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Spurious switching activity (SSA)

Let us define the RTL interconnects connected to the output of a datapath nit in the datapath as its output network. A functional unit with more than one operation or a register with more than one variable bound to it outputs values of multiple variables onto its output network in different clock cycles. In general, the output value has to be routed to only a small portion of the output network in a given clock cycle. If care is not taken, many other parts of the network may also experience switching activity. Many functional units, for example, a ripple-carry adder, output a lot of glitches before the output finally settles down to the correct value [46]. These glitches also contribute significantly to the output network power consumption. Fig. 3 shows the percentage of total interconnect power consumed through SSA for the eight benchmarks optimized for area and power. For both AO and IUPO circuits, SSA consumes a significant percentage of total interconnect power, with the average being 62.6% and 29.2%, respectively, even when glitches are not taken into account. AO circuits incur a higher percentage of SSA power in interconnects because their datapath units are usually heavily shared.
Figure 3: Percentage of total interconnect power consumed by SSA.
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next up previous
Next: Motivational example Up: Observations Previous: Interconnect power consumption
Lin Zhong 2003-10-11