In Slave Serial Mode Configuration of an xc4000XL device if the
CCLK starts out at a logic LOW, and is parked LOW after
configuration, the device may not configure. This was a known
problem with the xc3000 and xc4000 FPGA families, but was fixed
in the xc4000E family. However, some xc4000XL parts
exhibit this same problem.
Older FPGA familes had a maximum low time specification for
CCLK, `Tccl', however, this has not yet been characterized for
the xc4000XL. This has been fixed in the xc4000XLA devices
negating the characterization of a maximum Tccl.
Applications exhibiting this kind of behavior with xc4000XL
parts should be encouraged to invert the sense of the