Hi engwei,
1) The usage of variables on the FPGA VI will take up a certain amount of FPGA resources, e.g. flip-flops and LUTs. Thus, excessive use of variables (especially to hold large data items like big arrays) will use up significant amount of FPGA resources, leaving lesser amounts for your other functions on the FPGA VI. But variables used in moderation on small data types (like the SPI example) should be fine.
2) As long as the FPGA has enough resources to handle the variables and the other functions, the performance of the FPGA would not be affected. This is indicated by a successful compile report (At the end of each compile, there will be a Compile Report dialog box showing FPGA resource usage amounts).
3) Although variables are a convenient way of passing data, it is important to note that they are easily subjected to race conditions (e.g. having two subVIs writing to the same global variable at the same time), so the final value of the global variable might not be correct. Care must be taken in your programming to avoid this situation.
4) I do not quite understand the usage that you have explained in point (2) of your post, but local variables and global variables both take up FPGA resources, so I think performance wise there should be not much difference between using the two.
Hope the information helps,
Best regards,
Victor
NI ASEAN
