In this paper, we propose an advanced structure of the interface circuit, called a wrapper, for Globally Asynchronous Locally Synchronous (GALS) systems. The proposed wrapper is composed of a sender module and a receiver module. The sender module carries out data transfers in an efficient way by decoupling dependency between an external handshake protocol and an internal clock. The decoupling effect allows the external handshake protocol and the internal clock to be executed in a concurrent way and hence allows the wrapper to show better performance. We have designed our wrapper at the transistor level with 0.35-µm technology. When we compare our decoupled wrapper with two conventional wrappers based on pausible clocking scheme, our simulation results show that performance improvement is about 8-13% and 13-56%, respectively.