VHDL Design Review And Presentation

Dr. Rod Barto 3312 Moonlight El Paso, Texas 79904 915-755-4744 [email protected]

[email protected]

Basic Design Rule

• The designer should know and be able to prove: – The design meets the spec – The design passes a worst case analysis • The designer presents this proof to the reviewer • The reviewer verifies the proof May 30, 2002 RLB 2

Problems In Design Review

• Biggest problem: inadequate design documentation, giving rise to questions such as – What does this thing do?

– How does this implement the spec?

– How does this work?

• Documentation is the designer’s responsibility May 30, 2002 RLB 3

Special VHDL Problem

• Poorly written code – Endless, mindless structural and spaghetti VHDL • Writing good code is difficult • Understanding design by reading code extremely difficult • Documentation and comments crucial May 30, 2002 RLB 4

Results of Poor Documentation

• Reviewer asks a lot of questions – Of the designer – Of the system engineer – Of the scientists • Reverse engineering • The reviewer should not automatically assume that the designer understands the design.

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Designer’s Responsibilities

• Make the design reviewable – Documentation • Theory of operation • Proof that spec and WCA are met – Organization • Partitioning into logical components – Presentation • Readability of schematics, VHDL, etc.

• How would you, the designer, explain your design to someone else?

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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?

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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 May 30, 2002 RLB 8

VHDL Hides Design Details

• Connectivity hard to follow in VHDL files – Especially true for translations from schematics • Behavior of sequential circuits can be hard to follow through processes • Interactions between logical blocks can be difficult to understand • Spelling errors → undetected circuit errors May 30, 2002 RLB 9

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?

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E.g., meaning of names

-- ADDRESS DECODE LOGIC VALUES IF (ADDRCOUNT >= "000001000") THEN ADCGE8_I <= '1';

[note name ends in “8” and comparison value is 8]

ELSE ADCGE8_I <= '0'; END IF; IF (ADDRCOUNT >= "000000110") THEN ADCGE6_I <= '1';

[note name ends in “6” and comparison value is 6]

ELSE ADCGE6_I <= '0'; END IF; IF (ADDRCOUNT = "000110101" OR LOADAC = '1') THEN ADCGE36_D <= '1';

[note name ends in “36” but comparison value is 35]

• Lesson: Be careful with your names!

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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, MAPLD 2000 • The designer gives up some control over the design to unvalidated software May 30, 2002 RLB 12

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 May 30, 2002 RLB 13

E.g., part of a 16 page process

-- V1.02 & V2.2

-- 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)))); May 30, 2002 RLB 14

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??

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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 May 30, 2002 RLB 16

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 May 30, 2002 RLB 17

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??

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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! May 30, 2002 RLB 19

Solution to VHDL Problem

• Detailed design review • Make sure designs are reviewable and transferable • Don’t use VHDL May 30, 2002 RLB 20

May 30, 2002

VHDL Review Tools and Techniques

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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 May 30, 2002 RLB 22

.srr files

• Flip-flop replication • State machine encoding and illegal state protection • Inferred clocks • Resource usage May 30, 2002 RLB 23

.adb files

• Check timing • External part timing • I/O pin options May 30, 2002 RLB 24

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 May 30, 2002 RLB 25

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 May 30, 2002 RLB 26

Most Important Tool:

• Your thought and logical reasoning • There is no algorithm for design review • Based on the type of work you have to do (simple review or reverse engineering), – Partition the design into simple blocks – Start with what you understand and move out – Ask all the questions you need to – Model and simulate as necessary May 30, 2002 RLB 27

Suggestions for FPGA Design Presentation

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 May 30, 2002 RLB 29

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 May 30, 2002 RLB 30

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 May 30, 2002 RLB 31

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 • Model: MSI part data sheet May 30, 2002 RLB 32

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 FPGA-level VHDL simulation May 30, 2002 RLB 33

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 May 30, 2002 RLB 34

5. Describe state machines

• Encoding chosen • Protection against lock-up states • Homing sequences • Reset conditions May 30, 2002 RLB 35

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!

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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 May 30, 2002 RLB 37

8. Provide timing spec for external timing analysis

• Tsu, Th with respect to clock • Clock to output Tpd • Tpw for signals connected to flip-flop clocks • Clock symmetry requirements if both edges of clock used May 30, 2002 RLB 38

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 May 30, 2002 RLB 39

References

• Design guidelines: http://klabs.org/DEI/References/design_guidelines/nasa_guidelines/index.htm

• Design tutorials http://klabs.org/richcontent/Tutorial/tutorial.htm

• Design, analysis, and test guidelines: http://klabs.org/DEI/References/design_guidelines/design_analysis_test_guides.htm

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