US 7,680,874 B2Granted

Adder

AssigneeTexas Instruments Incorporated

InventorsAkihiro Takegama, Tsuyoshi Tanaka, Masahiro Fusumada

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Abstract

An adder that can detect the generation of overflow at a high speed. Carry signal c14 from the 15th digit to the 16th digit in the result of addition from the 1st digit to the 16th digit of the input data is generated on the basis of bit signals (a0-a15, b0-b15) for the portion from the 1st digit to the 15th digit of the input data, and of carry signal CIN input to the 1st digit, and it is output from CLA 204. Then, carry signal c15 from the 16th digit to the 17th digit is generated based on said generated carry signal c14 and bit signals (a15, b15) of the 16th digit of the input data, and this is output from CIA 205. Exclusive-NOR circuit 206 then operates on said carry signals c14 and c15, and overflow detection signal OVF16 is generated.

Claims3

Dependent claims

An adder having an overflow output indicating when a sum of a first input number and a second input number overflows comprising:

a logic operation circuit receiving respective bits of said first input number and said second input number and generating a carry propagate signal and a carry generate signal for each i-th bit of said first input number and said second input number;
a first carry look ahead adder receiving a least significant bit carry input signal, carry propagate signals and carry generate signals for a first predetermined number of least significant bits, said first carry look ahead adder generating a carry output for a most significant of said first predetermined number of least significant bits;
a second carry look ahead adder receiving said carry output for said most significant of said first predetermined number of least significant bits, carry propagate signals and carry generate signals for a second predetermined number of more significant bits, said second carry look ahead adder generating a carry output for a most significant of said second predetermined number of more significant bits;
a first carry increment adder receiving said least significant bit carry input signal, carry propagate signals and carry generate signals for said first predetermined number of least significant bits, said first carry increment adder generating a sum output for each of said first predetermined number of least significant bits by propagating local carry signals;
a second carry increment adder receiving said carry output for a most significant of said first predetermined number of least significant bits, carry propagate signals and carry generate signals for said second predetermined number of more significant bits, said second carry increment adder generating a sum output for each of said second predetermined number of least significant bits by propagating local carry signals;
a carry select adder receiving said carry output for a most significant of said second predetermined number of more significant bits, carry propagate signals and carry generate signals for a third predetermined number of most significant bits, said carry save adder including:
a first chain of bit adders, each bit adder receiving a carry propagate signal and a carry generate signal of a corresponding bit from said logic operation circuit and a carry input, each bit adder generating a first preliminary sum output and a first preliminary carry output for said corresponding bit, a first bit adder in said first chain of bit adders receiving a 0 carry input and subsequent bit adders in said first chain of bit adders receiving a carry input from a prior bit adder in said first chain of bit adders,
a second chain of bit adders, each bit adder receiving a carry propagate signal and a carry generate signal of a corresponding bit from said logic operation circuit and a carry input, each bit adder generating a second preliminary sum output and a second preliminary carry output for said corresponding bit, a first bit adder in said second chain of bit adders receiving a 1 carry input and subsequent bit adders in said second chain of bit adders receiving a carry input from a prior bit adder in said second chain of bit adders,
