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Operand Isolation

Disabling datapath computation when not enabled


Often, datapath computation elements are sampled only periodically. This sampling is controlled by an enable signal.
In design with low-power intent, synthesis tools automatically perform a variety of power optimization techniques, including operand isolation.

Operand isolation reduces dynamic power dissipation. When the enable is inactive, the datapath inputs are disabled so that unnecessary switching power is not wasted in the datapath.

In the digital system shown as Before Operand Isolation, register C uses the result of the multiplier when the enable is on. When the enable is off, register C uses only the result of register B, but the multiplier continues its computations. Because the multiplier dissipates the most power, the total amount of power wasted is quite significant.

One solution to this problem is to shut down (isolate) the function unit (operand) when its results are not used, as shown in After Operand Isolation. The synthesis engine inserts AND gates at the inputs of the multiplier and uses the enable logic of the multiplier to gate the signal transitions. As a result, no dynamic power is dissipated when the result of the multiplier is not needed.

For a sample of designs operand isolation provided less than 5% dynamic power savings with little or no impact on any other aspects of the design.

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