4.8.1. Reducing Power Consumption With Multiple Clock Domains

Quartus® Prime Pro Edition User Guide: Platform Designer

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Date 4/01/2024

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4.8.1. Reducing Power Consumption With Multiple Clock Domains

When you use multiple clock domains, you should put non-critical logic in the slower clock domain. Platform Designer automatically reconciles data crossing over asynchronous clock domains by inserting clock crossing logic (handshake or FIFO).

You can use clock crossing in Platform Designer to reduce the clock frequency of the logic that does not require a high frequency clock, which allows you to reduce power consumption. You can use either handshaking clock crossing bridges or handshaking clock crossing adapters to separate clock domains.

You can use the clock crossing bridge to connect host interfaces operating at a higher frequency to agent interfaces running at a lower frequency. Only connect low throughput or low priority components to a clock crossing bridge that operates at a reduced clock frequency. The following are examples of low throughput or low priority components:

PIOs
UARTs (JTAG or RS-232)
System identification (SysID)
Timers
PLL (instantiated within Platform Designer)
Serial peripheral interface (SPI)
EPCS controller
Tristate bridge and the components connected to the bridge

By reducing the clock frequency of the components connected to the bridge, you reduce the dynamic power consumption of the design. Dynamic power is a function of toggle rates and decreasing the clock frequency decreases the toggle rate.

Figure 179. Reducing Power Utilization Using a Bridge to Separate Clock Domains

Platform Designer automatically inserts clock crossing adapters between host and agent interfaces that operate at different clock frequencies. You can choose the type of clock crossing adapter in the Platform Designer Project Settings tab. Adapters do not appear in the Connections column because you do not insert them. The following clock crossing adapter types are available in Platform Designer:

Handshake—Uses a simple handshaking protocol to propagate transfer control signals and responses across the clock boundary. This adapter uses fewer hardware resources because each transfer is safely propagated to the target domain before the next transfer begins. The Handshake adapter is appropriate for systems with low throughput requirements.
FIFO—Uses dual-clock FIFOs for synchronization. The latency of the FIFO adapter is approximately two clock cycles more than the handshake clock crossing component, but the FIFO-based adapter can sustain higher throughput because it supports multiple transactions simultaneously. The FIFO adapter requires more resources, and is appropriate for memory-mapped transfers requiring high throughput across clock domains.
Auto—Platform Designer specifies the appropriate FIFO adapter for bursting links and the Handshake adapter for all other links.

Because the clock crossing bridge uses FIFOs to implement the clock crossing logic, it buffers transfers and data. Clock crossing adapters are not pipelined, so that each transaction is blocking until the transaction completes. Blocking transactions may lower the throughput substantially; consequently, if you want to reduce power consumption without limiting the throughput significantly, you should use the clock crossing bridge or the FIFO clock crossing adapter. However, if the design requires single read transfers, a clock crossing adapter is preferable because the latency is lower.

The clock crossing bridge requires few logic resources other than on-chip memory. The number of on-chip memory blocks used is proportional to the address span, data width, buffering depth, and bursting capabilities of the bridge. The clock crossing adapter does not use on-chip memory and requires a moderate number of logic resources. The address span, data width, and the bursting capabilities of the clock crossing adapter determine the resource utilization of the device.

When you decide to use a clock crossing bridge or clock crossing adapter, you must consider the effects of throughput and memory utilization in the design. If on-chip memory resources are limited, you may be forced to choose the clock crossing adapter. Using the clock crossing bridge to reduce the power of a single component may not justify using more resources. However, if you can place all of the low priority components behind a single clock crossing bridge, you may reduce power consumption in the design.

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Quartus® Prime Pro Edition User Guide: Platform Designer

4.8.1. Reducing Power Consumption With Multiple Clock Domains