Document: NUREG-0800
Document ID: 5e40cf60-028d-44c3-b9b8-735a6a68a48a
Document Type: srp
Title: Revision 6 - August 2016
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1602/ML16020A036.pdf
Revision Date: 2023-06
Chapter: 7
Section ID: 7
CFR Part: 
CFR Title: 

Content:
tly known methods and representative equipment. Design timing feasibility may be demonstrated by allocating a timing budget to components of the system architecture so that the entire system meets its timing requirements. See NUREG/CR-6083, Sections 2.2, 2.3.1, and 2.3.2, and NUREG/CR-6082. The timing budget should include internal and external communication delays, with adequate margins. Any non-deterministic delays should be noted and a basis provided that such delays are not part of any safety functions, nor can the delays impede any protective action. Software architectural timing requirements should be addressed in a software architectural description as described in SRP BTP 7-14. Databases, disk drives, printers, or other equipment BTP 7-21-8 Revision 6 – August 2016 or architectural elements subject to halting or failure should not be able to impede protective system action. Design Commitments Design-basis documents should describe system timing goals. Timing requirements should be satisfied by design commitments. A design should consider data rates, data bandwidths, and data precision requirements for normal and off-normal operation, including the impact of environmental extremes. There should be sufficient excess capacity margins to accommodate likely future increases in demands or software or hardware changes to equipment. Design basis documents should identify design practices that the applicant/licensee will use to avoid timing problems. Risky design practices such as non-deterministic data communications, non-deterministic computation, use of interrupts, multitasking, dynamic scheduling, and event- driven design should be avoided. When such practices are allowed, the applicant or licensee should describe methods for control of the associated risk. NUREG/CR-6082 and NUREG/CR-6083 describes risky design practices in more detail. Performance Verification The means proposed, or used, for verifying a system's timing should be consistent with the design.