# Implementing Carry look ahead Unit in logism

I'm quite new to logism and circuits and I have an assignment about implementing a carry look ahead unit. I Just want some help in regards to clearing some of my confusions to carry look ahead units. So from what I understand, A CLA has 3 main parts to it, a Carry input, a Propagator and a Generator. Carry input can be calculated via the following formula:

$$\C_{i+1} = G_i + P_iC_i\$$

And $$\G_i =\$$ A XOR B, $$\P_i=AB\$$

And the what I really need to find out the rest of the carry inputs is just $$\C_0\$$

Now, as I am implementing my CLA unit in Logism, am I supposed to make different circuits for each carry input? Like for example, for $$\C_1\$$ I would implement the circuit of $$\G_0 + P_0C_0\$$ and then another for $$\C_2\$$, I would implement the circuit $$\G_1 + P_1C_1\$$, and then concatenate them via a splitter. I ask this because I tried just plugging every variable up to $$\C_4\$$ and implementing the long expression, only to get things wrong. Any helpful hints is appreciated. Please don't outright give a picture of the CLA implementation as I want to actually do this myself :)

edit: I managed to figure it out. I did indeed have to calculate each carry in and make a circuit for each expression.

• please post an answer, then you can accept it Commented Feb 3, 2020 at 5:04
• Do you know the concept of subcircuits in Logisim? Read the built-in tutorial for details. It might help you minimize the work. Commented Feb 3, 2020 at 7:42