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If multiplexing is used then we need to add timing constraints for the mux and interconnects so that the implementation tools will consider them along with those for the rest of the design.
The most obvious constraint is the clock constraint for the mux clock. Based on the multiplexing scheme used and the required performance of the prototyping system, it should be possible to analyze the maximum possible transfer clock speed and then work backwards to calculate the optimum constraint accordingly. Some partitioning tools can perform timing estimation if they can be given information about the board trace delays. The derived clock constraint for the transfer clock can then be used during synthesis and place & route.
We can calculate the maximum design speed based on the maximum transfer clock speed. Remember that the mux transfers exactly one set of its inputs to the destination FPGA within a design clock period but the ratio of mux clock to design clock depends not only on the multiplexing ratio but also on the multiplexing scheme used and any skew and jitter margin added between design clock and transfer clock. The clock-to-clock ratio, rather than just the mux ratio, should be provided by the designer of the multiplexing scheme in use.
Other required constraints are the maximum delay constraints between the design clock domain and the transfer clock domain. As we have seen before, the ratio between design clock and transfer clock depends on the delay between design and the multiplexing components. If we assume that there is a defined delay like 10ns we have to give a constraint to the place & route tools to ensure that all multiplexed connections are within this range. This kind of constraint is often missing but it is important to guarantee full functionality. See the worked example above for how to calculate the correct constraint.
If we perform multiplexing correctly, all timing will be met, a high performance will be achieved and the end user of the prototype should not even be aware that multiplexing has been used.
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