Lockup latch
Beginner
A small holding cell placed where a test chain crosses between two parts of the chip that keep slightly different time, so values don’t race ahead.
Novice
A latch (a simple memory cell that lets data through while its clock is at one level and holds it while the clock is at the other) placed between two scan flip-flops whose clock signals arrive at different times. It holds the bit back so it can’t skip a place in the chain.
Expert
Clocked on the opposite phase of the launching flop, so the shifted bit waits about half a cycle and the receiving flop’s capture edge, even if it arrives late, sees the old value. Lockups also act as break points that scan reordering can’t move flops across, so too many of them lengthen scan wiring and add congestion.