In-flight upset - 154 km west of Learmonth, WA, 7 October 2008,

• A non-destructive, unrecoverable change, or firm error. 222 These include singleevent functional interrupts (SEFI), in which the device ceases to functionproperly until its power is cycled.• A destructive, permanent change, or hard error. These include single eventlatchups (SEL) and are the result of permanent damage to a component that isnot recoverable even by cycling the power OFF and ON.In common usage, both soft and firm errors can be referred to as SEU.About 1% of SEEs affect more than one binary digit (bit) with a single particleinteraction, known as a multiple bit upset (MBU).Sensitivity to SEEThe probability of an SEE event occurring to any given system is dependent onnumerous factors, including the:• flux, energy and type of particles present at the system’s position• energy, location and direction of a striking particle• sensitivity of a struck chip, including the number and size of internal transistors,and the amount of charge accumulation required to affect a transistor.Any active electronic component or device can be susceptible to SEE, especiallydigital devices such as CPUs, memory, and other digital integrated circuits. As thechip density of integrated circuit components has increased greatly in recentdecades, digital systems have generally become more sensitive to SEEs thanpreviously. In particular, memory chips and central processing units (CPUs) aretypically most sensitive as they have the highest transistor densities, although anykind of silicon chip may experience SEEs. Chip designs with higher componentdensities can also be more susceptible to MBU.An integrated circuit’s sensitivity to SEE varies throughout production due tonormal variations in materials and dimensions from time to time. For integratedcircuits produced in the late 1980s and early 1990s, the sensitivity to SEE betweenbatches can be as much as a factor of 10, and a factor of two to three betweenintegrated circuits within the same batch. Sometimes a specific variation will beunintentionally introduced into a batch that will make the components significantlymore or less susceptible. A component that has had an SEE can sometimes becomemore vulnerable to subsequent SEE due to physical changes in its internal structure.Passive devices such as analogue filters, capacitors, and resistors are not generallyconsidered susceptible since their behaviour does not change significantly with thesmall charge transfers typical of SEE events.Effects of SEEComputer systems are generally very complex, incorporating large amounts ofprocessing, transmission/reception, memory, and other devices. Within a single chipsuch as a CPU, thousands of computations are conducted every second and different222Elsewhere in this report, usage of the term ‘soft error’ includes firm errors.- 262 -