Current mode logic (CML), or source-coupled logic (SCL), is a digital design style used both for
logic gates and for
board
Board or Boards may refer to:
Flat surface
* Lumber, or other rigid material, milled or sawn flat
** Plank (wood)
** Cutting board
** Sounding board, of a musical instrument
* Cardboard (paper product)
* Paperboard
* Fiberboard
** Hardboard, a ty ...
-level digital signaling of
digital data
Digital data, in information theory and information systems, is information represented as a string of discrete symbols each of which can take on one of only a finite number of values from some alphabet, such as letters or digits. An example i ...
.
The basic principle of CML is that
current from a
constant current generator is steered between two alternate paths depending on whether a logic zero or logic one is being represented. Typically, the generator is connected to the two sources of a pair of differential
FETs, with the two paths being their two drains.
Bipolar
Bipolar may refer to:
Astronomy
* Bipolar nebula, a distinctive nebular formation
* Bipolar outflow, two continuous flows of gas from the poles of a star
Mathematics
* Bipolar coordinates, a two-dimensional orthogonal coordinate system
* Bipolar ...
equivalents operate similarly, with the output being taken from the collectors of the BJT transistors.
As a
differential PCB
PCB may refer to:
Science and technology
* Polychlorinated biphenyl, an organic chlorine compound, now recognized as an environmental toxin and classified as a persistent organic pollutant
* Printed circuit board, a board used in electronics
* ...
-level interconnect, it is intended to transmit data at speeds between 312.5
Mbit/s and 3.125 Gbit/s across standard
printed circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
s.

The transmission is point-to-point, unidirectional, and is usually
terminated at the destination with 50
Ω resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
s to
Vcc on both differential lines. CML is frequently used in interfaces to fiber optic components. The principle difference between CML and
ECL as a link technology is the
output impedance The output impedance of an electrical network is the measure of the opposition to current flow (impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal'' to the electrical source. The ...
of the driver stage: the
emitter follower of ECL has a low resistance of around 5 Ω whereas CML connects to the drains of the driving transistors, that have a high impedance, and so the
impedance of the
pull up/down network (typically 50 Ω resistive) is the effective output impedance.
Matching this drive impedance close to the driven
transmission line's
characteristic impedance
The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in ...
greatly reduces undesirable ringing.
CML signals have also been found useful for connections between modules. CML is the
physical layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip.
The ...
used in
DVI
Digital Visual Interface (DVI) is a video display interface developed by the Digital Display Working Group (DDWG). The digital interface is used to connect a video source, such as a video display controller, to a display device, such as a comp ...
,
HDMI and
FPD-Link III video links, the interfaces between a
display controller and a
monitor.
In addition, CML has been widely used in high-speed integrated systems, such as for
serial data
Serial may refer to:
Arts, entertainment, and media The presentation of works in sequential segments
* Serial (literature), serialised literature in print
* Serial (publishing), periodical publications and newspapers
* Serial (radio and televisi ...
transceivers and
frequency synthesizers in
telecommunication systems.
Operation
The fast operation of CML circuits is mainly due to their lower output voltage swing compared to the static
CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
circuits, as well as the very fast current switching taking place at the input differential pair transistors. One of the primary requirements of a current-mode logic circuit is that the current bias transistor must remain in the saturation region to maintain a constant current.
Ultra low power
Recently, CML has been used in ultra-low power applications. Studies show that while the leakage current in conventional static CMOS circuits is becoming a major challenge in lowering the energy dissipation, good control of CML current consumption makes them a very good candidate for extremely low power use. Called subthreshold CML or subthreshold source coupled logic (STSCL),
[ the current consumption of each gate can be reduced down to a few tens of picoamps.
]
See also
* Low-voltage differential signaling (LVDS) A differential standard used primarily for signals between modules.
* Positive-referenced emitter-coupled logic
In electronics, emitter-coupled logic (ECL) is a high-speed integrated circuit bipolar transistor logic family. ECL uses an overdriven bipolar junction transistor (BJT) differential amplifier with single-ended input and limited emitter current to ...
, a differential signaling standard for high speed inter-module communications
References
* System Interface Level 5 (SxI-5): Common Electrical Characteristics for 2.488 – 3.125 Gbit/s Parallel Interfaces. OIF, October 2002.
* TFI-5: TDM Fabric to Framer Interface Implementation Agreement. OIF, September 16, 2003
* Introduction to LVDS, PECL, and CML, Maxim, https://pdfserv.maximintegrated.com/en/an/AN291.pdf
* http://www.ee.iitm.ac.in/~nagendra/videolectures/doku.php?id=ee685:start
* Interfacing between LVPECL, VML, cml and LVDS Levels, http://focus.ti.com/lit/an/slla120/slla120.pdf
* For more details on design automation and low power design of CML circuits, see: http://lsm.epfl.ch
External links
JESD204B - a JEDEC Standard for serial data interfacing
- Analog Devices
JESD204B Overview (slides)
- Texas Instruments
{{Logic Families
Logic families