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Tip of the week : Always-on Buffers

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Always-on buffers
 
As Low power designs have started using the power shutoff technique to reduce
leakage power, the need of using always-on buffer arises.
Always-on buffers are just like regular buffers but have a secondary always-on pin
to keep the cell on even when the primary power is switched off in a block/domain.
Always-on buffers are primarily used in the following scenarios:
 
1. Buffering  feedthrough nets  crossing a shut off power domain
2. Buffering high fanout nets like the control net of an State Retention Power Gate (SRPG) cell
or the enable net of an isolation cell
 
Cadence currently supports the above techniques in the low power flow if the library
has always-on cells. "bufferTreeSynthesis" is the command which is used for this method.
This command can take in a list of always-on buffers for each domain and the net name which
needs to be buffered.
 
 
Command:
 
bufferTreeSynthesis -nets   -powerDomainBufList { {PDname Buflist} {PDname Buflist}} –bufPowerDomainReport -maxDelay -maxTran -maxSkew
 
Example:
 
bufferTreeSynthesis -nets enable0  -fixedBuf -fixedNet -powerDomainBufList { {AO BUFFD2BWP BUFFD4BWP BUFFD8BWP} {TDSPCore PTBUFFD2BWP}}
 
BufferTreeSynthesis can be used after the placement step in a lower flow followed by routing of the secondary always-on pins to the always-on power grid. The always-on pin hook up is need to keep the cell always-on during the shutoff period.
In addition to the support of always-on buffer, Cadence also supports the replacement of regular buffers with always-on buffers.
 
Thanks
Sundar


Originally posted in cdnusers.org by sundar_shanmugam

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