Hardware Reference
In-Depth Information
Parity and ECC
Partofthenatureofmemoryisthatitinevitably fails.Thesefailuresareusuallyclassified
as two basic types: hard fails and soft errors.
Thebestunderstoodarehardfails,inwhichthechipisworkingandthen,becauseofsome
flaw, physical damage, or other event, becomes damaged and experiences a permanent
failure. Fixing this type of failure normally requires replacing some part of the memory
hardware, such as the chip, SIMM, or DIMM. Hard error rates are known as HERs.
The other, more insidious type of failure is the soft error, which is a nonpermanent failure
that might never recur or could occur only at infrequent intervals. Soft error rates are
known as SERs.
Inthelate1970s,Intelmadeadiscoveryaboutsofterrorsthatshookthememoryindustry.
Itfoundthatalphaparticleswerecausinganunacceptablyhighrateofsofterrorsorsingle
event upsets (SEUs, as they are sometimes called) in the 16KB DRAMs that were avail-
able at the time. Because alpha particles are low-energy particles that can be stopped by
something as thin and light as a sheet of paper, it became clear that for alpha particles to
cause a DRAM soft error, they would have to be coming from within the semiconductor
material.Testingshowedtraceelementsofthoriumanduraniumintheplasticandceramic
chippackagingmaterialsusedatthetime.Thisdiscoveryforcedallthememorymanufac-
turers to evaluate their manufacturing processes to produce materials free from contamin-
ation.
Today, memory manufacturers have all but totally eliminated the alpha-particle source of
soft errors, and more recent discoveries prove that alpha particles are now only a small
fraction of the cause of DRAM soft errors.
As it turns out, the biggest cause of soft errors today is cosmic rays. IBM researchers
began investigating the potential of terrestrial cosmic rays in causing soft errors similar to
alpha particles. The difference is that cosmic rays are high-energy particles and can't be
stopped by sheets of paper or other more powerful types of shielding. The leader in this
lineofinvestigation wasDr.J.F.ZiegleroftheIBMWatsonResearchCenterinYorktown
Heights, New York. He has produced landmark research into understanding cosmic rays
andtheirinfluenceonsofterrorsinmemory.Oneinterestingsetofexperimentsfoundthat
cosmic ray-induced soft errors were eliminated when the DRAMs were moved to an un-
derground vault shielded by more than 50 feet of rock.
Cosmic ray-induced errors are even more of a problem in SRAMs than DRAMS because
theamountofchargerequiredtoflipabitinanSRAMcellislessthanisrequiredtoflipa
DRAMcellcapacitor.Cosmicraysarealsomoreofaproblemforhigher-densitymemory.
As chip density increases, it becomes easier for a stray particle to flip a bit. It has been
