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FPGA Configuration - Can CCLK run before data is sent?
Users who download bitstreams from a microcontroller may want
to give the FPGA a free-running clock that would also be used
as the configuration clock (CCLK). This may result in the FPGA
seeing extra CCLKs before the configuration data is sent.
The problem with this situation is that the FPGA keeps a
running tally of how many configuration clocks have been
received since power-up. This number is matched against the
length count in the bitstream to determine when configuration
has been completed. The length count in the bitstream will not
account for the extra configuration clocks; thus, when all
the data has been sent, the length count and FPGA internal
count will not match (the FPGA's internal counter will contain
a greater value than the length count). The result is that the
FPGA will continue to wait for data until it has been provided
with enough configuration clocks to force its internal counter
to wrap back around to zero, and then up to the number in the
bitstream length count. At this point, the FPGA would end
configuration and assert the DONE pin.
The symptoms of this problem are a device that will not
configure but gives no indication of an error (if the
microcontroller stops providing cclks after sending all the
data), or a device that takes an unusually long time to
configure (if the microcontroller continues to provide cclks
after sending all the data).
Express Mode will ignore the CCLKs if the CS1 pin is LOW.
There are three resolutions.
Re-assert the PROG pin such that there are no more than four
(4) CCLKs after the rise of INIT and the start of the
configuration data. The configuration data starts with a byte
of dummy '1' fill bytes. These may also be trimmed to lessen
the number of extra CCLKs.
Modify the 24 bit length count number after the preamble in the
Bitstream Header to account for the extra CCLKs.
After downloading the bitstream, send the extra CCLKs required
to roll over the internal counter and count up to the length
count again (~18,000,000 CCLKs).