Global Clock Enables - 2024.1 English

Vivado Design Suite User Guide: Power Analysis and Optimization (UG907)

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2024.1 English

In general, dealing with Clock Enables is less complex than dealing with Reset. In most of the design usage, it is obvious and straightforward. However, Clock Enables can grow as complex as Reset on power aware designs in which Clock Enables are extensively used and are controlled using special logic circuits. Dynamic power on logic cells depends on the switching activity of Clock Enables. If the activity rates are not set properly, it will easily result in inaccurate numbers.

For example, Enable is expected to be asserted (active) throughout the run and remains inactive only when the logic cell is not being used - if at all explicitly controlled to save power. This could be denoted in terms of switching activity as:

Report Power identifies primary ports which are found to be global enables and applies the above switching activity. It uses a very conservative and safe way to identify the global enables: the ports which are directly connected to CE pins of leaf primitives.

The Power Report also helps identify such Enable nets in the design, so that you can quickly validate the switching information on these nets and take corrective action. You can run a first trial run of Report Power using the default settings to analyze the activity on Enable nets.

Figure 1. Clock Enable Net in the Power Report
Note: The Power Report also gives information about the number of logic cells that are affected by this Enable net, in the Fanout and Logic Type columns. If the initial switching activity estimation does not seem correct, you can select the net in the Power Report and edit Power properties in the Net Properties window.