docs: def write_fpga_reg(self, fpga_num, addr, value, cabinet, frame, board):
          """Write the value of an FPGA (SPI) register.
  
          See the SpI/O project's spinnaker_fpga design's `README`_ for a listing
          of FPGA registers. The SpI/O project can be found on GitHub at:
          https://github.com/SpiNNakerManchester/spio/
  
          .. _README: https://github.com/SpiNNakerManchester/spio/\
                      blob/master/designs/spinnaker_fpgas/README.md#spi-interface
  
          Parameters
          ----------
          fpga_num : int
              FPGA number (0, 1 or 2) to communicate with.
          addr : int
              Register address to read or write to (will be rounded down to the
              nearest 32-bit word boundary).
          value : int
              A 32-bit int value to write to the register
          """
          arg1 = addr & (~0x3)
          arg2 = 4  # Write a 32-bit value
          arg3 = fpga_num
          self._send_scp(cabinet, frame, board, SCPCommands.link_write,
                         arg1=arg1, arg2=arg2, arg3=arg3,
                         data=struct.pack("<I", value), expected_args=0)