False Path in VLSI: Simplified Technology

False Path in VLSI: Simplified Technology

One of the most difficult tasks for RTL designers is Identifying full-timing exceptions upfront. In intricate designs, this becomes an iterative process in which more timing exceptions are detected based on the critical path or failed path analysis using timing data. This method leaves a considerable number of timing pathways that may or may not be genuine, but they are never recognized since they do not appear in the: crucial path report. Synthesis and timing tools, on the other hand, will continue to waste resources optimizing these pathways when they are not required. At the same time, it may affect the device’s area and power consumption.

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What Is a False Path in VLSI?

False path in VLSI is a word that is frequently used in STA. It refers to a timing path that does not need to be optimized for timing since it is never required to be recorded in a limited period when stimulated in regular chip operation. In a typical instance, the signal released from one flip-flop must be collected by another flip-flop in just one clock cycle. A false path in VLSI is an exceptional path that is never exercised in the design.  As a result, a bogus path does not need to be timed and may be ignored during timing analysis. 

False Paths Are Classified Into Design Topologies such as

Static False Path:

Static false paths are temporal arcs in VLSI design where the excitation of the source register has no effect or modification on the destination register. Even though both the source and destination flops are running on the same clock domains, the timing route in these topologies cannot be sensitized by any input vector.

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False Reset Timing Arc: 

During device startup, all of a device’s application modules don’t need to be activated. As a result, the clock to such modules is gated by default. The reset de-assertion to those modules occurred during the system reset de-assertion procedure in the absence of the clock. Because there is no clock, there is no possibility of metastability owing to a reset de-assertion timing violation. As a result, asynchronous reset recovery/de-assertion pathways to such modules can be regarded as false.

Asynchronous False Path (CDC path):

The path is deemed asynchronous or Clock Domain Crossing path if the clock domain of the source register is asynchronous to the clock domain of the destination register. There can be no temporal connection in these pathways since there is no established link between the clock edges of the launching and capturing domains. For timing analysis, these pathways might be considered false paths. In this circumstance, it is the designer’s responsibility to avoid any instances of setup/hold violations when capturing domain registers.

Read more about what is clock domain crossing and how it works.

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Why do we set a false path in VLSI?

A false path exists topologically in the design but is neither functional nor does it need to be timed. As a result, during time analysis, the incorrect pathways should be ignored. During the max frequency computation, we check for crucial paths. When the longest path between two pipeline stages is a false path, all longest pathways do not need to be crucial paths. A false path in VLSI is one in which data flow between pipeline stages is impossible. Designing false or multicycle paths and employing restrictions during timing analysis helps to close the timing of a high-frequency system. At the same time, setting incorrect limitations might result in catastrophic device failure. Hence, VLSI designers should use extreme caution when creating limitations for synthesis or time analysis.


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