Classification
Revision
Public
PA6
Document Number
Date
17-1957
2018-06-07
Note
Another critical point is the queue interface which takes user provided references to memory and
returns the accumulation results.
Please be aware that the number of buffers needed to ensure stable operation is highly dependent on the
use case since the buffers circulate in the queue interface at a rate determined by the user application.
For example, 20 buffers may be enough to allow stable operations when writing the incoming records to
disk using a binary format while 50 may be required if instead, the data is formatted as ASCII.
There are strategies which could be used to design the user application in such a way that 20 queued
buffers will be sufficient in both cases.
Note
The number of queued buffers needed ensure stable operation of the system is highly dependent on
buffer handling in the user application.
4.7.1
Overflow Behavior
An overflow is caused by a stall of the data transfer interface for an extended period of time. This may in
turn be caused by an imbalance between the transfer bandwidth of the device-to-host interface and the
output data rate of the digitizer.
During an overflow, data collected up until that point (waiting to be transferred) remains intact and
incoming data is discarded in a well-defined manner. What ’well-defined’ means in detail is beyond
the scope of this document. However, the resulting user space behavior is worth commenting on. An
overflow will manifest itself in two possible ways, depending on the WFA settings and current work load
partitioning. Both of these events are completely discernible to the user by reading the record header
information.
1.
A user space record will contain fewer number of accumulated waveforms than the defined number
of accumulations.
2.
One or several user space records will be missing completely.
What is guaranteed not to happen is the corruption of data, e.g. that some sections of a user space
record are the result of
X
accumulated waveforms while
Y
is the number of accumulated waveforms for
other sections.
A different type of overflow occurs if the trigger rate is not well-matched to the record length. For
example, if the trigger period is 8
µ
s and the record length is 10
µ
s, the digitizer will still be recording
data for the previous record when the new trigger arrives. In this case the trigger is simply ignored,
causing the effective trigger period to be 16
µ
s.
Note
Triggers occurring while the previous record is not yet completed are ignored by the digitizer.
4.7.2
Status Codes
The status value reported in the data struct (Tab.
4
) provides information on the overall health of the
WFA. The exact meaning of the value can be interpreted using Tab.
5
.
ADQ7-FWATD User Guide
www.teledyne-spdevices.com
Page 13 of 26
