Static timing analysis is critical in ensuring timing closure in current IC designs. Additional pessimism(a procedure to observe the wrong aspects in the design), on the other hand, might considerably increase the difficulties of achieving time closure. And CRPR in VLSI is a common step in achieving precise timing signoff.
What is CRPR in VLSI all about?
Clock reconvergence pessimism (CRP) is a delay difference between the launching and capturing clock pathways. The most prevalent causes of CRP are convergent pathways in the clock network and varying minimum and maximum delays of clock network cells. CRP is a negative consequence. Clock reconvergence pessimism is a general accuracy restriction of STA.
During the fabrication of chips on the same die, differences in process, voltage, or temperature might occur, causing transistors to be quicker or slower in various dies. The variation might be predictable or random. Some examples of random variations are Oxide thickness changes, implant dosages, and metal or dielectric thickness variations.
What’s The Mechanism of CRPR in VLSI?
Assume there are two inverters with identical properties on a single chip, but their delay propagation differs owing to manufacturing, voltage, and temperature variations. Static timing analysis proposes a notion called On-Chip Variation (OCV) to compensate for these changes. OCV adds a set derate value to all cells or nets in the launch clock path, data path, and capture clock path, increasing pessimism in timing analysis and correcting for volatility. As a result, OCV introduces some pessimism into the shared path of launch and capture, resulting in two delays, min, and max, for the same cell. We eliminate unnecessary pessimism from the common path in this idea. In general, we add the delay to each buffer in the OCV process. However, adding extra delay reduces chip performance and may result in violations. To address this, we are reducing delays from the common route in the CRPR process. The delay difference between the launching and capturing clock routes is known as Clock Reconvergence Pessimism Removal or CRPR in VLSI.
Also read, Everything you need to know about VLSI microprocessor
CPPR and CRPR in VLSI STA:
CRP and CRP Removal (CRPR) is sometimes used interchangeably with CPPR (Common Path Pessimism Removal); however, CRPR and CPPR are not interchangeable and represent two distinct perspectives on clock route pessimism. CPPR is caused mainly by OCV fluctuations, whereas CRPR is an architectural artifact. If a designer sets the OCV derates to 0%, there will be no CPPR involved since the common components will have the same delay for launch and capture, resulting in zero skews.
Even in this case, CRP will be relevant since it eliminates the common path, and so there will always be a skew in clock re-convergent pathways until they are balanced with accuracy. This is highly implausible for multi-mode-multi-corner analysis.
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