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Electrical Isolation Technique for Tree Branches in CVS Circuit Family

IP.com Disclosure Number: IPCOM000038683D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 2 page(s) / 80K

Publishing Venue

IBM

Related People

Hanafi, HI: AUTHOR

Abstract

In this publication a technique is described wherein the performance of a CVS tree is greatly enhanced by electrically isolating the loading of the "off" tree branches within the tree from its output node. A design problem in the SCVS/DCVS circuit family greatly affects its performance. This may be explained, utilizing a DCVS design (the same explanation is also valid for SCVS trees), as follows: Fig. 1 shows an example of a DCVS tree with inputs A, AN, B, Bn, C, CN,..... and outputs NT and NC. A tree branch is defined as a combination of series and/or parallel n-channel devices (performing a specific Boolean expression) connecting node NC or NT to ground. (Image Omitted) In the standby state, PC is low, turning off device TG and enabling nodes NT and NC to charge to full VDD through P-channel devices TV1 and TV2.

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Electrical Isolation Technique for Tree Branches in CVS Circuit Family

In this publication a technique is described wherein the performance of a CVS tree is greatly enhanced by electrically isolating the loading of the "off" tree branches within the tree from its output node. A design problem in the SCVS/DCVS circuit family greatly affects its performance. This may be explained, utilizing a DCVS design (the same explanation is also valid for SCVS trees), as follows: Fig. 1 shows an example of a DCVS tree with inputs A, AN, B, Bn, C, CN,..... and outputs NT and NC. A tree branch is defined as a
combination of series and/or parallel n-channel devices (performing a specific Boolean expression) connecting node NC or NT to ground.

(Image Omitted)

In the standby state, PC is low, turning off device TG and enabling nodes NT and NC to charge to full VDD through P-channel devices TV1 and TV2. Tree branches internal nodes are charged to VDD - VT, where VT is the threshold voltage for n-channel devices either by explicit internal node precharging or by implicit precharging due to the previous input state. In the active cycle, if the proper inputs to a tree change to the high (VDD) state, node NT (or NC) will be discharged to ground. The performance of the circuit is determined by the speed of discharging node NT (or NC) by the "on" branch. This depends on the capacitance seen by node NT (or NC). Since node NT (or NC) connects several tree branches, the capacitance seen by node NT (or NC) will include all device capacitance of all other "off" branches. The capacitance can have a very high value specially for the branches when all devices are turned on except the bottom device whose source is...