A first logic gate has a first input coupled to a first circuit input or a second circuit input, a second input selectively coupled to a third circuit input or a fourth circuit input, and a first output. The first output has a signal with a duty cycle that is a function of a phase difference between a first signal on the first input and a second signal on the second input. A second logic gate has a third input coupled to the third circuit input or the fourth circuit input, a fourth input coupled to the second circuit input or the fourth circuit input, and a second output. The second output has a signal with a duty cycle that is a function of a phase difference between a third signal on the third input and a fourth signal on the fourth input.