Intel® Quartus® Prime Standard Edition User Guide: Design Recommendations

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ID 683323
Date 9/24/2018
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1.2.2.5.1. Recommended Clock-Gating Methods

Use gated clocks only when your target application requires power reduction and gated clocks provide the required reduction in your device architecture. If you must use clocks gated by logic, follow a robust clock-gating methodology and ensure the gated clock signal uses dedicated global clock routing.

You can gate a clock signal at the source of the clock network, at each register, or somewhere in between. Since the clock network contributes to switching power consumption, gate the clock at the source whenever possible to shut down the entire clock network instead of further along.

Figure 7. Recommended Clock-Gating Technique for Clock Active on Rising Edge

To generate a gated clock with the recommended technique, use a register that triggers on the inactive edge of the clock. With this configuration, only one input of the gate changes at a time, preventing glitches or spikes on the output. If the clock is active on the rising edge, use an AND gate. Conversely, for a clock that is active on the falling edge, use an OR gate to gate the clock and register

Pay attention to the delay through the logic generating the enable signal, because the enable command must be ready in less than one-half the clock cycle. This might cause problems if the logic that generates the enable command is particularly complex, or if the duty cycle of the clock is severely unbalanced. However, careful management of the duty cycle and logic delay may be an acceptable solution when compared with problems created by other methods of gating clocks.

In the Timing Analyzer, ensure to apply a clock setting to the output of the AND gate. Otherwise, the timing analyzer might analyze the circuit using the clock path through the register as the longest clock path and the path that skips the register as the shortest clock path, resulting in artificial clock skew.

In certain cases, converting the gated clocks to clock enable pins may help reduce glitch and clock skew, and eventually produce a more accurate timing analysis. You can set the Intel® Quartus® Prime software to automatically convert gated clocks to clock enable pins by turning on the Auto Gated Clock Conversion option. The conversion applies to two types of gated clocking schemes: single-gated clock and cascaded-gated clock.

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