Design Article
Automatic shape-based routing to achieve parasitic constraint closure in custom design
Mark Williams, co-founder and CEO, Pulsic Ltd.
2/9/2011 10:43 AM EST
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Figure 2, below, shows a signal being driven by one integrated circuit cell I1 and being received by another cell I2. The critical path involves I1, I2 and the wiring joining them. For a large fraction of the total wire length of the signal connecting the two cells, the wire is adjacent to two other signals on the same metal layer and will therefore have a significant capacitance to both of them. It is seen that the wiring of the critical signal is of minimum length and therefore if it is rerouted it is likely to follow exactly the same path and thus rerouting will not improve the performance of the signal.
There are two changes that could reduce the parasitics of the critical signal: increase the spacing to the other wires on the same metal layer (as shown in Figure 3); or, find another path for the signal that is of non-minimum length but has lower parasitics than the existing path (shown in Figure 4).
Figure 2, below, shows a signal being driven by one integrated circuit cell I1 and being received by another cell I2. The critical path involves I1, I2 and the wiring joining them. For a large fraction of the total wire length of the signal connecting the two cells, the wire is adjacent to two other signals on the same metal layer and will therefore have a significant capacitance to both of them. It is seen that the wiring of the critical signal is of minimum length and therefore if it is rerouted it is likely to follow exactly the same path and thus rerouting will not improve the performance of the signal.
Figure 2: Plan view of integrated circuit wiring
There are two changes that could reduce the parasitics of the critical signal: increase the spacing to the other wires on the same metal layer (as shown in Figure 3); or, find another path for the signal that is of non-minimum length but has lower parasitics than the existing path (shown in Figure 4).
Figure 3: Illustration of the effects of increasing spacing
Figure 4: Illustration of the effects of finding alternative path (parasitic constraint driven routing)
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