Avionics hardware DO-254 clearly explained. DO-254 facts, myths, challenges, and successes described in this paper. DO-254 can be expensive, sometimes increasing hardware development/documentation costs by 150%. This DO-254 paper provides facts to hopefully reduce DO-254 costs by 20-50%.
DO-254 has been called “DO-178’s Little Sibling.” Like many little brothers and sisters worldwide however, the term “little” is often wrong and in a few areas the relationship itself is suspect …
The following provides an overview of DO-254 plus relevant differences with DO-178C.
DO-254, or more properly, “Design Assurance Guidance for Airborne Electronic Hardware,” was created as an obvious response to two simultaneous and related events:
1. Firmware was playing a larger role in avionics, with developers rapidly leveraging increased silicon-based complexity, and
2. The avionics firmware development process was unregulated, with certification performed after-the-fact at a high level.
While embedded avionics software engineering made huge strides and inroads in the eighties and nineties, firmware development was considered an informally adjunct art form. But what is “firmware”? When does “soft” become “firm” become “hard”? Time for a review …
Twenty years ago, firmware was relegated to specialized functions within avionics as compared to its highly varied role today. There were multiple reasons early firmware was more limited in aviation:
1. Firmware development tools provided limited flexibility compared to software
2. Firmware was difficult to change once burned or loaded in silicon devices
3. Firmware was considered a less-desirable option than software due to a seemingly more difficult debugging and update process
While there was truth in each of the aforementioned reasons, smart engineers coupled with more capable development environments assured the evolution and adoption of firmware within avionics. In particular, great strides in Field-Programmable Gate Arrays (FPGA’s) brought such firmware to the forefront of aviation. With FPGA’s, all of the aforementioned restrictions on firmware adoption were dramatically reduced. FPGA’s increasingly have very modern development tools, were easy to update, and allowed for potential flexibility and execution speed advantages over software-based logic.
As a result of this evolution (or almost “revolution”) in silicon based logic, avionics developers increasingly exercised the choice of implementing logic via silicon- instead of software. However, DO-178 did not strictly apply to silicon based logic and there was no regulatory counterpart for such; thus silicon-based logic could potentially avoid the certification rigor associated with software, even though the logic may have a nearly identical purpose between software and silicon. Thus the need for DO-254: to ensure that logic developed specifically for avionics purposes underwent a similar development and certification regimen as if it had been implemented via software using DO-178 …
DO-254 then covers Complex Electronic Hardware (CEH), e.g. hardware with embedded logic. DO-254 is:
• A flexible framework for the development of airborne hardware containing avionics-specific logic.
• Able to accommodate almost all types of hardware ranging from sensors, multiplexers, and switches to full-featured FPGA’s and ASIC’s.
• A guideline which tries to cover a nearly infinite spectrum of applications thus lacks specificity for particular projects.
Like software, the term “airborne electronic hardware” from DO-254’s title is wide-ranging. At the beginning and end of the day, hardware is part of a system or more specifically an aviation eco-system. Therefore, DO-254 is normally preceded by a safety assessment per ARP-4761 and an avionics system development process per ARP-4754A. And the hardware itself will typically be required to undergo environmental testing via DO-160. Therefore, DO-254 is merely one link within the avionics certification chain. Avionics will be neither safe nor compliant without this safe foundation which precedes DO-254.
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