How do I reconfigure one 3K/4K device out of a daisy chain?
It is sometimes necessary to reconfigure only one device in a daisy-chain of FPGAs. This is possible, although it requires some modification of the design.
The user-configurable pins that are also used as DIN and DOUT before configuration should be wired together in the design, so that the connectivity exists after configuration as well. Then, to reconfigure a device, provide input at the first device in the chain (the usual source before configuration), and pull program (4K) or done/program (3K) low on the device you wish to reconfigure. CCLK should be provided to the chips as it was during power-on configuration. The data should ripple through the configured devices until it is received by the device that is being reprogrammed.
If you use this method on an XC3000, XC3100, or XC3000A part, do not connect the done/program lines together so that done/program on one device may be pulled low independent of the others. On the XC4000 devices, the DONE pins may be connected together, as PROGRAM is a separate pin.