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Fixed-Size Array Requirement in Optimized FPGA VIs

Last Modified: January 2, 2018

Arrays in Optimized FPGA VIs must be of a fixed size to ensure proper memory allocation on the FPGA at compile time.

Array Inputs

The type of input you wire to an array determines whether you need to take additional steps to designate a fixed size for the array:

  • Array constants—No additional steps needed. The size of the array is set equal to the number of elements in the array constant.
  • Array controls—You must manually designate a fixed size for an array control.
    1. Select the control on the diagram.
    2. Enable Fixed in the Array dimensions section of the Item tab.
    3. Specify the size of the array.

Array Outputs

When you wire an indicator to an array, the compiler automatically sets a fixed size for the output that scales to the size of the array. This behavior is the default setting and you can view this setting in the Adapt to source section of the Item tab after you select the indicator on the diagram.

In some situations, the compiler may not be able to determine the size of the array, in which case you can manually set the indicator to be a fixed size. Use the manual setting on an indicator only when the compiler cannot determine the size of the array because the setting can cause coercion dots to appear later in development.


When you manually set an indicator to fixed size, the Auto Adapt option is set to Don't Auto Adapt. If you then switch the indicator back to a variable size, the option does not automatically switch back to Auto Adapt, meaning the compiler no longer automatically sets the size of the indicator to the size of the array, as per the default behavior. Therefore, when you switch an indicator from fixed size back to variable size, you must manually enable Auto Adapt to allow the compiler to automatically determine the size of the array output.

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