Lookahead carry unit

A lookahead carry unit (LCU) is a logical unit in digital circuit design used to decrease calculation time in adder units and used in conjunction with carry look-ahead adders (CLAs).

A single 4-bit CLA is shown below: By combining four 4-bit CLAs, a 16-bit adder can be created but additional logic is needed in the form of an LCU.

The LCU accepts the group propagate (

have the following expressions for each CLA adder:[1] The LCU then generates the carry input for each CLA.

from the ith CLA then the output carry bits are Substituting

yields the expanded equations:

corresponds to the carry input into the second CLA;

In addition, the LCU can calculate its own propagate and generate: By combining 4 CLAs and an LCU together creates a 16-bit adder.

Four of these units can be combined to form a 64-bit adder.

An additional (second-level) LCU is needed that accepts the propagate (

) from each LCU and the four carry outputs generated by the second-level LCU are fed into the first-level LCUs.