Availability is defined as the ratio of the time a system is working, divided by the time the system is working and not working. So, for example, a system with .99999 availability has about five minutes and 26 seconds a year unavailability. “5-nines” is a common requirement in the telecommunications industry, usually applied to the trunk side of the equipment. The line side often has a smaller availability, as it is allowed to be out of service more often. Traditionally, out of service meant a hardware failure, and a hardware failure meant replacement of a circuit card. In order to meet the requirement there was a “hot-standby” circuit card ready to take over in the event of a failure.
Enter the Soft Error
Cosmic Rays from Novae (super and not so super) and protons from our Sun impact our atmosphere and create streams of neutrons (and a few protons) that make it all the way to the earth’s surface where they may disrupt microelectronics.
The following link details the latest family’s robustness to these soft error effects:
For the latest data on Single Event Upset (SEU), see:
I will not go over the Rosetta program here, but it is the best, most accurate data, published quarterly. No other manufacturer does this, which should tell you that Xilinx is far ahead of everyone (ASIC, ASSP, FPGA).
Remember to always use ug116.pdf (above) for the latest data.
Not every particle strike which results in a transient or flipped bit causes a functional failure. De-rating refers to the ratio of particle strikes to functional failures. In an FPGA device, the ratio of hits to functional failures is from one in ten (1:10) to one in eighty (1:80), depending on the nature of the customer’s design. For example, a 256-bit AES encryption and decryption core fully unrolled (all actions performed in parallel), if it filled an entire FPGA device, is about 1:11. An FPGA device programmed to watch how you are driving and help you avoid dangerous situations (like a car stopped ahead of you) is about 1:80. Why is this?
Depending on the number of signals that are critical to the function of the design at a clock edge, the de-rating can vary between the above two extremes. Since in any design roughly 90% of the FPGA device is completely unused (all those potential interconnects are not used), it is sensible that the de-rating cannot be worse than 1:10. But, how can it be 1:80? Not every signal was critical on each clock edge. Examples of this are: self-check logic that has no effect on function if it fails, startup logic that is used only once, error handling (like ‘404’ “page not found”) which is used only in exceptions, and bits of wires which just do not matter (like address bit 4 of the stack pointer when the stack is never deeper than 7).
SEU IP Core
Released recently, and now fully supported, the SEU monitor IP core finds and fixes upsets:
Additionally, through the use of an additional SPI Flash device, the core identifies bits which are essential to the operation of the FPGA device. Unfortunately, Xilinx is only able to determine if these bits would affect the hardware operation of the FPGA device; we cannot determine if the bit flip would affect your design. This feature identifies roughly one in three upsets as “essential.” Identifying the upset as “critical” to your design is still a subject of research. If needed, the core may be used to walk through the list of essential bits, with you keeping track of which bits really were critical. A new map can then be made which informs you only when a functional bit flip has occurred. This is done by using the “error-injection” feature of the SEU Monitor IP core which allows you flip any bit, and then see what your system does as a consequence of the flip.
OK – But How Does this Affect Availability?
Imagine if every time an upset occurs, you take yourself out of service, reconfigure, and start over. Instead of causing a functional failure every one in 10 to every one in 80 times, you are out of service one in one, or every upset. This might be prudent if the system can tolerate a short outage. The driver assistance design is a good case in point: 100 milliseconds of outage is OK, not detecting that stopped truck ahead of you is not OK. However, in the majority of cases, the already low level of upsets does not warrant getting your customers upset with each and every upset (Pun intended: it can be very upsetting).
The most common level of mitigation for upsets is just to log that it happened. The system log then contains the time and date of the event, and now in perhaps the one in 10 to one in 80 times there is an upset there is a matching outage, while the system at a higher level took some remedial action. The upset is also repaired, as repair is shown to improve the mean time to functional failure by an additional 30% or so.
If the logging strategy does not meet the availability requirement, then using the essential bits feature is unlikely to meet the requirement, unless the design can transfer operation smoothly over to a spare unit while the unit struck repairs itself and is ready to go back into service.
“I Do Not Have Redundancy”
Some legacy systems made no use of redundancy, as you can quickly surmise; they probably never met their availability requirement, and never will. A hardware failure alone would be sufficient to violate the requirement, as dispatching a technician for repair and replacement means many minutes of down-time before service is restored. Soft errors, while more frequent than hardware failures (see page 20 of ug116.pdf above), rarely cause a non-redundant system to fail to meet the availability criteria--the lack of redundancy is the cause.
Even if triple-modular redundancy with triplicated voters is used to fully mitigate the effects of the soft error, the first hardware failure will cause the system to exceed its availability requirement.
Bottom line: if availability is a requirement, consider a redundant architecture. Use the SEU Monitor IP to find and fix, log, and transfer function gracefully to the spare unit in the event of an essential bit upset.
If availability is not the problem, but reliability is (mean time between failures), then consider find and fix, and log; do not attempt to stop, or reconfigure, or start over. In the case where you must know if you are OK or not, consider the use of the essential bits as the starting point to find the actual critical bits by flipping them one by one, and looking for functional failure. This might only be needed for safety critical systems where redundancy to a spare unit is too costly, takes too much power, or takes too much space.
With the addition of a separate module of RTL (Verilog or VHDL) to perform a watch-dog function, the restart option may be only taken when the watch-dog notes that the repair has not returned the system to a functional state. The last “single point of failure” then becomes the watch-dog, which is usually made as small as possible, and as simple as possible. For example, the SEU Monitor IP itself fails roughly 1:1500 upsets (as measured in a neutron beam), so that the likelihood of the SEU Monitor itself failing drops below that of a hardware failure.
As soon as the cause of failure drops below the hardware failure rate you are “done,” as nothing you can do will improve matters. Adding anything only increases the hard failure rate--more hardware, more probability of hardware failure! Often, engineers do not realize that there is a point of diminishing returns and do not know when to stop. It has been suggested that once any large source of failures is reduced to the background, underlying hardware failure rate, you are done.
If the resulting failure rate is too high, or availability too low, then you must be designing a human space flight vehicle or a nuclear reactor, in which case there are other clear requirements and methods to use, and criteria to meet. Remember, if you do have a safety critical system which may affect human life or the environment if it fails, you need to contact Xilinx. We need to inform you of the best methods to use the device if it is used to do these sorts of tasks, and what standards apply to its intended use that must be met.
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