Soft Error

In electronics and computing, a soft error is an error in a signal or datum that is wrong. Errors may be caused by a defect, usually understood either to be a mistake in design or construction, or a broken component. A soft error is also a signal or datum which is wrong, but is not assumed to imply such a mistake or breakage. After observing a soft error, there is no implication that the system is any less reliable than before. In the spacecraft industry this kind of error is called a single-event upset.

In a computer's memory system, a soft error changes an instruction in a program or a data value. Soft errors typically can be remedied by cold booting the computer. A soft error will not damage a system's hardware; the only damage is to the data that is being processed.

There are two types of soft errors:

• chip-level soft error:

These errors occur when the radioactive atoms in the chip's material decay and release alpha particles into the chip.

Because an alpha particle contains a positive charge and kinetic energy, the particle can hit a memory cell and cause the cell to change state to a different value. The atomic reaction is so tiny that it does not damage the actual structure of the chip. Chip-level errors are rare because modern memory is so stable that it would take a typical computer with a large memory capacity at least 10 years before the radioactive elements of the chip's materials begin to decay.

• system-level soft error:

These errors occur when the data being processed is hit with a noise phenomenon, typically when the data is on a data bus. The computer tries to interpret the noise as a data bit, which can cause errors in addressing or processing program code. The bad data bit can even be saved in memory and cause problems at a later time.

If detected, a soft error may be corrected by rewriting correct data in place of erroneous data. Highly reliable systems use error correction to correct soft errors on the fly. However, in many systems, it may be impossible to determine the correct data, or even to discover that an error is present at all. In addition, before the correction can occur, the system may have crashed, in which case the recovery procedure must include a reboot.

Soft errors involve changes to data — the electrons in a storage circuit, for example — but not changes to the physical circuit itself, the atoms. If the data is rewritten, the circuit will work perfectly again.

Soft errors can occur on transmission lines, in digital logic, analog circuits, magnetic storage, and elsewhere, but are most commonly known in semiconductor storage.