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ADM1168
Each fault record contains eight bytes, with each byte taking
typically about 250 μs to write to EEPROM, for a total write
time of about 2 ms. After the black box begins to write a fault
record into EEPROM, the ADM1168 ensures that it is complete
before attempting to write any additional fault records. This means
that if consecutive sequencing engine states are designated as
black box write states, then a time delay must be used in the first
state to ensure that the fault record is written before moving to
the next state.
When the ADM1168 powers on initially, it performs a search
to find the first fault record that has not been written to. It does
this by checking the flag bit in each fault record until it finds
one where the flag bit is 1. The first fault record is stored at
Address 0xF980, and at multiples of eight bytes after that, with
the last record stored at Address 0xF9F8.
The fault recorder is only able to write in the EEPROM. It is not able
to erase the EEPROM prior to writing the fault record. Therefore,
to ensure correct operation, it is important that the fault record
EEPROM is erased prior to use. When all the EEPROM locations
for the fault records are used, no more fault records are written.
This ensures that the first fault in any cascading fault is stored and
not overwritten and lost.
To avoid the fault recorder filling up and fault records being lost, an
application can periodically poll the ADM1168 to determine if there
are fault records to be read. Alternatively, one of the PDOx outputs
can be used to generate an interrupt for a processor in the fault
record write state to signal the need to come and read one or more
fault records.
After reading fault records during normal operation, two things
must be done before the fault recorder will be able to reuse the
EEPROM locations. First, the EEPROM section must be erased.
Data Sheet
BLACK BOX WRITES WITH NO EXTERNAL SUPPLY
In cases where all the input supplies fail, for example, if the card
has been removed from a powered backplane, the state machine
can be programmed to trigger a write into the black box EEPROM.
The decoupling capacitors on the rail that power the ADM1168
and other loads on the board form an energy reservoir. Depending
on the other loads on the board and their behavior as the supply
rails drop, there may be sufficient energy in the decoupling
capacitors to allow the ADM1168 to write a complete fault record
(eight bytes of data).
Typically, it takes 2 ms to write to the eight bytes of a fault record. If
the ADM1168 is powered using a 12 V supply on the VH pin, then
a UV threshold at 6 V can be set and used as the state machine
trigger to start writing a fault record to EEPROM. The higher the
threshold is, the earlier the black box write begins, and the more
energy available in the decoupling capacitors to ensure it completes
successfully.
Provided the VH supply, or another supply connected to a VPx pin,
remains above 3.0 V during the time to write, the entire fault record
is always written to the EEPROM. In many cases, there should be
sufficient decoupling capacitors on a board to power the
ADM1168 as it writes into the EEPROM.
In cases where the decoupling capacitors are not able to supply
sufficient energy after the board is removed to ensure a complete
fault record is written, the value of the capacitor on VDDCAP
may be increased. In the worst case, assuming that no energy is
supplied to the ADM1168 by the external decoupling capacitors,
but that VDDCAP has 4.75 V on it, then a 47 μF is sufficient to
guarantee that a single complete black box record can be written
to the EEPROM.
The fault recorder must then be reset so that it performs its search
again for the first unused location of EEPROM that is available to
store a fault record.
Rev. A | Page 18 of 28
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