Probability of latching single event upset errors in VLSI circuits

Abstract

The ability of radiation to cause transient faults in space borne as well as ground based computers is well known. with the density of VLSI circuits increasing every year, the probability of an upset by radiation is becoming more likely. However, research in this area has matured over the last decade, and the mechanisms which cause such faults are better understood. This understanding enables us to propose ideas to eliminate or lessen the effects of radiation on VLSI circuits. <p>Most of the research to date has concentrated on the effect of transient faults on flip-flops rather than combinational logic. This is due to several reasons. First, transient faults, also known as Single Event Upsets (SEU), were first observed in memory circuits located on board satellites. Second, an SEU can leave a lasting effect on a circuit if it occurs in a flip-flop, and third, SEUs can cause the output of a flip-flop to change state more easily if it occurs directly in the flip-flop rather than in the combinational logic. <p>In combinational logic, the node struck by the radiation is completely disjoint from the flip-flops output node. This in effect causes the SEU to satisfy more criteria in order to change the flip-flops output state. The criteria that the SEU must satisfy tend to be complex, and this complexity has caused many researchers to believe that SEUs that occur in combinational logic cause negligible errors in the state of flip-flops. <p>Thus, in this thesis, the criteria for latching a SEU are discussed, and original methods are presented that can be used to determine the probability of an SEU occurring at any node in a circuit will cause a change in the output state of a flip-flop. The methods are then incorporated into a program, named SUPER II, that is able to evaluate the circuit to determine the nodes with the highest probability of having a SEU error latched. The results from the program show that SEUs that occur in combinational logic can have a significant probability of becoming latched.