loses understanding of what is in the design, then adds erroneous circuitry until simulation looks right

MAPLDVHDL Design Review E.g., found in a VHDL file: CASE BVALREG3A_Q IS WHEN "0000" => IF (DAVAIL_LCHA_Q = '1' ) THEN -- ISN'T THIS CONDITION ALWAYS TRUE -- AT THIS POINT??? PC Just how well did the designers understand the design??

MAPLDVHDL Design Review Worst Case Result A design that works in simulation for expected conditions, but with flaws that show up in unusual conditions Passed on with little documentation by engineers who become unavailable  A total programmatic disaster!! An common occurrence!

MAPLDVHDL Design Review Solution to VHDL Problem Make sure designs are reviewable and transferable –No such thing as too good an explanation of how a circuit works Don’t use VHDL (much less Verilog!!)

MAPLDVHDL Design Review VHDL Review Tools and Techniques

MAPLDVHDL Design Review Netlist Viewer Crucial because –Synthesizer output, not VHDL, is the final design –Easy to see asynchronous design items –Connectivity often more apparent in viewer than in VHDL Helps solve the non-WYSIWYG nature of VHDL

MAPLDVHDL Design Review.srr files Flip-flop replication –See examples in “Synthesis Issues” module. State machine encoding and illegal state protection Inferred clocks => possible asynchronous design techniques Resource usage

MAPLDVHDL Design Review.adb files Needed for netlist viewer Check internal timing Get timing parameters to perform board level timing analysis Check I/O pin options

MAPLDVHDL Design Review VHDL Simulator Simulate modules or extract parts of modules Try to break them: –Most simulations are success oriented, in that they try to show the module works when it gets the expected inputs –Try to simulate with the unexpected inputs –Easier to do with smaller modules

MAPLDVHDL Design Review E.g., breaking a FIFO Here’s the full flag, but we’ll keep writing Here we get the full flag while reading out Turned out to be a problem for the project

MAPLDVHDL Design Review Suggestions for FPGA Design Presentation

MAPLDVHDL Design Review Goals Detailed design review and worst-case analysis are the best tools for ensuring mission success. The goal here is not to make more work for the designer, but to: –Enhance efficiency of reviews –Make proof of design more clear –Make design more transferable –Improve design quality

MAPLDVHDL Design Review Steps to Preparing for Design Review 1.Modularize your design 2.Make a datasheet for each module 3.Show FPGA design in terms of modules 4.Describe internal circuitry 5.Describe state machines 6.Describe FPGA connections 7.Describe synthesis results 8.Provide timing spec for external timing analysis 9.Show requirements of external circuitry

MAPLDVHDL Design Review 1. Modularize your design Easier to do during design phase Goal is to describe design in terms of components that can be individually verified Each component, or module, is a separate VHDL entity Modules should be of moderate, e.g., MSI, size –E.g., FIFO, ALU –Counter, decoder probably too small –VME interface too big

MAPLDVHDL Design Review 2. Make a Datasheet for Each Module Describe the module’s behavior Show truth table Show timing diagrams of operation Provide test bench used to verify module –Try to break it Model: MSI part data sheet

MAPLDVHDL Design Review 3. Show FPGA Design in Terms of Modules Provide requirements spec for FPGA Draw block diagram Top-level VHDL entity shows FPGA inputs and outputs and ties component modules together Show necessary timing diagrams –Interaction between modules –Interaction with external circuitry Text for theory of operation Provide test bench for top level simulation

MAPLDVHDL Design Review 4. Describe internal circuitry Use of clock resources Discuss skew issues Describe deviations from fully synchronous design –Be prepared to show necessary analysis Show how asynchronism is handled –External signals –Between clock domains Glitch analysis of output signals used as clocks by other parts

MAPLDVHDL Design Review 5. Describe state machines Encoding chosen Protection against lock-up states Homing sequences Reset conditions

MAPLDVHDL Design Review 6. Describe FPGA Connections Use of special pins: TRST*, MODE, etc. Power supply requirements –Levels, sequencing, etc. Termination of unused clock pins Input and output options chosen for pins Discuss transition times of inputs POR operation and circuitry Critical signals and power-up conditions –Remember WIRE! (being presented next door in a parallel seminar).

MAPLDVHDL Design Review 7. Describe synthesis results Percentage of utilization Flip-flop replication and its effects on reliable operation Margin results from Timer Timing of circuits using both clock edges

MAPLDVHDL Design Review 8. Provide Timing Specification for External Timing Analysis t SU, t H with respect to clock Clock to output t PD t PW for signals connected to flip-flop clocks Clock symmetry requirements if both edges of clock used

MAPLDVHDL Design Review 9. Show Requirements of External Circuitry Provide data sheets for parts interfacing to FPGA Show timing diagrams of interactions of FPGA to other parts Show timing analysis of external circuitry

MAPLDVHDL Design Review References Design guidelines: Design tutorials Design, analysis, and test guidelines: