Get Number of Elements to Write (FIFO Method)

Returns the number of empty element slots available to be written to in the FPGA FIFO. You must use this node in the same clock domain as the clock domain containing the FIFO Method Node configured with the Write method.

Use the FIFO Method Node to implement this method.

Inputs/Outputs

Option Description
FIFO In FIFO In specifies the FIFO.

You can wire a FIFO control, FIFO constant, VI-Defined FIFO Configuration node, or the FIFO Out terminal of another FIFO Method Node to FIFO In.
FIFO Out FIFO Out returns FIFO In if FIFO In is wired. Otherwise, FIFO Out returns the FIFO that you specify in the FIFO Method Node.
Number Of Elements to Write Number Of Elements to Write—Returns the number of empty element slots available to be written to in the FPGA FIFO.The number of elements that this method returns is less than or equal to the actual number of elements available to write to the FIFO.
Note This method may generate different output values when executed in simulation mode versus on hardware. In simulation mode, the output value of this method is the actual number. On hardware, block memory FIFOs implemented using slice fabric control logic include an additional buffer. This additional buffer contains five elements when the FIFO reader and writer are in the same clock domain, or six elements when they are in different clock domains. On hardware, this method does not report the number of elements available in the additional buffer. If the FIFO contains less elements than the depth of the additional buffer, the output value of this method is less than the actual number of elements available to write. Otherwise, the output value of this method is the actual number.

You can use the following types of FIFOs in this method:

  • Target-scoped
  • VI-defined
  • DMA
  • Peer-to-peer

(Xilinx Vivado) This method does not support FIFOs with the built-in control logic or implemented using UltraRAM.

The sum of the values returned from this method and the Get Number of Elements to Read method is less than or equal to the total number of elements in the FIFO. You can use this method to ensure that the FIFO can handle bursts of elements before the Write method times out.

Note Within a single-cycle Timed Loop, the Get Number of Elements to Write and Get Number of Elements to Read FIFO methods will always return the number of elements available at the beginning of the cycle, prior to the occurrence of any Read or Write methods.