Acquisition Modes
Acquisition Run/Stop
The acquisi on can be started and stopped in different ways, according to bits[2:0] of register 0x8100
:
- SW CONTROLLED (bits[1:0] = 00): Start and Stop take place by so ware command. Bit[2] = 0 means
stopped, while bit[2] = 1 means running.
- GPI CONTROLLED (bits[1:0] = 01): bit[2] = 1 arms the acquisi on and the Start is issued as the GPI
signal is set high and the Stop occurs when it is set low. If bit[2] = 0 (disarmed), the acquisi on is
always off.
- FIRST TRIGGER CONTROLLED (bits[1:0] = 10): bit[2] = 1 arms the acquisi on and the Start is issued on
the first trigger pulse (rising edge) on the TRG-IN connector. This pulse is not used as a trigger; actual
triggers start from the second pulse on TRG-IN. The Stop acquisi on must be SW controlled (i.e. reset
of bit[2]).
Acquisition Triggering: Samples and Events
When the acquisi on is running, a trigger signal allows to:
- store a 31-bit counter value of the Trigger Time Tag (TTT).
The counter (represen ng a me reference), like the Trigger Logic Unit (see Fig.
), operates at a
frequency of 100 MHz (i.e. 10 ns or 1 ADC clock cycle). Due to the way acquired data is wri en 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 me stamp resolu on results in 20 ns (i.e. 50 MHz). Basing on
that, the LSB of the TTT is always “0”;
- increment the EVENT COUNTER;
- fill the ac ve buffer with the pre/post-trigger samples, whose number is programmable via register
address 0x8114; the acquisi on window width (also referred to as record length) is determined via
register addresses 0x800C and 0x8020; then, the buffer is frozen for readout purposes, while acqui-
si on con nues on another buffer.
An event is therefore composed by the trigger me tag, pre- and post-trigger samples and the event
counter.
Overlap between “acquisi on windows” may occur (a new trigger occurs while the board is s ll storing the
samples related to the previous trigger); this overlap can be either rejected or accepted (programmable
via so ware).
If the board is programmed to accept the overlapping triggers (by wri ng at register address 0x8000), as
the overlapping trigger arrives, the current ac ve buffer is filled up, then the samples storage con nues on
the subsequent one. In this case, not all events will have the same size (see Fig.
).
A trigger can be refused for the following causes:
- Acquisi on is not ac ve.
- Memory is FULL and therefore there are no available buffers.
- The required number of samples for building the event pre-trigger is not reached yet; this happens
typically as the trigger occurs too early either with respect to the RUN Acquisi on command (see
Sec.
) or with respect to a buffer emptying a er a Memory FULL status (see
Sec.
- The trigger overlaps the previous one and the board is not enabled for accep ng overlapped triggers.
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UM3247 - N6724 User Manual rev. 10
