I need to debug my bitstream. How can I use LOUT writes in serial modes for debugging purposes?
The LOUT configuration register is a pipeline register to the DOUT pin. Embedding LOUTwrites in the bitstream will allow you to see when the downloading has reached a certain point. Accordingly, it will also mean that the device is synchronized.
The best way to embed a LOUT write is to use a text editor to place it into the ".rbt" file; however, it must be inserted between two packets. Refer to (Xilinx XAPP138): "Virtex FPGA Series Configuration and Readback" for a description of packets and the bitstream format.
The packet header for an LOUT write of 1 32-bit word is 0x30010001. This must be followed by a single DWord of data. This data can be anything, which is useful when debugging. LOUT writes can be embedded anywhere in the bitstream, and can be invaluable for verification of successful downloads.
See (Xilinx Answer 7891) for further details.