Intel® Hyperflex™ Architecture High-Performance Design Handbook

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ID 683353
Date 10/04/2021
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2.4.2.3. Control Signal Backpressure

This section describes RTL design techniques to control signal backpressure. The Intel® Hyperflex™ architecture efficiently streams data. Because the architecture supports very high clock rates, it is difficult to send feedback signals to reach large amounts of logic in one clock cycle. Inserting extra pipeline registers also increases backpressure on control signals. Data must flow forward as much as possible.

Single clock cycle control signals create loops that can prevent or reduce the effectiveness of pipelining and register retiming. This example depicts a ready signal that notifies the upstream register of readiness to consume data. The ready signals must freeze multiple data sources at the same time.

Figure 63. Control Signal Backpressure

Modifying the original RTL to add a small FIFO buffer that relieves the pressure upstream is a straightforward process. When the logic downstream of this block is not ready to use the data, the FIFO stores the data.

Figure 64. Using a FIFO Buffer to Control Backpressure

The goal is for data to reach the FIFO buffer every clock cycle. An extra bit of information decides whether the data is valid and should be stored in the FIFO buffer. The critical signal now resides between the FIFO buffer and the downstream register that consumes the data. This loop is much smaller. You can now use pipelining and register retiming to optimize the section upstream of the FIFO buffer.

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