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A Rad Hard ASIC Design Approach: Triple Modular Redundancy (TMR)
By AsicNorth
TMR is not a new idea in the world of ASIC design. It was published as far back as 1962 in the IBM Journal of Research and Development. However, it has become an essential design solution for ASIC chips sent into space, a vast environment filled with radiation.
Most overviews of TMR within ASIC design are described at a high level. This discussion will center on a more detailed approach for integrating TMR within a physical design methodology using the Cadence Design Systems Innovus Tool.
What Is Triple Modular Redundancy and How Does It Work?
Electronic systems operating in a radioactive environment, such as outside the Earth’s atmosphere, will eventually encounter malfunctions. These malfunctions are due to the high energy particles from radiation passing through the circuit transistors.
This energy can alter the transistor’s performance, which can cause the circuit to shift voltage levels. The upset voltage shifts can affect data being processed and look like noise, while state machines could be upset and completely malfunction. The integrity of data values — whether for control, microcode, or measured data — becomes suspect.
TMR techniques can mitigate these system upsets. System changes will require additional logical structures and physical implementation techniques to prevent the radiation from altering the design function.
The Effects of Radiation on Electronic Systems
Radiation is energy or matter that moves through space. It can occur as electromagnetic or particle radiation. Radiation is classified into two categories — ionizing radiation and non-ionizing radiation. When ionizing radiation encounters materials such as semiconductor circuits, it can knock electrons out of the semiconductor’s atomic orbits and change the circuits’ operating properties.
Electromagnetic radiation is the transfer of energy from massless photon particles moving at the speed of light. It is assessed in the frequency spectrum. The higher the frequency, the higher the energy presented by the radiation. Electromagnetic radiation in the ultraviolet, X-ray, and gamma-ray spectrums poses the highest damage risk to semiconductors.
Particle radiation comprises fast-moving subatomic particles that have intrinsic mass, such as protons, electrons, and nuclei of atoms. The speed these particles travel can be at any magnitude up to almost the speed of light. These particles have great penetrating power, making shielding a challenge.
Radiation can impact electronic systems either as a Single Event Effect (SEE) or as a Cumulative Event Effect (CEE).
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