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XC2000/XC3000/XC4000/XC5200: What set, reset capabilities do the 2000. 4000, 5200 flip flops have?
Due to architectural differences in the flip flops in the 2, 3, and 4k families of devices, the same set and reset abilities are not available to them all;
2000: FFs can both set and reset asynchronously.
3000: FFs can only reset asynchronously.
4000E/EX/XL/XV: FFs can either set OR reset asychronously, but not both.
3000: Latches are not reliable because of potential timing
issues. User should be aware min timing specs are not
4000e: Latches maybe safely implemented using RAM. Look at
solution 122 for more details. However, the ones present in
the library may have the same problems as the XC3000.
4000ex/xl/xv: The CLBs in these devices contain latches and
can be safely used.