A method of transferring data from a first bus to a second bus across an asynchronous interface between said first and second buses (11, 13), using an asynchronous bridge comprising: 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; wherein the forward-channel initiator and the forward-channel terminator are in communication to form a status mutex for arbitrating access to signals used to transfer data from the first power and/or clock domain to the second power and/or clock domain, wherein the status mutex comprises a control request flag, a control acknowledgement flag, and a mutex lock flag, and wherein the status mutex is arranged such that: the forward-channel initiator raises the control request flag only when the control acknowledgement flag is not raised; the forward-channel terminator raises the control acknowledgement flag in response to the control request flag being raised only when the mutex lock flag is not raised; the status mutex is locked and the forward-channel initiator raises the mutex lock flag in response to the control acknowledgement flag being raised; the method comprising: if the status mutex is locked, using a forward data channel to transfer data between said first and second power and/or clock domains; if the status mutex is unlocked, said forward channel initiator toggling the control request flag and the forward channel terminator toggling the control acknowledgement flag in response; the status mutex thereby becoming locked; wherein if either side of the asynchronous bridge is reset or loses power thereby giving rise to a desynchronisation of the status mutex, detecting said desynchronisation by the forward-channel initiator or the forward-channel terminator on the other side. 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; wherein the forward-channel initiator and the forward-channel terminator are in communication to form a status mutex for arbitrating access to signals used to transfer data from the first power and/or clock domain to the second power and/or clock domain, wherein the status mutex comprises a control request flag, a control acknowledgement flag, and a mutex lock flag, and wherein the status mutex is arranged such that: the forward-channel initiator raises the control request flag only when the control acknowledgement flag is not raised; the forward-channel terminator raises the control acknowledgement flag in response to the control request flag being raised only when the mutex lock flag is not raised; the status mutex is locked and the forward-channel initiator raises the mutex lock flag in response to the control acknowledgement flag being raised; the method comprising: if the status mutex is locked, using a forward data channel to transfer data between said first and second power and/or clock domains; if the status mutex is unlocked, said forward channel initiator toggling the control request flag and the forward channel terminator toggling the control acknowledgement flag in response; the status mutex thereby becoming locked; wherein if either side of the asynchronous bridge is reset or loses power thereby giving rise to a desynchronisation of the status mutex, detecting said desynchronisation by the forward-channel initiator or the forward-channel terminator on the other side. the forward-channel initiator raises the control request flag only when the control acknowledgement flag is not raised; the forward-channel terminator raises the control acknowledgement flag in response to the control request flag being raised only when the mutex lock flag is not raised; the status mutex is locked and the forward-channel initiator raises the mutex lock flag in response to the control acknowledgement flag being raised; the method comprising: if the status mutex is locked, using a forward data channel to transfer data between said first and second power and/or clock domains; if the status mutex is unlocked, said forward channel initiator toggling the control request flag and the forward channel terminator toggling the control acknowledgement flag in response; the status mutex thereby becoming locked; wherein if either side of the asynchronous bridge is reset or loses power thereby giving rise to a desynchronisation of the status mutex, detecting said desynchronisation by the forward-channel initiator or the forward-channel terminator on the other side. if the status mutex is locked, using a forward data channel to transfer data between said first and second power and/or clock domains; if the status mutex is unlocked, said forward channel initiator toggling the control request flag and the forward channel terminator toggling the control acknowledgement flag in response; the status mutex thereby becoming locked; wherein if either side of the asynchronous bridge is reset or loses power thereby giving rise to a desynchronisation of the status mutex, detecting said desynchronisation by the forward-channel initiator or the forward-channel terminator on the other side.