More Than the Sum of Its Parts Design Partitioning

2005 MAPLD 61 Design Integrity Concepts

Agenda – Design Partitioning • Definitions and examples • Why partition an electronic design • Guidelines for partitioning an electronic design • Why isn’t it done more often?

• Summary

2005 MAPLD 62 Design Integrity Concepts

Definitions and Examples

• Partition – to divide into parts or shares – Examples: budgeting, outlining, WBS development • Design partitioning refers to the deconstruction of a design into various sub-designs in an ordered and logical manner • Goal – to simplify the whole by optimizing the parts and thus increase: – Efficiency – Reliability – Maintainability 2005 MAPLD 63 Design Integrity Concepts

Why Partition (General)?

• Complexity interferes with ready comprehension – Comprehension of a complex system depends on ability to impose order upon it – A given mind has a finite capability to impose order which depends on the quantity and structure of the data related to the system – The more complex the system, the more difficult its comprehension • Partitioning a design introduces a piece-wise reduction in complexity – Reduces quantity and complexity of design to manageable “chunks” – Improves comprehension of the various parts of the design – Increased comprehension of the parts leads to better comprehension of the whole • Better comprehension of the whole increases the ability to – Verify correctness of the design – Correct errors in the design – Update the design when necessary 2005 MAPLD 64 Design Integrity Concepts

Why Partition (cont.)?

• Programmatic advantages – Refines scope of work • Identifies unexpected effort • Identifies reuse possibilities • Identifies staffing requirements – Identifies schedule dependencies • Improves allocation of resources • Identifies parallelization and schedule enhancement opportunities – Promotes management visibility 2005 MAPLD 65 Design Integrity Concepts

Why Partition? - Design

• Improved interface organization / more formal structure – Interfaces between functions have more predictable characteristics – Expansion by addition of functions is more controlled • Enhanced functional encapsulation – Individual functions have predictable results when invoked – Functional design enhancements have limited side-effects when installed – Effects of faults are more easily predicted and mitigated • Efficient design implementation – Functional (fewer functions and types of functions) – Data flow (fewer, more sensible data busses) – Control flow (simpler address decoding and state machines) • All are side effects of additional thought put into “How?” 2005 MAPLD 66 Design Integrity Concepts

Why Partition? - Correctness

• Simpler inspection – Functionality may be “obvious at a glance” – Error space is more limited • Within the function or within the interconnect • Simpler Qualification – Verification can begin with encapsulated modules or circuit subsets – Overall functionality correctness becomes less of a “late” concern because subset functionality is proven correct “early” in the process • Simpler / more thorough review – Structure provides orientation to peer reviewers – Encapsulation allows easier review – Peer input more likely to be useful 2005 MAPLD 67 Design Integrity Concepts

Why Partition? – Maintainability

• Clearer documentation – Documentation has “smaller” parts to focus on – Structure of documentation grows from the design structure • Simpler maintenance – Changes affect only enhanced area – Interactions between changed area of design and remainder of original design is controlled by a formal structure • Enhanced reuse – Sub-circuits / functions usable in other applications as long as the interface structure is observed • Inherent capability of design better understood 2005 MAPLD 68 Design Integrity Concepts

Guidelines for Partitioning

• Take advantage of organizational strengths – Expertise (analog, digital, software, etc.) is seldom the same across organizations – Partition the design in accordance with organizational strengths according to primary functions – Divide auxiliary functions (those that can be assigned to multiple organizations) so that • Interfaces are simplified • Workload is equalized • Functions are easily tested without requiring all of the hardware 2005 MAPLD 69 Design Integrity Concepts

Guidelines for Partitioning (cont.)

• Example: Alice UV spectrograph electronics (Rosetta) • Core expertise – Sensor Sciences: Detector and front end electronics – Mobius Systems: Embedded software – SwRI ® space systems: Digital, low speed data acquisition – SwRI ® space science: LV and HV power supplies • Primary partition per core expertise 2005 MAPLD 70 Design Integrity Concepts

Guidelines for Partitioning (cont.)

• Alice example (work breakdown chosen) – Sensor Sciences: detector electronics through parallel digital output – simplified interface – Mobius: instrument control / protocol servicing (some error decoding partitioned to H/W) – Space systems: microcontroller; s/c interface; heater, motor, door digital control; analog high-voltage control – Space sciences: LVPS; HVPS; motor, door, and heater drive circuitry 2005 MAPLD 71 Design Integrity Concepts

Guidelines for Partitioning (cont.) • Alice example – partitioning decisions

– Auxiliary expertise split along sensible interfaces • C&DH to detector – analog or digital? (noise, ease of test) -

digital

• C&DH to HVPS – analog or digital? (space, noise susceptibility, ease of test)

- analog

• C&DH to S/W – protocol decoding? (performance margin, logic capability) –

hardware error decoding, s/w protocol encoding

2005 MAPLD 72 Design Integrity Concepts

Guidelines for Partitioning (cont.)

