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

ID 683323
Date 9/24/2018
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1.2.2.5. Use Gated Clocks

Gated clocks turn a clock signal on and off using an enable signal that controls gating circuitry. When a clock is turned off, the corresponding clock domain is shut down and becomes functionally inactive.
Figure 6. Gated Clock

You can use gated clocks to reduce power consumption in some device architectures by effectively shutting down portions of a digital circuit when they are not in use. When a clock is gated, both the clock network and the registers driven by it stop toggling, thereby eliminating their contributions to power consumption. However, gated clocks are not part of a synchronous scheme and therefore can significantly increase the effort required for design implementation and verification. Gated clocks contribute to clock skew and make device migration difficult. These clocks are also sensitive to glitches, which can cause design failure.

Use dedicated hardware to perform clock gating rather than an AND or OR gate. For example, you can use the clock control block in newer Intel FPGA devices to shut down an entire clock network. Dedicated hardware blocks ensure that you use global routing with low skew, and avoid any possible hold time problems on the device due to logic delay on the clock line.

From a functional point of view, you can shut down a clock domain in a purely synchronous manner using a synchronous clock enable signal. However, when using a synchronous clock enable scheme, the clock network continues toggling. This practice does not reduce power consumption as much as gating the clock at the source does. In most cases, use a synchronous scheme.