Please note...............It is possible to have a synchronous serial protocol without a separate clocking line going down the cable. It is also possible to have a synchronous protocol even without a cable at all !! Ie take for example the RF transmssions to/from a satellite or the photon bursts on a fiber optic bundle. In these applications the transfer protocol is designed to embedd the clock signal into the data stream along with the data. At the receiving end an appropriate filter and possibly a PLL are used to recover the clocking signal and recover the companion data. One very common synchronous serial protocol is called Bi-phase (aka Manchester) encoding.

It is also possible to use a UART in a mode where its output is encoded/decoded outside the UART into/from a bi-phase waveform for synchronous transmission/reception. I have done this several times. Note however that it is very difficult to get the on-board UART of a typical 8051 to cooperate with type of design because of two reasons. First off you need to have access to the 1X clock on the transmit side and you need to be able to provide eith the 1X or 16X clocking back to the receiver. Both of these are difficult to achieve with typical 8051's due to their desire to generate baud rates on board.

Michael Karas