• Use specific functionalities – Combine functionality with very similar characteristics • Low-level analog / discrete bi-level (comparator is difference) • Example: Spacelab flex interfaces – Cluster related functionalities • Shared data-flow direction, shared logical control • Examples – Constant level discretes / pulsed discretes – Low-level discretes / high-level discretes 2005 MAPLD 73 Design Integrity Concepts

Guidelines for Partitioning (cont.)

• Use specific functionalities (cont.) – Isolate related functional sub-groups from other items • Ex. – analog data acquisition group (multiplexers, converters, signal processing, control logic) • Isolate – Through appropriate data/address buffering – Through separate programmable logic sub-sets – Exploit directionality • Write functions; read functions; read/write functions – appropriate buffering • Examples – Write: Analog output, digital output, telemetry – Read: Analog input, digital input, command – R/W: Memory, bi-directional discretes, GSE 2005 MAPLD 74 Design Integrity Concepts

Guidelines for Partitioning (cont.)

• Exploit operational considerations – Operational considerations often determine the specific configuration of a set of common components – Example: memory • Components: memory, write control, read control, sequencing, buffering • Application: telemetry system – Packetized / unpacketized?

– Asynchronous timing / TDM ?

– Science data / engineering data?

– Pushed / pulled ?

• The type of telemetry system determines the partitioning 2005 MAPLD 75 Design Integrity Concepts

Example – Science Data Storage and Readback System • How should the logic be partitioned?

– Is write logic part of science data process or memory process?

– Is read logic part of telemetry system or memory process?

– How complicated is the arbitration • How it is partitioned depends on specific operational requirements 2005 MAPLD 76 Design Integrity Concepts

Example - New Horizons Alice Science Data

• • Alice Operation – Slow source accumulation relative to output speed – Push interface initiated by instrument Alice Implementation ( others likely in different circumstances ) – Block 1: handshake and latch data – Block 2: Ingest data, process, write to memory – Block 3: Read, serialize, send, blank memory – Arbitration simplified to a switched double buffered memory access (no real-time arbitration) 2005 MAPLD 77 Design Integrity Concepts

Guidelines for Partitioning (cont.)

• Ensure encapsulation is reflected in the form of the engineering documents – Functions contain many types of operations – Example: Telecommand interface • De-serialization, decoding, error determination, re-packetization, temporary storage • Real time functionality [level 0], forwarding to software • Storage access / arbitration, status log maintenance, data-bus handshaking – Partitioning should ensure that all reasonable aspects of a function are in one locality (we are ordering data for understanding) • One (or a few) pages in a schematic • One module in HDL • One object in software – Benefits: readability, error determination, testability, maintainability 2005 MAPLD 78 Design Integrity Concepts

Ordering Example – Bus Structure

• A logical bus structure – Simplifies data flow – Eases expansion / enhancement – Identifies bottlenecks / opportunities for efficiency – Ensures signal compatibility – Reduces timing uncertainty (capacitive loading) – Reduces power – Simplifies control logic / arbitration – Simplifies analysis 2005 MAPLD 79 Design Integrity Concepts

Ordering Ex. – Bus Structure (cont.)

Before: After: 2005 MAPLD 80 Design Integrity Concepts

Ordering Ex. – Bus Structure (cont.) • Directional data flow is clustered • High-speed access devices (SRAM) not buffered • Exclusive functions clustered (PROM/ EEPROM) • Simplification of

– Timing – Loading – Control 2005 MAPLD 81 Design Integrity Concepts

Why is Partitioning Not a Priority?

• It is – at the S/C, sub-system, and box level (sometimes) • Why not at the board level?

– Requires potentially significant planning effort (schedule / cost) – The tool syndrome (CAD / CAE) • Crush creativity by forcing an early start to design • Primarily a way to communicate between software packages rather than humans (schematic => PWB) – Too much junk being placed in too little space (simplification may not always be space efficient) – Lack of emphasis from senior engineers – Boards aren’t where the action is (FPGAs) – less effort placed on them 2005 MAPLD 82 Design Integrity Concepts

Why is Partitioning Not a Priority? (cont.)

• At the FPGA level – Lack of solid design methodology • Methodology must be tailored to a tool • VHDL / Verilog are functional descriptions • VHDL / Verilog don’t inherently enable data flow visualization or block oriented deconstruction • The synthesizer can understand non-partitioned design – Perception of expansive resources (no / few constraints) – The tool syndrome (“I must start coding”) – Lack of effective design quality gates prior to start of detailed design 2005 MAPLD 83 Design Integrity Concepts

Summary

• A design is only as solid as its weakest part • Proper planning and partitioning of a design: – Ensures individual functions are logical and complete – Ensures interconnects are ordered and efficient – Provides for improved reliability / verifiability – Allows easier modification and enhancement – Enhances detailed understanding of how things work • Partitioning requires ordering the design by considering – The capabilities of the team – The functionality of the modular pieces – How the design will operate – The individual components of a particular function • Partitioning a design ensures that all parts are solid – resulting in a solid whole 2005 MAPLD 84 Design Integrity Concepts