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CPU overhead when writing values to FPGA Host Interface Controls on FPGA side?

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Not sure this is the most appropriate group for posting above topic … but since it may affect best practices when building large LV applications and NI R&D folks in this group may have a quick answer (or point elsewhere) – I’ll give it a shot …

I need to get a better idea of CPU overhead added by RIO Driver as FPGA side code writes values to Top Level VI Controls:

Overhead Code Snippet.png

Would CPU overhead be the same in both cases? Or should my FPGA side code minimize number of Control writes if I need minimizing CPU overhead on the Host side? I expect RIO Driver may be using some CPU bandwidth even if my Host side code is not reading control values at all …

There is a really helpful description of DMA FIFO inner workings in the “NI LabVIEW High-Performance FPGA Developer’s Guide”. Unfortunately it doesn't tell much about how RIO Driver implements Host Interface Controls – only mentions that propagation delay is on the microsecond scale and that polling data this way is faster than using FPGA Interrupts (a ball park propagation delay of 25 microseconds- this may be from a different document though).

Any insights from LabVIEW R&D?



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There is no CPU overhead for the FPGA to write values to its Top Level Controls. In both of your examples, the CPU would not be used at all. The CPU is only used to read or wrote a value between the host processor and the FPGA. 

Your values for the time it takes to perform an I/O read and the propagation delay of FPGA interrupts seem about right, although they are highly processor and architecture dependent, i.e. they will vary quite a bit between a lower-end cRIO controller and a high-end PXI controller.

authored by
Christian L, CLA
Principal Development Manager - Partner Program
Applications Engineering Senior Manager - Data Acquisition, Control, and Real-Time Test
National Instruments - Austin, TX

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