a set of selectors having a first input receiving said first preliminary sum output of a corresponding bit, a second input receiving said second preliminary sum output, a selection input receiving said carry output for said most significant of said second predetermined number of more significant bits and an output, each selector outputting said first preliminary sum output or said second preliminary sum output dependent upon said carry output for said most significant of said second predetermined number of more significant bits,
a first exclusive NOR gate having a first input receiving said carry output of a most significant bit adder in said first chain of bit adders, a second input receiving said carry output of a next most significant bit adder in said first chain of bit adders, and
a second exclusive NOR gate having a first input receiving said carry output of a most significant bit adder in said second chain of bit adders, a second input receiving said carry output of a next most significant bit adder in said second chain of bit adders;
and an overflow detection output circuit including
a first bit value circuit including
a plurality of OR gates, each OR gate having a first input receiving a corresponding bit of said first input number, a second input receiving a corresponding bit of said second input number and an output,
a plurality of exclusive OR gates, each exclusive OR gate having a first input receiving said output of a corresponding OR gate, a second input receiving said carry propagate signal of a corresponding next bit and an output, and
a NOR gate having plural inputs each receiving said output of a corresponding exclusive OR gate and an output,
a second bit value circuit including
a plurality of exclusive OR gates, each exclusive OR gate having a first input receiving a carry generate signal of a corresponding, a second input receiving carry propagate signal of a corresponding next bit and an output, and
an AND gate having plural inputs each receiving said output of a corresponding exclusive OR gate and an output,
a first selector having a first input receiving said output of said first bit value detect circuit, a second input receiving said output of said second bit value detect circuit, a control input receiving said carry propagate signal of a least significant bit of said carry select adder and an output, said first selector outputting said output of said first bit detect circuit or said output of said second bit detect circuit dependent upon said carry output for said most significant of said second predetermined number of more significant bits,
a second selector having a first input receiving said output of said second bit value detect circuit, a second input receiving said output of said first bit value detect circuit, a control input receiving said carry propagate signal of a least significant bit of said carry select adder and an output, said second selector outputting said output of said second bit detect circuit or said output of said first bit detect circuit dependent upon said carry output for said most significant of said second predetermined number of more significant bits,
a third selector having a first input receiving said output of said first selector, a second input receiving said output of said first exclusive NOR gate of said carry select adder, a control input receiving a bit width mode signal and an output, said third selector outputting said output of said first selector or said output of said first exclusive NOR gate of said carry select adder dependent upon said bit width mode signal,
a fourth selector having a first input receiving said output of said second exclusive NOR gate of said carry select adder, a second input receiving said output of said second selector, a control input receiving a bit width mode signal and an output, said fourth selector outputting said output of said second exclusive NOT gate of said carry select adder or said output of said second selector dependent upon said bit width mode signal, and
a fifth selector having a first input receiving said output of said third selector, a second input receiving said output fourth selector, a control input receiving said carry output of said carry output for a most significant of said second predetermined number of more significant bits, said fifth selector outputting said output of said third selector or said output fourth selector dependent upon said carry output for a most significant of said second predetermined number of more significant bits, said output being said overflow output.

