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AR# 780

MAKEBITS - length count differs due to -lc=aligned_lc

説明

Keywords: MAKEBITS, length count, configuration

Urgency: Standard

General Decription:
MAKEBITS 5.2.x generates a bitstream which has a different length count than a bitstream created with previous versions of MAKEBITS.

ソリューション

The default options were changed for makebits between version 5.1 and version 5.2. The default options for makebits 5.2 is: -lc=aligned_lc. The default options for makebits 5.1/5.0 is: -lc=aligned_done.

The usage for makebits 5.2 still shows the default options as -lc=aligned_done though it is -lc=aligned_lc.

To fix the problem, explicitly specify the length count option on the command line:

makebits <design> -lc=aligned_done

It is important to note that, in spite of the length-count difference, both files resulted in a properly functioning part.

The default option is "Length-Count aligned" which adjusts the length-count value such that length-count match occurs during the first bit in the last configuration byte. This assures sufficient CCLK pulses to complete any selected type of start-up sequence.

The other option is "DONE-aligned", which adjusts length count value to make DONE go active at the end of a configuration data byte, which can cause problems in Peripheral mode.

So "Done-aligned" is sensitive to Peripheral mode.

The issue that led to the change is described on the last page of the Xilinx app note "FPGA Configuration Guidelines". It deals the appropriate length-count value for byte-parallel configuration. ( For byte serial configuration, the length count can be any value, as long as it is high enough, and there never was any problem with that.)

For byte parallel configuration, it is mandatory. From detecting the length-count match to the real end of the configuration process (reaching "F" in the drawing on page 4-54 of the '98 data book), all occurs within the same byte. Therefore it matters where the byte boundary is, and the length-count value is calculated appropriately.

The length-count calculation algorithm calculated a value that made "DONE go High" at the end of the last byte. This did work for most cases, but created problems with some parallel interfaces driving certain daisy-chains.

The "Length Count Aligned" offered a smarter way of calculating length count: Make sure that the "beginning of the end", the detection of "length-count match" occurs at the first bit of the last byte, then there will always be enough CCLK pulses to finish configuration. That is obviously a cleaner and simpler algorithm.
AR# 780
作成日 08/31/2007
最終更新日 09/30/2005
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