CAEN
Electronic Instrumentation
UM3148
–
DT5730/DT5725 User Manual rev. 2
27
Acquisition Triggering: Samples & Events
When the acquisition is running, a trigger signal allows to:
-
Store the 31-bit counter value of the Trigger Time Tag (TTT).
The counter (representing a time reference), like so the Trigger Logic Unit (see
Fig. 9.2
) operates at a
frequency of 125 MHz (i.e. 8 ns or 4 ADC clock cycles in case of DT5730, while 2 ADC clock cycles in case of
DT5725). Due to the way the acquired data are written into the board internal memory (i.e. in 4-sample
bunches), the TTT counter is read every 2 trigger logic clock cycles, which means the trigger time stamp
resolution results in 16 ns (i.e. 62.5 MHz).
Basing on that, the LSB of the TTT is always “0”;
-
Increment the EVENT COUNTER.
-
Fill the active buffer with the pre/post-trigger samples, whose number is programmable (Acquisition
window width), freezing then the buffer for readout purposes, while acquisition continues on another
buffer.
An event is therefore composed by the trigger time tag, pre- and post-trigger samples and the event counter.
Overlap between “acquisition windows” may occur (a new trigger occurs while the board is still storing the samples
related to the previous trigger); this overlap can be either rejected or accepted (programmable via software).
If the board is programmed to accept the overlapped triggers (by programming at register address 0x8000), as the
“overlapping” trigger arrives, the current active buffer is filled up, then the samples storage continues on the
subsequent one. In this case events will not have all the same size (see
Fig. 9.8
below)
TRIGGER
PRE
POST
ACQUISITION WINDOW
Recorded
Not Recorded
Overlapping Triggers
EVENT n
EVENT n+1
EVENT n+2
Fig. 9.8:
Trigger overlap
A trigger can be refused for the following causes:
-
Acquisition is not active.
-
Memory is FULL and therefore there are no available buffers.
-
The required number of samples for building the pre-trigger of the event is not reached yet; this happens
typically as the trigger occurs too early either with respect to the
RUN Acquisition
command (see §
Acquisition Run/Stop
) or with respect to a buffer emptying after a
Memory FULL
status (see §
Acquisition
Synchronization
).
-
The trigger overlaps the previous one and the board is not enabled for accepting overlapped triggers.
As a trigger is refused, the current buffer is not frozen and the acquisition continues writing on it. The EVENT COUNTER
can be programmed in order to be either incremented or not. If this function is enabled, the EVENT COUNTER value
identifies the number of the triggers sent (but the event number sequence is lost); if the function is not enabled, the
EVENT COUNTER value coincides with the sequence of buffers saved and readout.