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Dependencies are inferred from the claim text.

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Abstract

An adder having an overflow output indicating when a sum of a first input number and a second input number overflows comprising:

a logic operation circuit receiving respective bits of said first input number and said second input number and generating a carry propagate signal and a carry generate signal for each i-th bit of said first input number and said second input number;
a first carry look ahead adder receiving a least significant bit carry input signal, carry propagate signals and carry generate signals for a first predetermined number of least significant bits, said first carry look ahead adder generating a carry output for a most significant of said first predetermined number of least significant bits;
a second carry look ahead adder receiving said carry output for said most significant of said first predetermined number of least significant bits, carry propagate signals and carry generate signals for a second predetermined number of more significant bits, said second carry look ahead adder generating a carry output for a most significant of said second predetermined number of more significant bits;
a first carry increment adder receiving said least significant bit carry input signal, carry propagate signals and carry generate signals for said first predetermined number of least significant bits, said first carry increment adder generating a sum output for each of said first predetermined number of least significant bits by propagating local carry signals;
a second carry increment adder receiving said carry output for a most significant of said first predetermined number of least significant bits, carry propagate signals and carry generate signals for said second predetermined number of more significant bits, said second carry increment adder generating a sum output for each of said second predetermined number of least significant bits by propagating local carry signals;
a carry select adder receiving said carry output for a most significant of said second predetermined number of more significant bits, carry propagate signals and carry generate signals for a third predetermined number of most significant bits, said carry save adder including:
a first chain of bit adders, each bit adder receiving a carry propagate signal and a carry generate signal of a corresponding bit from said logic operation circuit and a carry input, each bit adder generating a first preliminary sum output and a first preliminary carry output for said corresponding bit, a first bit adder in said first chain of bit adders receiving a 0 carry input and subsequent bit adders in said first chain of bit adders receiving a carry input from a prior bit adder in said first chain of bit adders,
a second chain of bit adders, each bit adder receiving a carry propagate signal and a carry generate signal of a corresponding bit from said logic operation circuit and a carry input, each bit adder generating a second preliminary sum output and a second preliminary carry output for said corresponding bit, a first bit adder in said second chain of bit adders receiving a 1 carry input and subsequent bit adders in said second chain of bit adders receiving a carry input from a prior bit adder in said second chain of bit adders,
a set of selectors having a first input receiving said first preliminary sum output of a corresponding bit, a second input receiving said second preliminary sum output, a selection input receiving said carry output for said most significant of said second predetermined number of more significant bits and an output, each selector outputting said first preliminary sum output or said second preliminary sum output dependent upon said carry output for said most significant of said second predetermined number of more significant bits,
a first exclusive NOR gate having a first input receiving said carry output of a most significant bit adder in said first chain of bit adders, a second input receiving said carry output of a next most significant bit adder in said first chain of bit adders, and
a second exclusive NOR gate having a first input receiving said carry output of a most significant bit adder in said second chain of bit adders, a second input receiving said carry output of a next most significant bit adder in said second chain of bit adders;
and an overflow detection output circuit including
a first bit value circuit including
a plurality of OR gates, each OR gate having a first input receiving a corresponding bit of said first input number, a second input receiving a corresponding bit of said second input number and an output,
a plurality of exclusive OR gates, each exclusive OR gate having a first input receiving said output of a corresponding OR gate, a second input receiving said carry propagate signal of a corresponding next bit and an output, and
a NOR gate having plural inputs each receiving said output of a corresponding exclusive OR gate and an output,
a second bit value circuit including
a plurality of exclusive OR gates, each exclusive OR gate having a first input receiving a carry generate signal of a corresponding, a second input receiving carry propagate signal of a corresponding next bit and an output, and
an AND gate having plural inputs each receiving said output of a corresponding exclusive OR gate and an output,
a first selector having a first input receiving said output of said first bit value detect circuit, a second input receiving said output of said second bit value detect circuit, a control input receiving said carry propagate signal of a least significant bit of said carry select adder and an output, said first selector outputting said output of said first bit detect circuit or said output of said second bit detect circuit dependent upon said carry output for said most significant of said second predetermined number of more significant bits,
a second selector having a first input receiving said output of said second bit value detect circuit, a second input receiving said output of said first bit value detect circuit, a control input receiving said carry propagate signal of a least significant bit of said carry select adder and an output, said second selector outputting said output of said second bit detect circuit or said output of said first bit detect circuit dependent upon said carry output for said most significant of said second predetermined number of more significant bits,
a third selector having a first input receiving said output of said first selector, a second input receiving said output of said first exclusive NOR gate of said carry select adder, a control input receiving a bit width mode signal and an output, said third selector outputting said output of said first selector or said output of said first exclusive NOR gate of said carry select adder dependent upon said bit width mode signal,
a fourth selector having a first input receiving said output of said second exclusive NOR gate of said carry select adder, a second input receiving said output of said second selector, a control input receiving a bit width mode signal and an output, said fourth selector outputting said output of said second exclusive NOT gate of said carry select adder or said output of said second selector dependent upon said bit width mode signal, and
a fifth selector having a first input receiving said output of said third selector, a second input receiving said output fourth selector, a control input receiving said carry output of said carry output for a most significant of said second predetermined number of more significant bits, said fifth selector outputting said output of said third selector or said output fourth selector dependent upon said carry output for a most significant of said second predetermined number of more significant bits, said output being said overflow output.

21 more elements