Acquisition Considerations for DDR

DDR acquisition has some additional considerations for your application.

Edge Triggers

In DDR acquisitions, while data channels are sampled on both clock edges, triggers are only stored once per Sample clock period. For example, refer to the following figure. Consider if PFI Trigger A was a digital edge Start trigger, and the device were configured for rising edge data position. Whether PFI Trigger A arrives before the rising edge of the clock or before the falling clock edge, the trigger has the same effect—the first sample acquired is that of the rising clock edge.

Pattern Match Trigger

In DDR acquisitions, while data channels are sampled on both clock edges, triggers are only stored once per Sample clock period. In the following figure, consider if the device were configured for a pattern match Start trigger and rising edge data position. Whether the data in sample A or B matches the pattern configured for the pattern match trigger, sample A is the first acquired sample.

Special Considerations for Using the Pause Trigger

If a Pause trigger is asserted on either edge of the Sample clock, the acquisition is paused for the samples that occur on both edges of the clock.

Data Width

Data width is a function of your data rate multiplier. Since data width refers to how large your sample is in bytes, using DDR mode effectively halves your allowable data width. For example, on a device with 16 channels, you can generate or acquire data on all 16 channels. For the same device with its data rate multiplier configured for DDR, you can generate on only eight channels and acquire on the other eight.

Memory Usage

Memory usage is effectively doubled per channel since the data width and channel count are halved.