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ALU and Carry Circuit in Differential Cascode Current Switch Technology

IP.com Disclosure Number: IPCOM000045905D
Original Publication Date: 1983-May-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 67K

Publishing Venue

IBM

Related People

Leininger, JC: AUTHOR

Abstract

The Arithmetic and Logic Unit (ALU) circuit provides Add, Subtract, AND, OR, Exclusive OR, Shift Right, Flush A and Flush B functions in Differential Cascode Current Switch (DCCS) technology. The height of DCCS trees is limited by semiconductor technology and circuit considerations. If differential transistor pairs are used throughout the tree, the height of the tree determines the maximum number of variables that may be present in any current path of the tree. This means that current flowing from the resistor to the current source should pass through a maximum of M devices, where M is 5 for existing 5-volt semiconductor processes.

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ALU and Carry Circuit in Differential Cascode Current Switch Technology

The Arithmetic and Logic Unit (ALU) circuit provides Add, Subtract, AND, OR, Exclusive OR, Shift Right, Flush A and Flush B functions in Differential Cascode Current Switch (DCCS) technology. The height of DCCS trees is limited by semiconductor technology and circuit considerations. If differential transistor pairs are used throughout the tree, the height of the tree determines the maximum number of variables that may be present in any current path of the tree. This means that current flowing from the resistor to the current source should pass through a maximum of M devices, where M is 5 for existing 5-volt semiconductor processes.

When it is desirable to have additional variables in a tree, any differential pair of transistors may be replaced by a set of 3 or 4 transistors. Precautions must be taken so that at all times there will be one and only one input to the transistor set at a positive (+) level relative to the other inputs so that there may be only one path that conducts current.

The ALU circuit shown in the drawing provides for a plurality of functions, namely, Add, XOR, Subtract/Compare, AND/Test, Shift Right, OR, Flush A, and Flush B, to be obtained. The ALU DCCS circuit also has a carry-generation circuit which provides a ripple carry from one bit of the ALU to the next.

The operation of the ALU and carry circuits is as follows. If a Subtract operation is to be performed, the Subtract/Compare line into the four-transistor (4 TX) set is + with respect to the other inputs. This allows current to flow through this path to the current source. The differential pairs of transistors higher in the tree would have complementary inputs with current flowing through the transistors with the most positive relative input to each pair so that there will exist one and only one current path from the + sum or - sum resistor. The resistor with current flowing in it will have a voltage drop across it and its level will be - with respect to the output level at the resistor with no current flow; thus complementary outputs are generated.

Similarly, the Add,XOR functions cause the second input to the 4 TX set to be at an up level. These functions are the same in the ALU except that the XOR function, like all logical functions, does not allow carries so that the inhibit carry input to the carry DCCS trees would be active and no carries would be allowed within the ALU.

The Test/AN...