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output delay max = board delay (max) - board clock skew (min) + tsu (external device)
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tCO Requirement set_output_delay -max <latch − launch − tCO requirement>
why does <latch - launch> come in to it? Isn't "set_output_delay - max" equivalent to defining the tcomax?
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For a common simple case, the source register driving out from the FPGA and the destination register receiving in the external device are using the same clock, and a single clock period is allowed (no multicycle). For that case, "latch - launch" is one clock period. set_output_delay -max is one clock period minus the FPGA's tco. If the board delays are ignored, then set_output_delay -max is also the same as the tsu of the external device--FPGA tco plus external device tsu equals one clock period.
For set_output_delay -min in this simple case, "latch - launch" is zero.
Table 7-4 in the Quartus handbook gives you the conversion between set_output_delay and tco/min tco in terms of latch and launch to cover more cases like where a PLL in the FPGA makes "latch - launch" be something different from a simple single clock period for max or zero for min.
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