Source-synchronous
Source-Synchronous clocking refers to a technique used for timing symbols on a digital interface. Specifically, it refers to the technique of having the transmitting device send a clock signal along with the data signals. The timing of the unidirectional data signals is referenced to the clock (often called the strobe) sourced by the same device that generates those signals, and not to a global clock (i.e. generated by a bus master). Compared to other digital clocking topologies like system-synchronous clocks, where a global clock source is fed to all devices in the system, a source-synchronous clock topology can attain far higher speeds.
This type of clocking is common in high-speed interfaces between micro-chips, including DDR SDRAM, SGI XIO interface, Intel Front Side Bus for the x86 and Itanium processors, HyperTransport, SPI-4.2 and many others.
A reason that source-synchronous clocking is useful is that it has been observed that all of the circuits within a given semiconductor device experience roughly the same process-voltage-temperature (PVT) variation. This means signal propagation delay experienced by the data through a device tracks the delay experienced by the clock through that same device over PVT. This advantage allows higher speed operation as compared to the traditional technique of providing the clock from a third device to both the transmitter and the receiver. Another benefit is that higher complexity data-recovery or clock-data-recovery circuits (such as PLLs) are not required when this technique is used.
Or rather than higher clock speeds, large systems that take advantage of source-synchronous clocking can have the benefit of a higher tolerance of PVT variation of its individual components.
Synchronous logic elements such as flip-flops have static timing criteria that must be satisfied in order for them to work correctly. In a system-synchronous clock topology where a skew-aligned clock is fed to all devices, the criteria are
...
Wikipedia