Motorola MVME172 - Manual (Page 297)

Functional Description
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Notify the local MPU via interrupt if so enabled.
Double Bit Error (Cycle Type = Scrub)
Do not perform the write portion of the cycle. This causes the location to
continue to indicate non-correctable error when accessed.
Log the error if one has not already been logged.
Notify the local MPU via interrupt if so enabled.
Triple (or Greater) Bit Error (Cycle Type = Scrub)
Some of these errors are detected correctly and are treated the same as a
double bit error. The rest could show up as "no error" or "single bit error",
both of which are incorrect.
Error Logging
ECC error logging is facilitated by the MCECC because of its internal
latches. When an error (single or double bit) occurs in the DRAMs to
which an MCECC is connected, it freezes the address of the error and the
syndrome bits associated with the data that is in error. Each MCECC
performs this logging function independently of the other. Once an
MCECC has logged an error, it does not log any new errors that occur until
the ERRLOG control/status bit has been cleared by software.
Scrub
The MCECC pair contains programmable registers and circuitry that
provide the scrubbing function. Programmable registers determine how
often the entire DRAM is scrubbed. During a scrub, the scrubber holds the
memory for a programmable amount of time, then releases it for the local
bus, or refresher if one of them is requesting local bus mastership. The
scrubber then refrains from using the DRAM again for a programmable
amount of time. Each scrub cycle is made up of a full 144-bit read of
DRAM, a correction of any single bit errors, and a write of the full 144
corrected bits back to the same location. If a single or double bit error