Zero Length Encoding ZLE
Once the event is acquired, the Zero Length Encoding mode is able to tag the samples as ”good” or ”skipped”.
Samples are defined as ”good” if they are over/under threshold according to the logic polarity (see bit[31]
of register 0x1n24
). ”Skipped” events are those under/over the threshold and they are rejected.
Only the number of skipped events is reported in the output data.
It is also possible to acquire data before the signal crosses the threshold (N
LBK
, look back) and/or data a er
the over-threshold (N
LFWD
, look forward). Those samples are tagged as good and reported in the output
data. N
LBK
and N
LFWD
can be set through register 0x1n24 .
The channel event format in ZLE mode has the following structure:
•
Total size of the event (total number of transferred data per channel)
•
Control word
• [stored valid data if control word is ”good”]
• ...
The total size is the number of 32-bit data that compose the event (including the size itself).
The control word has the following format:
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0
RESERVED
NUMBER OF SKIPPED SAMPLES
‘SKIP’ CONTROL WORD
1
RESERVED
NUMBER OF GOOD SAMPLES
‘GOOD’ CONTROL WORD
Fig. 7.7:
Control Word format.
A ”good” control word is followed by as many sample words as those indicated in the ”number of good
samples” field; if the control word type is “skip”, then it is followed by a “good” control world, unless the
end of event is reached.
Note
: The maximum allowed number of control words is 62 (14 for mezzanine FPGA release 0.6 and ear-
lier); therefore the ZLE is ac ve un l the 62
th
transi on between a “good”/“skip” zone is reached. All the
subsequent samples are considered “good” and stored.
Fig.
shows an example of Zero Length Encoding with posi ve logic. The top picture shows the skipped
and good areas (N
SKIP
and N
GOOD
). Within the good area there are the samples back, the over-threshold
and the samples forward (N
LBK
, N
OVT
, and N
LFWD
respec vely). The bo om figure shows the corresponding
event structure.
34
UM3247 - N6724 User Manual rev. 10
