The NASA ASIC Guide: Assuring ASICs for SPACE
Chapter One: Technical Specification
Objective:To provide ASIC managers and designers a method of mapping a set of system and organizational requirements into a detailed set of device functional, performance, and test specifications.
A well-written ASIC specification creates an unambiguous ASIC device description that guides the ASIC designer's development work, the ASIC vendor's manufacture, test and screening work, and the system implementor's work. Using this description, which may exist at many levels, ASIC program managers can measure the technical progress of the ASIC design. The specification also forms the basis for contracting and procuring the ASIC parts.
Generally, the ASIC designer and a parts specialist jointly create a detailed technical specification. Once generated, the design team negotiates the specification with the ASIC vendor for any exceptions that a vendor may choose to take. Besides the ASIC designer, any ASIC device user needs this document. We recommend that it provide the kind of information typically found in any standard VLSI parts handbook.
We limit the focus of this chapter to the technical specification. Please refer to the management section and the procurement support appendix for information on other types of contract information. The guide's Appendix Six shows a technical specification case study of one of the gate arrays used in the JPL Attitude and Articulation Control Subsystem for the Cassini spacecraft. Below, we will discuss major areas of an ASIC Technical Specification.
ScopeThe "Scope" section of the technical specification provides a clear definition of the document, including what the specification covers and how to use it.
Applicable DocumentsThe list of "Applicable Documents" together with the technical specification contain all requirements levied on the ASIC, such as government documents, NASA center or prime contractor documents, vendor documents, and the project requirements.
Conventions"Conventions" covers any special notation used to make the technical specification easier to read and understand. This part of the specification usually refers to the conventions used for signal names.
Additional RequirementsThis section contains technical specifications not covered in the other sections of the technical specification document and may include CAD tools, processing, packaging areas, etc.
Chip OverviewThis section offers a brief description of the ASIC's functions and features. It also includes a block diagram of an ASIC device and how it may fit into its intended target application.
ASIC Signal Designations and DescriptionThis part of the specification provides a pin-out and pad designation (useful when probing--but not a must), type of a signal (input, output, tri-state, etc.), its drive when active, I/O receiver and driver macro names, and signal description.
Functional DescriptionThe "Functional Description" describes ASIC functions in detail. As a specification typically describes an ASIC design in a hierarchical fashion to ease of understanding and verification, describe each module and sub module in detail including control and data flow. Also describe any internal buses, any internal registers, all internal and external timing diagrams, various modes of operations and external interfaces, and define all acronyms used.
Testability ConstructsUse this section to point out any testability features implemented to make a device more testable. Describe the list of I/O signals (if applicable) used to implement boundary scan. To facilitate DC tests, also specify here all testability enhancements, such as synchronous full scan, partial scan, built-in self-test methods, and NAND tree approach.
Electrical CharacteristicsThis portion of the specification concerns various parameter limits and how they can be used, measured, and verified, including:
ELECTRICAL TEST REQUIREMENTSList all required burn-in tests, how delta limits are calculated per MIL-STD-883, and applicable subgroups in "Electrical Test Requirements."
ABSOLUTE MAXIMUM RATINGThe vendor usually has this section readily available. It states the absolute electrical and thermal limits to which a device can be subjected and that a vendor will guarantee. Designers must calculate maximum power dissipation and maximum output current as they will vary from ASIC to ASIC. The project drives the electrostatic discharge voltage (VESD) requirement, which must to comply with vendor's capability.
RECOMMENDED OPERATING CONDITIONSThe vendor data books on cell characteristics can provide this part of the specification, excepting the device dependent maximum operating frequency parameter.
DC CHARACTERISTICSThis section lists all vendor guaranteed DC parameters such as VIL, VIH, VOL, VOH, IOZ, IOS, CIN and COUT parameters. Use this section to point out any exceptions or any special condition concerning device testing.
IDDQ TESTINGIf your project requirement calls for negotiating with the vendor IDDQ testing of ASIC devices, source of the test vectors, the number of measurement points and measurement limits for IDDQ, explicitly state that here. If your device requires IDDQ testing, avoid using pull-up or pull down resistors altogether, since use of such resistors will reduce the amount of toggle coverage during IDDQ testing.
PULL-UP AND PULL-DOWN RESISTORSList all pull-up or pull-down resistors used either internally or for I/Os here. Also list any signals which, for example, may have been used for testability enhancement and must be held either in low or high state during flight.
AC CHARACTERISTICSBased on the simulation results signed off at Critical Design Review (CDR) time, the ASIC vendor will guarantee all functional testing at slow speed (typically at 1 or 2 MHz) and "AC tests" at system speed under extreme voltage and temperature conditions. Make sure the vendor checks and screens any parts not meeting critical timing parameters, such as, propagation delays, setup and hold times and pulse widths with either minimum or maximum limits. Adjust all AC parameters for modified tester output levels and loading. Also describe applicable switching test circuits.
BURN-INThe parts specialist defines and negotiates with the vendor on various static and dynamic burn-in conditions, as well as terminal connections.
DELTA LIMITSEstablish these limits per MIL-STD-883, for parameters such as input and output leakage currents (in high and low states) for all used I/O buffers, sinking and sourcing currents for output buffers, output low and high voltage levels, and for quiescent current.
Physical CharacteristicsThis section contains information on pin assignment, bonding diagram, package outline, and marking diagram.
PIN ASSIGNMENTTaking number of simultaneously switching outputs per VSS/VDD pair into account, the ASIC designer comes up with a preferred pin assignment, obeying rules imposed by the vendor for power and ground pins placement. The vendor must approve this preferred assignment before it can be incorporated into the specification.
BONDING DIAGRAMThis diagram shows the package pin-to-die pad assignment. Some vendors will have a fixed assignment that a designer has to use; other vendors will give some freedom in this assignment.
PACKAGE OUTLINEThe vendor provides the package outline information, which shows all mechanical dimensions and tolerances.
MARKING DIAGRAMProduce this diagram according to MIL standard documents for marking diagrams. Besides information such as part number, date code, pin number 1 indicator and ESD identifier, etc., include traceability capability to die information.
DEVICE STATISTICSThis provides a quick glance at device statistics such as total number of used gates, number of input, output and bi-direct signals, use of testability constructs, package type, fault coverage, etc.
EXCEPTIONS TO ANY PROCUREMENT DOCUMENTNegotiate any variations and deviations that may apply to documents mentioned earlier in this chapter with the vendor and list them in this section.
- The ASIC designer and his manager take responsibility for the technical specification. While they should seek appropriate help from others, the final responsibility must remain with them.
- Technical specifications amount to much more than paper descriptions of a circuit, completed mainly after the vendor delivers a chip. The many roles a technical ASIC specification may fill include:
- a contract between ASIC manager and designer(s), which defines designers' deliverables
- a detailed "data book" for other device users
- the important contract between the ASIC design group and any other internal (quality assurance, test, reliability, etc.) or external (ASIC vendor) groups
- A complete technical specification must include all applicable documents (see Appendix Six), so the reader can find further information on any particular subject.
- Remember the technical specification is part of a contract package including the general contract and the statement of work (SOW). Keep all three contracts in mind when creating the technical specification. Try to support procurement by integrating the technical specification with them. This will also reduce potential errors that the ASIC vendor may generate in translating your technical specification into their own internal circuit and device descriptions.
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