Fast Carry Sum Adder
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2005-Feb-02
An improved complementary metal-oxide semiconductor (CMOS) carry sum adder (CSA) which is 35-50 percent faster than existing CSAs is described. This CSA requires no more chip area than conventional CSAs and uses two less devices. Referring to the figure, operation of the EXOR and EXNOR is essentially identical. If A0 is high and B0 is high, T5 and T6 are turned off and T4 is turned on. The input inverter's (T1 and T2) output is low, so T3 is turned off. A low is provided to the source of T4, and C1 is low. When A0 falls, T4 is turned off and T6 is turned on, driving C1 high. The delay is approximately equal to that of an inverter. When B0 falls, T5 turns on, the input inverter's output rises, turning on T3, and C1 falls. The total delay is close to that of an inverter.