Look-ahead carry for counters

Claim 1

A circuit for producing a carry signal for a counter upon the presence of a predetermined one of a series of timed count signals, said circuit comprising first logic means having an input operatively connected to said counter effective to produce a precarry signal upon the presence of a predetermined condition of said counter, second logic means receiving a count signal, and switch means actuated in the absence of a count signal and deactuated in the presence of a count signal, operatively interposed between said first and second logic means, and effective when actuated to operatively connect said second logic means to said first logic means, thereby to establish a precarry condition at the former in response to said precarry signal when present and when deactuated, to electrically isolate said second logic means froM said first logic means, said second logic means being effective when once established in a precarry condition and upon the application thereto of said predetermined count signal, to produce a carry signal substantially simultaneously with said predetermined count signal, in which said second logic means comprises gating means having an input operatively connected to said switch means and an output node, and means effective to charge said output node to a first operative level in the absence of a precarry condition at said second logic means, and in which said gating means comprises second and third switch means having their outputs operatively connected between said output node and a reference point at said first operative level, the control terminal of said second switch means defining said input, the control terminal of said third switch means receiving the complement of a count signal and effective when actuated thereby to operatively connect said output node to said reference point.