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XC2000/XC3000/XC4000/XC5200: Tying two output pins to enhance current drive
Special considerations must be taken into account when tying
two outputs to enhance drive capabilities.
Even though the two signals that are being tied come from the
same net, it is possible there maybe a few nanoseconds of
difference between the arrival of a signal in the two
pins. This means that for a few nanoseconds
there will be signal contention that may likely reduce
the life expectancy of the device.
The best way to reduce contention time to a fraction of
a nanosecond is to register the outputs using the output
flip flops inside the IOBs with a clock signal coming from a
global buffer. Or in the case of the 5200, use
the CLB flip flop in the slice that is right next to the IOB
that is going to be used so the direct lines can be used.