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High Speed Synchronous Counter with Improved Look Ahead Carry

IP.com Disclosure Number: IPCOM000115524D
Original Publication Date: 1995-May-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 55K

Publishing Venue

IBM

Related People

Kato, Y: AUTHOR

Abstract

In the case of the conventional counter, the wider the bit width, the lower the maximum frequency of the synchronous counter becomes. This performance degradation is due to the carry-generation circuit having a long path. This invention uses the improved carry-generation circuit so that the performance degradation is reduced to a minimum.

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High Speed Synchronous Counter with Improved Look Ahead Carry

      In the case of the conventional counter, the wider the bit
width, the lower the maximum frequency of the synchronous counter
becomes.  This performance degradation is due to the carry-generation
circuit having a long path.  This invention uses the improved
carry-generation circuit so that the performance degradation is
reduced to a minimum.

      The diagram is shown in Fig. 2.  CCELL is the basic counter
element of this invention and using n CCELLs will produce an n-bit
synchronous counter.  This CCELL has two carry inputs.  One is ripple
carry which is referred to as CI in the diagram and the other is look
ahead carry, LCI.  As shown in the diagram, CI of MSB CCELL is
connected to Carry-A, and LCI of MSB CCELL is connected to Carry-B.
Carry-A is logically equivalent to Bit0 and Carry-B to AND of Bit1
through Bit(n-1).  Both carries are ANDed inside the CCELL to
generate the real carry input.  In order to improve the carry
propagation delay, this invention has two carry paths on purpose.

      As shown in Fig. 1, Carry-B is asserted from Cycle-3, while
Carry-A is asserted in only  Cycle-4.  This means that the delay
requirements are different from each other.  The delay permitted to
Carry-B is almost two times of cycle time, but that to Carry-A is
about one cycle time.  On the other hand, to look into the actual
circuit, the path length of Carry-A is very short, and that of
Carry-...