In a 4 bit ripple carry adder 4 full adders are connected serially, one FA waits for the carry input from the previous FA. My question is, when calculating the propagation delay, should we assume that the subsequent FAs won't start functioning until it receives carry from previous FA, or can at least the first XOR gate compute A XOR B even without carry input from previous FA?
The addition of each A and B can start in the first XOR gate without the carry being valid. However, propagation of the correct value to S needs the carry input valid, and then some further time for the propagation of the second XOR gate.
You simply have to add up all the gates that the signal has to pass through, in series, add up all the delays, and take the worst case, for the propagation delay of the entire circuit.