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Pins cannot be selected appropriately without a good knowledge of the placement of components.
To illustrate the situation, consider a trivial example: two FPGAs interfacing with four DDR2 memories (Figure 1). Given this, what's the best pin assignment for the entire system?
Figure 1 - One Design, Three PCB Layout Options
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The answer is, without at least a crude understanding of the PCB topology, it's impossible to tell! Board-optimal pin assignments cannot be made while simultaneously ignoring the board. But in a misguided, zealous desire to get the job done, many teams regularly allow FPGA designers to do exactly that: pick the pins without completely appreciating the PCB-related ramifications of their decisions. In other words, they discount the physical constraints of the design.
7Circuits enables users to indicate their intent clearly by placing components on a 'canvas' or the virtual board.
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Logical Constraints
