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Section 7 - Ram Error Detection
RAM Error Detection and Corrective Action
1.1 Non-volatile Memory (NVM)
256 bytes of non-volatile RAM are
maintained in the CPU by the standby power supply. Errors
can occur within this RAM due to loss of battery voltage
to the ECM or due to electrical noise originating within
or external to the ECM This software feature provides a
means by which loss of the contents of the NVM is
detected and appropriate remedial action taken.
1.1.1 Memory Loss Detection
Loss of non-volatile memory due to loss
of standby voltage to the CPU, NVM is tested by a
checksum test on the three diagnostic memory cells
(MALFFLG1, MALFFLG2, and MALFFLG3) located in
non-volatile RAM. In operation, each time the system
software stores information in one of the diagnostic
memory locations, a checksum calculation is made on all
three diagnostic locations, and the result Plus 1, is
stored in the checksum NVM location (DCKSUM). Upon each
reset initialization sequence, the same checksum
calculation is performed and the result compared with the
contents of DCKSUM. If the two differ, it is concluded
that an NVM loss has occurred.
1.1.2 Memory Loss Remedial Action
Upon detection of a non-volatile memory
loss, the ECM software performs the following operations:
1.The three diagnostic
bytes are cleared, and the checksum byte is
set equal to one.
2.All memory utilized as
NVM by the ECM software, except the block
learn table and idle air control variables
3.The block learn table and
idle air control variables shall be
initialized to starting values.
4.Upon detection of the NVM
loss, the ECM shall set the NVM fail flag
(Bit 6 of CLCCMW) for use by the IM to
indicate this condition. This NVM fail flag
is reset on any power-up sequence where an
NVM alteration is not detected.
1.2 RAM Refresh
RAM cells may change state if the cell
is not periodically refreshed. To prevent this
possibility, the software reads and stores all active RAM
at a rate of not less than once each 100 msec.