AR# 102: FPGA Configuration - DONE does not go High; general XC4000 debugging hints
FPGA Configuration - DONE does not go High; general XC4000 debugging hints
What are some general XC4000 Configuration debugging hints?
This section assumes you have understanding of how configuration works in our XC4000 devices, so before you read the following, Xilinx strongly recommends that you carefully read the configuration section in our "Programmable Data Book."
For the DONE pin on an XC4000 device to go High, two conditions must be met:
1. The FPGA's internal configuration memory must be full.
2. The configuration length count must be met, EXACTLY.
This is important because the counter that determines when the length count is met begins counting with the very first cclk, not the first one after the preamble.
This means if a stray bit is inserted before the preamble or the data source is not ready at the time of the first cclk, the internal counter that holds the number of cclks will be one ahead of the actual number of data bits read. Thus, at the end of configuration the second condition will not be met until the 24-bit counter can roll past terminal count and then back up to the lengthcount number stored in the lengthcount register. This causes the FPGA to wait for no more than [ 2^24 * cclk frequency] seconds.
If it is not possible to have the data ready at the time of the first cclk, the problem could be circumvented by adding a few buffer bits to the number in the length count.
Also, check the following:
Remember that the xchecker expects to DRAW power from the pwr and ground connections, not provide it.
If using the download cable, is the LCA configured in Slave mode (M0=M1=M2=High)?
Is the DONE pin configured with the internal pullup or pulled up externally? Note that the download cable does not pull up DONE.
Are the PROG and INIT pins pulled up?
The default MakeBits options are shown below:
CRC Disable (1)
Donepin Nopullup (2)
1. With CRC enabled, the INIT pin is pulled Low during configuration if a data frame error is detected.
2. Activate Pullup if an external pullup is not used.
The sections in the back of The User Guide and Tutorials book are good sources for further information.