VXI SVM2608 user manual Linear Mode, Pre-Trigger, Delayed Trigger

VXI Technology, Inc.

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.

14 SVM2608 Introduction