A method of transferring data from a first bus to a second bus across an asynchronous interface using an asynchronous bridge. The bridge comprises a bus slave module, connected to the first bus, comprising a forward-channel initiator in a first power and/or clock domain; and a bus master module, connected to the second bus, comprising a forward-channel terminator in a second power and/or clock domain. The forward-channel initiator and terminator are in communication to form a forward lockable mutex for arbitrating access to signals used to transfer data from the first domain to the second domain. If the mutex is locked, a forward data channel is used to transfer data between the domains. Otherwise if the mutex is unlocked, the forward channel initiator toggles a status request signal and the forward channel terminator toggles a status acknowledge signal in response, the mutex thereby becoming locked.