Use of "Open FPGA VI Reference" function --- Build Specification vs VI vs Bitfile

Question

When using the "Open FPGA VI Reference" function in a LV2012 cRIO application, there are 3 options: Build Specification, VI, or Bitfile. What would be the reasons for selecting one over the others? Does it affect the resulting startup.rtexe when the cRIO application is built? I searched through the help and in these forums, but I don't see criteria for selecting one over the others; maybe I missed it.

Answer 1

Hello Chris,
Apologies in advance for a long reply.  
The reference method won't change the functionality of your rtexe.exe.  They all end up dropping a bitstream, based on a bitfile, onto the cRIO's FPGA.
To a degree, the method used to reference the FPGA code is a matter of taste, but there are situations where one method is better suited than the others.
Reference by VI:
Setting the configuration options to open reference by VI is helpful during development when you are making changes to an FPGA VI often and building/testing using the same spec.  When this option is used, a bitfile is selected based on the default build specification for the project.  A project may have only one default build specification.  You can make any build the default by checking the option under the Source Files category in the build properties.  The default build is indicated in the project explorer by the green box around the builds icon.  
Reference by Bitfile:
This option references a bitfile directly.  Through the configuration window, you can select one specific bitfile to open a reference to (this is not dynamic and does not change unless you physically go make a change to that path).  If you're using this method, it helps to give your bitfiles more meaningful names than the ones that are automatically generated by LabVIEW.  When you run subsequent compilations off of the same build specification and do not change the bitfile nname or path in the build configuration, the old bitfile is overwritten and replaced with the new one.  When you are using this option, it is critical that you keep up with which bitfile is the one you want to be using.  There is an option now that will help alleviate any problems referencing by bitfile through the Open FPGA VI Reference function.  There is a new VI called Open Dynamic Bitfile Reference.  It is typically used when you want to chose a specific bitfile to load depending on something in your host code (a configuration option etc) - but it allows you to dynamically reference a bitfile on the block diagram by path.
Referency by Build Specification:
This option is good for when you want to always use a bitfile that is associated with/compiled with the same build configuration.  Say you have two options for top level FPGA VIs in your project (each with its own build spec).  Both of these VIs have the same interface (read/write controls, DMA) but they run different algorithms or something.  This is nice because you can easily switch your host application between them by picking the build spec associated with the FPGA VI you want to use.  In this type of sutation, referencing by VI is no good because you can only have on default build spec.
cheers.
Matthew H.
Applications Engineer
National Instruments