Hi all, In TVPACK packing tool
- posted
19 years ago
Hi all, In TVPACK packing tool
The values are above are estimates of the relative value of the delay through a BLE (LUT), the delay to use local routing within a cluster (LAB in Altera terminology), and the average delay to route a connection between clusters (LABs). Since placement and routing hasn't been performed yet (you're clustering the circuit before placement and routing) you can only crudely estimate the delay of a net that isn't captured within a cluster. In the equations above, it is saying that
LUT delay = local routing delay inside a LAB Average routing delay for connections between LABs = 10x LUT delay.
The quality of the packing you get isn't too sensitive to those parameters, so it isn't worth getting too hung up on their precise values.
I would say that in modern FPGAs, the delay between clusters/LABs above is a bit too pessimistic for connections that are on or near the critical path. They probably usually get routed with a delay that is ~3X - 5X the delay of an "average" path through a LUT (each path through a LUT has a different delay, but the T-Vpack delay model doesn't take that into account). Local interconnect within a cluster is still about 1X - 1.5X the delay of an "average" path through the LUT, at least for the FPGAs I'm most familiar with.
Vaughn
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