Tuesday, 20 November 2018

Big CMOS

( CPU / Memory / FPGA )    

1.Fanout    2. Path Delay Calculations   3. Beta Ratio   4. Keepers ( Write-ability / Off Leakage )    5.Aggressors, Noise Injection 


Fanout Checks

Fanout is the comparison of drive strength vs total load. Insight Analyzer solves fanout at the primitive MOSFET level, including the effects of:

  • Transistor geometry, driver and receiver
  • Passgate and body characteristics
  • Parasitic loading / extracted C, R-C

 

Path Delay Calculations

Fanout leads way to discussion of R/C delay calculations. While it’s not “fanout” in a textbook sense, it is a natural extension to fanout.
R.ds.on of MOSFETs is not used.

  • R/C topology is converted by Elmore, simplified by Savarino-Kashyap to yield a delay.
  • Wire delay is added to per-unit Fanout delay of the process (essentially accounting for R.ds.on).
  • Intrinsic (baseline) delay of the process is added, to yield a final delay value.

 

Beta Ratio

The Beta Ratio check finds CMOS structures that are not sized at appropriate P:N ratios.
Sizing is done at the primitive transistor level, without using textbook topologies. Sizing rules adapt to virtually any CMOS topology.
This check reports cases where values are outside a given threshold window.

 

Keepers

Write-ability

Logic keepers are checked for strength against drivers and passgates. A keeper may be too strong to be changed by driver or passgate.
In the Keeper checks, the strength of a keeper is compared to drivers and passgates that face it. A violation is reported if the keeper, driver, or passgate is not sized appropriately.

 

 

Off Leakage

Keepers are checked against leakage in off devices.
NMOS devices on ground have highest leakage if they’re at minimum length. This check compares:

  • Width of keeper (“keeper strength”).
  • Width of leakers, each at its given length, all combined in parallel (“leaker strength”).
  • Threshold of failure is where leaker strength becomes a significant fraction of keeper strength.

 

Aggressors, Noise Injection

Storage nodes can be the victim of noise injection. Especially risky are passgates. A nearby aggressor signal can flip the state of the stored value.

  • Finds all passgate and storage nodes.
  • Finds nearby aggressors through parasitic cross-coupling C values (SPF, DSPF, etc).
  • Compares victims own C, aggressor C, aggressor driver strength.
  • Ranks violations according to these factors. There is not a definitive threshold of failure, but the ranking will bring the most unsafe cases to the top.


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