OCCcm09291 - October 31, 2013

ACIS considers this Anomaly closed as it's happened only twice in 15
years; is unsure if it will happen again; if it does, it would have to
occur during a COMM, and with software procedures ready in order to
even begin to diagnose the problem. Lastly, the amomaly is cleared by
power cycling.  Details below. 



Anomaly
-------

this anomaly occurred twice:

1) On day 066, 2011, during obsid 12934, the ACIS flight instrument
reported no events from an active 
CCD. For this obsid, the anomaly came from CCD_I0 through FEP_5 at the
very start of event processing. 
The frame-averaged overclock values from nodes A through C were all
very low (consistent with zero), while those from node D were very
high (consistent with 4095, which would normally indicate an overflow
condition in the ADC for that output node.) Hence the name “Hi-Lo
Pixel” for this anomaly. Everything returned to normal after the
boards were power-cycled.

2) On day 304 of 2013, during obsid 16496, FEP1, reading out the I3
CCD chip, stopped returning event data. Overclock information
indicated that the bias was zero for quads A, B, and C, and 4095
for quad D. Other chips and FEPs continued to function normally.
The anomaly cleared when power was cycled to start the following
science run.

Command loads are being prepared to dump the relevant portions of FEP
and DEA board memories should the anomaly occur again. Further analysis
and proposed contents of the CLDs can be found in a Flight Software Report
entitled "The ACIs Hi-Lo Pixel Anomaly (v 1.3) dated March 24, 2014.

Analysis and Conclusion
------------------------

It is unclear whether Hi-Lo Pixel anomalies result from bad video
board output or from a temporary failure in the FEP’s firmware
thresholder. All that is known for sure is that the anomaly is: 

(a) infrequent,
(b) transient, i.e., goes away after power cycling, and
(c) appears unrelated to other ACIS hardware anomalies.

In the two known instances, it began at the start of an exposure
frame. The anomalous behavior, with three nodes reporting zero and the
fourth overflowing, has never been seen in the laboratory in the ACIS
engineering unit (although most ground testing does not use the DEA
video boards.) A “susceptibility” in a hardware component might be
triggered in the flight unit by some random event – e.g., ionizing
radiation, static (dis)charge, power-supply surge, etc. – to cause the
observed behavior. Little further progress can be made unless the
anomaly recurs and is recognized during a realtime contact with
sufficient time for the ACIS instrument team to react by executing one
or more realtime diagnostic procedures. These procedures are presently
being developed.