Logic

 

Here, we outline eight differential logic standards and their properties.

1. ECL (Emitter-Coupled Logic)

This is the oldest differential logic standard, dating from the times when all logic was done with BJT. As a result, it is sometimes incorrectly used to refer to any differential logic. ECL originally works off a negative 5.2V supply; this would now be referred to as NECL (Negative Emitter-Coupled Logic). ECL is no longer common.  Outputs have a sharper rise time than fall time. It has high fanout ability and uses significant power. The sharp unbalanced rise time causes noise, making ECL designs more prone to EMC radiation.

2. PECL (Positive Emitter-Coupled Logic or Pseudo Emitter-Coupled Logic)

The Pseudo name is more appropriate now as most drivers use FET and not BJT. PECL runs off +5.2V. Like ECL, PECL is no longer common. Other PECL properties are similar to ECL: PECL outputs have a sharper rise time than fall time, PECL has high fanout ability and uses significant power, the sharp unbalanced rise time results in noise causing PECL designs to suffer EMC radiation.

3. SCL (Source-Coupled Logic)

This the FET equivalent of ECL. The term “Source” refers to a FETS driver.  Outputs have a sharper rise time than fall time. For PCBs, SCL is not a term commonly used. However, many LVPECL compatible drivers are in fact implemented via SCL.

4. CML  (Current Mode Logic) 

This uses pull up termination resistors and a constant current sink driver around 16mA. Steering current between the two lines is quiet on supply noise but consumes significance power. CML is great for reducing EMC.  Table 1 outlines the typical properties of CML.

Table 1: Typical Properties of CML

Typical Property CML  (Current Mode Logic)
Supply Voltage (Vcc) 3.3V
Common mode voltage Vcc – 0.4V
Differential Voltage 0.8V
Rise time 300ps
Load / termination. 50 ohm to VCC

 

5. LVDS (Low Voltage Differential Logic)

This is one of the quietest high speed logic families, having the lowest drive voltage swing and current.  The constant common mode driver voltage means moving between different supply rails is simplified. The single load terminating resistor reduces PCB components. Many drivers will have a match drive impedance, resulting in transmission lines being terminated at both ends. The low drive levels limits tolerance of capacitance and losses in long lines.  LVDS is common for frequencies up to 700MHz.  Table 2 demonstrates the typical properties of LVDS.

Table 2: Typical Properties of LVDS

Typical Property LVDS (Low Voltage Differential Logic)
Supply Voltage (Vcc) 3.3V, 2.5V, 1.8V
Common Mode Voltage 1.2V
Differential Voltage 0.35V
Rise time/ fall time 300ps
Load / termination 100 ohm differentially

 

6. LVPECL (Low Voltage PECL)

This is designed for 3.3V. LVPECL outputs have a sharper rise time than fall time. It has high fanout ability and uses significant power. The sharp unbalanced rise and fall time procedures cause noise. LVPECL is prone to EMC radiation. The termination to VCC -2 causes some difficulty. Most designs use a Thevenin equivalent load, doubling the load resistors required. For a design using a lot of LVPECL, an extra supply rail can be used; however, it must be a current sink not source, hence a positive LDO will not work. LVPECL is common between 700MHz and 1.5GHz.  Refer to Table 3 for typical properties of LVPECL.

Table 3: Typical Properties of LVPECL

Typical Property LVPECL (Low Voltage PECL)
Supply Voltage ( Vcc) 3.3V
Common Mode Voltage Vcc – 1.1V
Differential Voltage 1.6V
Rise time/fall time 160ps / 300ps
Load /termination. 50 ohm to VCC – 2V

 

7. LVECL (Low Voltage Emitter-Coupled Logic)

This is no longer used. LVPECL is used instead as negative supplies offer no advantages for logic.

8. HSTL (High-Speed Transceiver Logic)

HSTL can be single or differential. HSTL is an attempt to provide a single logic standard to interface between different supply devices. HSTL has not gained common acceptance. Table 4 shows typical properties of HSTL.

 

Typical Property HSTL (High-Speed Transceiver Logic)
Supply Voltage ( Vcc) 3.3V,2,5V,1.8V,1.5V
Common Mode Voltage 0.7
Differential Voltage 1.3V
Rise time/fall time 400ps
Load/termination 50 ohm to gnd