a transient error (soft error) occurs. As for the case of a memory cell, a bit error
occurs unless the charge amount is restored to the predetermined amount which
corresponds to the pre-stored value.
3. Single event latch-up
Single event latch-up (SEL) is another phenomenon caused by radiation of high
energy particles like SEU described above. Sometimes a semiconductor device such
as an LSI often has an unexpectedly formed structure which resemble a thyristor due
to a combination of circuit configurations. When high-energy particles hit a thyristor
like structure formed on this semiconductor substrate, the circuit of this portion is
turned on, and an excessive power supply current flows. It is sometimes referred to
as “hard error” because it can lead to permanent failure rather than transient error.
In addition to the above described SEU and SEL, there are other radiation effects
such as Single Event Transient (SET) and Single Event Functional Interrupt (SEFI),
etc. SET is a transient large current, and the duration of large current is short.
However, if a successive flip-flop captures the pulse at the edge of clock, it can
lead to SEU. SEFI causes malfunction in the vicinity of high-density memories like
flash memories because recent control circuitries inside high density memory
devices are complex and are affected by SEEs. Once SEFI occurs, it must be reset
or reboot to resume operation. The control circuitries are sometimes harmed permanently by SEFI. These are collectively called Single Event effect (SEE). It is
necessary to select radiation hardened devices in which excessive charge can be
suppressed. Data refreshing by rewriting the correct data as much as possible is often
used to prevent any successive influence from spreading and to eliminate these
failures caused by SEE. Various techniques are taken to protect devices from SEEs.
Regarding radiation environments to which space systems are exposed, they were
conventionally treated as being unique to outer space. However, due to recent
miniaturization of semiconductor processes, soft errors as transient data errors
found in semiconductor memory cells caused by natural background radiation on
the Earth draw attention in addition to failures caused by radiation emitted from the
package of the device [13]. For this reason, opportunities are increasing in which
spacecraft onboard equipment design engineers and consumer electronics equipment
designers share information about radiation effects. Countermeasures against soft
errors that can be employed by the developers of consumer electronics facing with
the miniaturization of semiconductor processes are becoming close to the techniques
used by spacecraft onboard equipment design engineers. In consequence, not only
spacecraft onboard equipment design engineers but also consumer electronics
designers have become to be interested in atomic switch. Even though design
techniques like various error correction codes are employed, removing the amount
of semiconductor memories used as program memories of MCUs and configuration
memories of FPGAs is one of the most effective ways to reduce soft errors. Atomic
switch is a most promising candidate to reduce such semiconductor memories.
Therefore, atomic switch contributes directly to improve reliability of onboard
equipment of satellites. It will also be a subject shared with engineers in other
industrial domains. In addition, by withdrawing from a stored memory architecture,
Atomic Switch FPGA: Application for IoT Sensing Systems in Space
41
occurs unless the charge amount is restored to the predetermined amount which
corresponds to the pre-stored value.
3. Single event latch-up
Single event latch-up (SEL) is another phenomenon caused by radiation of high
energy particles like SEU described above. Sometimes a semiconductor device such
as an LSI often has an unexpectedly formed structure which resemble a thyristor due
to a combination of circuit configurations. When high-energy particles hit a thyristor
like structure formed on this semiconductor substrate, the circuit of this portion is
turned on, and an excessive power supply current flows. It is sometimes referred to
as “hard error” because it can lead to permanent failure rather than transient error.
In addition to the above described SEU and SEL, there are other radiation effects
such as Single Event Transient (SET) and Single Event Functional Interrupt (SEFI),
etc. SET is a transient large current, and the duration of large current is short.
However, if a successive flip-flop captures the pulse at the edge of clock, it can
lead to SEU. SEFI causes malfunction in the vicinity of high-density memories like
flash memories because recent control circuitries inside high density memory
devices are complex and are affected by SEEs. Once SEFI occurs, it must be reset
or reboot to resume operation. The control circuitries are sometimes harmed permanently by SEFI. These are collectively called Single Event effect (SEE). It is
necessary to select radiation hardened devices in which excessive charge can be
suppressed. Data refreshing by rewriting the correct data as much as possible is often
used to prevent any successive influence from spreading and to eliminate these
failures caused by SEE. Various techniques are taken to protect devices from SEEs.
Regarding radiation environments to which space systems are exposed, they were
conventionally treated as being unique to outer space. However, due to recent
miniaturization of semiconductor processes, soft errors as transient data errors
found in semiconductor memory cells caused by natural background radiation on
the Earth draw attention in addition to failures caused by radiation emitted from the
package of the device [13]. For this reason, opportunities are increasing in which
spacecraft onboard equipment design engineers and consumer electronics equipment
designers share information about radiation effects. Countermeasures against soft
errors that can be employed by the developers of consumer electronics facing with
the miniaturization of semiconductor processes are becoming close to the techniques
used by spacecraft onboard equipment design engineers. In consequence, not only
spacecraft onboard equipment design engineers but also consumer electronics
designers have become to be interested in atomic switch. Even though design
techniques like various error correction codes are employed, removing the amount
of semiconductor memories used as program memories of MCUs and configuration
memories of FPGAs is one of the most effective ways to reduce soft errors. Atomic
switch is a most promising candidate to reduce such semiconductor memories.
Therefore, atomic switch contributes directly to improve reliability of onboard
equipment of satellites. It will also be a subject shared with engineers in other
industrial domains. In addition, by withdrawing from a stored memory architecture,
Atomic Switch FPGA: Application for IoT Sensing Systems in Space
41