In this paper, we propose a new concept of testing only functionally testable transition faults in Broadside Transition testing via a novel constrained ATPG. For each functionally untestable transition fault , a set of illegal (unreachable) states that enable detection of is first computed. This set of undesirable illegal states is efficiently represented as a Boolean formula. Our constrained ATPG then incorporates this constraint formula to generate Broadside vectors that avoid those undesirable states. In doing so, our method efficiently generates a test set for functionally testable transition faults and minimizes detection of functionally untestable transition faults. Because we want to avoid launching and propagating transitions in the circuit that are not possible in the functional mode, a direct benefit of our method is the reduction of yield loss due to overtesting of these functionally untestable transitions. 1