Data strobe encoding and recovered clock.

Data strobe encoding (or D/S encoding) is an encoding scheme for transmitting data in digital circuit

Digital circuit

Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...

s.
It uses two signal lines (e.g. wires in a cable or traces on a printed circuit board

Printed circuit board

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

), Data and Strobe. These have the property that either Data or Strobe changes its logical value in one clock cycle, but never both.
This allows for easy clock recovery

Clock recovery

Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop...

 with a good jitter

Jitter

Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...

 tolerance by XORing the two signal line values.

There is an equivalent way to specify the relationship between Data and Strobe.
For even-numbered Data bits, Strobe is the opposite of Data.
For odd-numbered Data bits, Strobe is the same as Data.
From this definition it is more obvious that the XOR of Data and Strobe will yield a clock signal. Also, it specifies the simplest means of generating the Strobe signal for a given Data stream.

Data strobe encoding originated in IEEE 1355

IEEE 1355

IEEE Standard 1355-1995, IEC 14575, or ISO 14575 is a data communications standard for Heterogeneous Interconnect . It is a low-cost, low latency, scalable serial interconnection system, originally intended for communication between large numbers of inexpensive computers. It lacks many of the...

 Standard and is used on the signal lines in SpaceWire

SpaceWire

SpaceWire is a spacecraft communication network based in part on the IEEE 1355 standard of communications. It is coordinated by the European Space Agency in collaboration with international space agencies including NASA, JAXA and RKA...

 and the IEEE 1394 (also known as FireWire 400) system.

Gray code

Gray code

The reflected binary code, also known as Gray code after Frank Gray, is a binary numeral system where two successive values differ in only one bit. It is a non-weighted code....

is another code that always changes one logical value, but never more than one.