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VXI Technology, Inc.
14 SVM2608 Introduction
In absence of a Trigger signal, the acquisition can be forced by setting a control bit, the FORCE
bit. Forcing an acquisition on a channel only starts acquisition on that channel. Each channel has
its own corresponding FORCE bit.
Linear Mode
In Linear mode, the total number of samples collected (also referred to as Sample Points) is
determined by the value programmed into the Sample Points register. The first sample (also
referred to as Sample Zero) is stored in memory when a Trigger Event occurs. Sample Zero is
from the value read from the channel’s base address at offset zero (0x000000 for Channel 0,
0x200000 for Channel 1, 0x400000 for Channel 2 and 0x600000 for Channel 3).
Pre-Trigger
In Linear mode, it is also possible to store samples that occur before a Trigger Event. When a
channel is armed and the Pre-Trigger register is programmed with a value other than zero, that
channel will begin sampling immediately, without waiting for an External Trigger. After it stores
the number of samples specified in the Pre-Trigger register (also referred to as Pre-Trigger
Points), it begins monitoring the Trigger Event. Until the Trigger Event occurs, the channel
continues sampling and storing. When the Trigger Event occurs, Sample Zero is stored. After the
Trigger Event, the number of data points collected is determined by the following equation:
AFTER TRIGGER POINTS = SAMPLE POINTS – PRETRIGGER POINTS
When the user reads from offset zero of a channel, the data returned is Sample Zero followed by
Sample Zero + 1, etc. The Pre-Trigger Points can be read from the top of that channel’s memory.
For example, if 0x100 Pre-Trigger Points were sampled on Channel 0 after the acquisition is
completed, the samples can be retrieved from locations 0x1FFE00, 0x1FFE02 …0x1FFFFE with
the data at 0x1FFFFE being the last Pre-Trigger Sample before the trigger event.
Delayed Trigger
In Linear mode, it is also possible to delay storing Sample Zero by a number of sample clocks,
where a sample clock is defined by the Sample Clock Rate register. The number of sample clocks
an acquisition is delayed (also referred to as Delayed Points) is programmed in the Delayed
Trigger register. Samples are taken and stored immediately when the Trigger Event occurs, but
Sample Zero will be stored only after the specified number of Delayed Points passes. Data stored
during the Delayed period can be viewed by the user at the top of the memory space of the
respective channel (same as in Pre-Trigger mode as above), assuming that the following
condition is observed:
SAMPLE POINTS < 1 MSamples
As opposed to the Pre-Trigger acquisition, the number of samples taken after the Trigger Event is
not affected by the number of samples taken before it:
AFTER TRIGGER POINTS = SAMPLE POINTS
If the following condition is met:
DELAYED POINTS < 1 M – SAMPLE POINTS
Then all the samples collected before the Trigger Event are available to the user.