Intel® Hyperflex™ Architecture High-Performance Design Handbook

ID 683353
Date 10/04/2021

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8.2.2. Retiming with Clock Enables

Like synchronous resets, clock enables use a dedicated LAB-wide resource that feeds a specific function in the ALM register. Similarly, Intel® Hyperflex™ architecture FPGAs support special logic that simplifies retiming logic with clock enables. However, wide broadcast control signals, such as clock enables (and synchronous clears), are difficult to retime.

Figure 139. ALM Representing Clock EnablesThe following figure shows the sequence of retiming moves for the asynchronous clears in the Synchronous Resets and Limitations section.

The top circuit contains a dedicated Hyper-Register on the clock enable path. To push back the register, the Compiler must split the register, so that another register pushes up the clock enable path. In this case, the Hyper-Register location absorbs the register without problem. These features allow the Compiler to easily retime an ALM register with a clock enable backward or forward (middle circuit), to improve timing. A useful feature of a clock enable is that logic usually generates by synchronous signals, so that the Compiler can retime the clock enable path alongside the data path.

Figure 140. Retiming Steps and Structure with an ALM register and Hyper-Registers

The figure shows retiming of the clock enable signal clken typical broadcast type control signal. In the top circuit, before retiming, the circuit uses an ALM register. The circuit also uses the Hyper-Registers on the clock enable and data paths. In the middle circuit, the ALM register retimes forward into a Hyper-Register outside the ALM, into the routing fabric. The circuit still uses the ALM register, but the register is not on the data path through the ALM. The ALM holds the previous value of the register. The clock enable mux now selects between this previous value and the new value, based on the clock enable. The diagram shows retiming forward of a second register from the clock enable and data paths into the ALM register. The circuit now uses the ALM register in the path. You can repeat this process and iteratively retime multiple registers across an enabled ALM register.