482005 maplddesign integrity concepts letting constraints work for you proportional design

12005 MAPLD Design Integrity Concepts

Letting Constraints Work For You

Proportional Design


Agenda – Proportional Design

• Conceptual background

• Types of constraints

• Examples

• The proportional design mindset

• Summary


Conceptual Background

• Three parts to solving a problem:– Need, solution set, constraints

• All parts have a role to play in the solution

• Ignoring any of them will lead to problems


Conceptual Background (cont.)• Example

– Need: means of conveyance to work– Solution set: Skateboard, bicycle, bus, jogging shoes,

mid-size sedan, luxury car, helicopter– Constraints: Distance (6 miles), $, not on bus route, $,

not in very good shape, $– Solution: 1992 Honda Accord (120 kmiles, 4 k$) – The constraints guide selection of the solution from the solution set

• The particular solution is not necessarily -– The cheapest (roller skates)– The most desired (Lexus LS400)– What is perceived as best for society (bus)

• But … the best overall fit to the needs


Conceptual Background (cont.)• Definitions

– Constraint: the state of being checked, restricted, or compelled to avoid or perform some action (AH)

– Proportional: corresponding in some degree or intensity (AH)• Proportional design is design that results in a product

“sized” appropriately to the needs and restrictions of the specification

• The concept of proportional design:– Accepts the reality of constraints– Attempts to optimize the solution given the constraints– Accepts that the constraints provide benefits (more later)

• More efficient designs• More thorough designs• More correct designs• Caveat – All other things being equal


Types of Constraints

• External (mass, power, cost, quality)• Internal

– Derived (packaging, architecture, component availability, maximum clock speed)

– Self-imposed• Design rules/guidelines (free space, clock use, logic structure,

HDL language)

• Documentation style (pre-design, post design)

• Component acceptability (maturity of part, limited use of various features


Examples (1)• Problem: provide decoding logic for memory map

– 0-3FFF = SRAM; 4000-4FFF = Peripheral; E000-FFFF = PROM

• Constraint: use minimum amount of logic

• But what about …– Unused addresses, future expansion, etc.

• Doesn’t matter – given the constraints


Examples (2)• Problem: provide all combinational / sequential logic for

the RADARSAT ACP• Constraint: Only low density high speed logic available

(16X8 PALs, MSI/SSI logic)• What was forced by the constraint?

– Careful mapping of peripherals into available address space– Careful partitioning between:

• Programmable logic and MSI/SSI• MSI/SSI functionality

– Efficient data bus partitioning (tri-state enable issues)– Special attention to component delays at the gate level


The Proportional Design Mindset• Constraints inevitably foster attention to detail (creativity

“inside the box”)– With respect to methodology– With respect to level of planning– With respect to implementation

• Attention to detail is of inherent value because it produces carefully structured, well-thought out designs– Improved up-front correctness – Decreased design post-processing time (simulation, verification,

validation, lab time)– Efficient designs that meet the stated requirements– Increased reliability

• Therefore, constraints are welcomed, whether externally imposed or self-imposed


The Proportional Design Mindset (cont.)

• Examples of self-imposed constraint– Ignoring achievable flexibility (when not

necessary)– Removing non-specified capability– Avoiding gratuitous cleverness (especially with

abstract design techniques)– Rejecting brute force solutions without analysis



• Characteristics of the right mind set– Planning before starting– Reviewing before finalizing– Simplifying ruthlessly– Making the design do only what it must– Viewing resources as precious commodities to be used

only to the extent needed– Understanding the implication of the design’s level of

abstraction– Being satisfied with the result



• Why aren’t self-imposed constraints more common?– They aren’t absolutely essential because we have:

• Lots of logic space [FPGAs, ASICs]• Lots of memory space [DOS file systems, complicated operating

systems]• Lots of bandwidth [fast data busses, general purpose communications

protocols]– They don’t match the current paradigm

• Flexibility is all-important [re-use, re-configure, adapt]• Specifications are malleable late in the game• Software changes, why can’t hardware?• We can catch problems in simulation and reprogram the part

– They aren’t fun– We don’t train people to value constraints and work within them

• This is unfortunate because constraints can make our job easier without degrading the end product


Summary

• The proportional design mindset is important because it:– Focuses on fulfilling needs, not wants

[specification orientation]– Deepens understanding of the final design

[ownership oriented]– Avoids unnecessary effort [efficiency oriented]– Fosters simplicity that aids verification and

validation [quality oriented]

482005 maplddesign integrity concepts letting constraints work for you proportional design

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