By Juergen Fedrich, Director of System Engineering
Next generation design for radiation environments in space
Space is no longer the sole domain of well-funded governments. More and more commercial applications, such as telecom satellites, are finding homes above the earth. But with the increasing commercialism of space, there is a migration away from the use of traditional rad-hard components. Design teams are asking, "How rad-hard do components need to be for the system to work?"
In general, this has to be defined on a project-to-project basis depending on its mission, environment, budget and criticality. In mission critical systems, such as satellite guidance computers, true rad-hard components need to be used. Other systems may be able to use radiation enhanced, radiation tolerant, or even commercial parts if the system and board-level design take radiation into account. In all cases, designing with radiation in mind is essential.
SBS' Government Group, which specializes in designing and producing hardware for harsh environments, has supplied a number of boards and systems for usage on the International Space Station (ISS). One of the radiation-tolerant boards it produced is an Ethernet/High Speed Serial I/F (HLCU), shown in Figure 2. The major functional components are dual Ethernet controllers with 10BaseT interfaces, a 1 Mbyte, 4-port SRAM, 10 Mbit/sec serial links, and control FPGAs. The board was designed with dual control interfaces for redundancy. The board needed to meet the ISS radiation requirements:
|Total Ionizing Dose
||Below 1,4 krad (Si)
|SEU Threshold Linear Energy Transfer (LET) for Heavy Ions
||Above 36 MeVcm^2/mg
|SEU Threshold for Heavy Ions
||Above 10 MeVcm^2/mg
||Above 110 MeVcm^2/mg
The first step to meeting the radiation requirements was to analyze all its parts regarding availability of radiation data. For parts without known data, SBS identified functional replacement parts that had known data. The second step was to analyze all parts regarding radiation sensitivity. Simpler parts, such as buffer/drivers, met the radiation requirements. For all parts showing sensitivity at required levels, SBS sought replacement parts with better data. Due to market availability, it was not possible to eliminate/replace all sensitive devices. The remaining critical parts were handled on a case-by-case basis.
Radiation data showed that the Ethernet controller was sensitive to Latch-Up effects. As no other/better parts were available on the market, SBS designed a delatcher into the board. This delatcher controls the current consumption of the SONIC device. Whenever the SONIC device current exceeds a programmed threshold, the delatcher turns off the SONIC device and generates an interrupt to inform the software application about the occurred latch-up condition. After a defined time the delatcher reapplies the supply power to the SONIC and the board can continue with nominal operation.
The radiation data also showed SEU sensitivity in the memory and FPGA devices. To protect the memory, SBS chose and designed in an EDAC (error detection and correction) device that detects two-bit errors and corrects single-bit errors. For the FPGA, which was programmed to be a state machine, it was possible to overcome the SEU by using a "majority-vote" design. Instead of one register cell this design uses three registers cells, which are coupled by a voting circuitry in parallel. Whenever one cell content is flipped (e.g. due to SEU) the two remaining cells will out-vote the changed one and thus the system becomes SEU resistant.
While this example is for a space application, the same approach can be used for any design that might be subject to radiation effects. That may be more designs than people think. With the industry's move to reduce the size of device structures, radiation is becoming an important element to consider on sea level applications. By using the right technology, qualified parts and proper design, it is possible to create the right solution for radiation-sensitive applications.