28
•
Operation Theory
There are 4 trigger modes (pre-trigger, post-trigger, middle-trigger, and
delay-trigger) working with the 4 trigger sources to initiate different scan
data acquisition timing when a trigger event occurs. They are described as
follows. For information of trigger sources, please refer to section 4.5.
Pre-Trigger Acquisition
Use pre-trigger acquisition in applications where you want to collect data
before a trigger event. The A/D starts to sample when you execute the
specified function calls to begin the pre-trigger operation, and it stops when
the trigger event occurs. Users must program the value M in
M_counter
(16 bits) to specify the amount of the stored scans before the trigger event.
If an external trigger occurs, the program only stores the last M scans of
data converted before the trigger event, as illustrated in figure 10, where
M_counter = M =3, PSC_counter = 0. The post scan count is 0 because
there is no sampling after the trigger event in pre-trigger acquisition. The
total stored amount of data = Number of enabled channels * M_counter.
Scan_start
ADCONV
(M_counter = M = 3, PSC_counter=0)
Acquired data
Acquired & stored data
(M samples)
Acquisition_in_progress
Operation start
Trigger
Figure 10: Pre-trigger (trigger occurs after at least M scans
acquired)
Note that If the trigger event occurs when a conversion is in progress, the
data acquisition won’t stop until this conversion is completed, and the
stored M scans of data include the last scan, as illustrated in figure 11,
where M_counter = M =3, PSC_counter = 0.
