Suppose N=3; duty not 50%: you can use a ring counter, or a Moore FSM; duty 50%, first build a counter 0->2; then generate two enable signals, one active at time n=0, the other active at n=(N+1)/2; apply the two enable signals to two T-FF, the fiest one triggered on posedge, the second one triggered on negedge; the xor the T-FF outputs