Space Electronics
cell or a logic flip-flop. For earth-orbiting satellites, the vast majority of charged particles that satellites encounter are electrons and protons. Heavy ions are also encountered, and have the highest probability of interfering with the operation of electronic components. The effect that the charge build-up has
on a circuit depends on its response. The capacitance, C, of the node approximately determines how large a voltage swing, dV, results from the increase in charge, dQ, according to dV=dQ/C. The positive feedback loops in latches and SRAMs will cause a bit flip once the collected charge reaches a critical value, Qcrit, that is large enough to drive the node’s voltage past the switching voltage. The Qcrit has dropped with Moore’s Law scaling. SEUs in static latches and SRAMs became a problematic issue after feature sizes dropped below 10µm and the corresponding Qcrit fell below 1pC. Error-detection and correction circuitry can address the problem for data cells. Logic circuits can require more extensive protection mechanisms - the use of triple modular redundancy (TMR) is not uncommon in space-borne designs.
FPGA technology Field-programmable gate array (FPGA) technology is often used for custom circuitry as it provides more development flexibility than hardwired application- specific integrated circuits (ASICs), not only in one-off projects but also higher-volume space programmes. But FPGA technologies are not equal when it comes to the threats from space-borne radiation. The configuration cells in SRAM-based FPGAs are not typically protected from SEUs. As a result, a strike can change the state of a memory cell and the design of the circuit it controls, potentially leading to a catastrophic failure. FPGAs based on non-volatile technologies such as antifuse or flash, both manufactured by Microsemi, are effectively immune to these radiation- induced configuration memory upsets: the captured charge dissipates harmlessly without altering the programme state of the cell or its accompanying circuit trace. Non-volatile FPGAs still contain flip-flops so parts aimed at space applications such as the Microsemi RTAX-S/SL/DSP family has built-in triple modular redundancy (TMR) to guard against changes caused by radiation. If the particle has high enough energy to flip the state of a flop, the other two flip-flops in the redundant circuit will
www.cieonline.co.uk
‘outvote’ it, ensuring correct operation. This provides the assurance that many satellite users demand.
The five payloads that UKube-1 contains represent a mix of commercial and academic projects. One payload is designed to demonstrate the application of cosmic rays to random-number demonstration; another the long-term effects of that same radiation on electronic circuit behaviour. Joining those payloads is a programmable host for experiments that will let schoolchildren access the capabilities of the satellite remotely.
A key module in the 3U satellite is the Mission Interface Computer (MIC) developed by Steepest Ascent, one of the partners in the UKube-1 project. The MIC performs all the housekeeping tasks the satellite payloads need: gathering data; processing it; transferring the data to ground stations; and relaying commands from Earth-bound users.
Steepest Ascent has put a lot of time
and effort into finding ways to use predominantly commercial-grade components in a system that is to be deployed into a high cosmic-ray environment. TMR offers an approach but it is expensive to implement across the board - making the use of commercial- grade components less attractive from a cost standpoint. As a result, the company has focused its use of TMR and chosen non-volatile FPGAs over SRAM-based parts.
Steepest Ascent chose to use a
Microsemi antifuse-based FPGA for the core logic functions. For a separate signal- processing FPGA that is used to filter and compress data from the other experiments onboard UKube-1, Steepest Ascent picked another member of the Microsemi family - the flash memory-based ProASIC3L. The use of flash memory technology also makes it possible to reprogram the signal-processing algorithms, although the ProASIC3L in the initial UKube-1 launch will have fixed functionality. With both antifuse-based and flash- based FPGAs in place, the UKube-1 project demonstrates how non-volatile FPGA technologies can span the needs of an increasingly diverse space industry.
Microsemi Corporation |
www.microsemi.com
Ken O'Neill is Director, Space Marketing at Microsemi SOC Products Group
Components in Electronics September 2012 19
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44