Logic Families

Overview

Logic families are groups of basic digital logic electronic circuits (and, or, nor gates e.t.c), where each group consists of a different semiconductor technology from which the logic gates are built from. Within a family the voltage levels and signalling should be compatible between the various inputs and outputs.

Logic families are sorted in alphabetical order. Take note that there may be a low-voltage equivalent of many signal types under the prefix “LV” (e.g. TTL and LVTTL).

CML

See PECL.

CMOS

Overview

CMOS (complementary metal-oxide semiconductor) is the most widespread logic family in use today. CMOS logic is built from P-channel (PMOS) and N-channel (NMOS) MOSFETs.

Latch-up

CMOS logic suffers from a phenomenon known as latch-up. This is when a short occurs between a power-rail and ground in an CMOS-based IC, usually causing serious operation problems if not destruction of the IC. Once latch-up is triggered, it cannot be removed until the circuit is power-cycled.

Latch-up occurs because the PN junctions that form the PMOS and NMOS switching elements form parasitic PNPN thyristors (SCRs)[^ti-latch-up-white-paper]. A latch-up has to be initially triggered by an over-voltage/current condition which causes the voltage on a pin to go at least one diode drop above the rail voltage, or one voltage drop below the ground.

Hot-plugging can cause latch-up issues.

ICs are sometimes tested against the EIA/JESD 78A IC latch-up standard and the IC is provided with a latch-up class. The tests involve:

• Applying a supply overvoltage condition to the ICs power pins
• A current injection to the ICs I/O pins

Powered Off Protection

By default, most CMOS based devices contain diodes from the I/O pins to the voltage rails and ground (this is an inherent part of the fabrication of a MOSFET and the intrinsic body diodes). Additional ESD protection diodes may be also added in parallel. Whilst normally a good thing, these diodes can cause problems, notably a problem called backfeeding. If the local power rail collapses, but signals are still present on the I/O pins (from other parts of the circuitry which are still powered), the diode from the I/O pin to \(V_{CC}\) will conduct and keep the power rail “propped up”.

Some logic gates include “powered-off protection” to prevent this, and allow external signals to be present when the power rail is at \(0V\). Texas Instruments calls this \(I_{OFF}\) protection¹². The figure below shows how the standard CMOS totem-pole driver is modified to provide “powered-off protection”.

Some examples of ICs with “powered-off protection” include the SN74LVC1G125: Single Bus Buffer Gate With 3-State Output, the SN74CB3Q3125: 3.3-V, 2:1 (SPDT), 4-channel FET bus switch and the ADG5248F: 8:1 Analog Multiplexers.

AUC

Advanced ultra-low CMOS (AUC) is a CMOS logic family. It is optimised for 1.8V operation and voltage tolerant up to 3.6V.

LVCMOS

Voltage specifications:

CSEF

See PECL.

CSL

See PECL.

DCS (Differential Current Switch)

DTLL (Differential Transistor-Transistor Logic)

DTLL is a differential signal type that is similar to standard to TTL. Because of it’s differential nature, DTLL is preferred over TTL for communications over long cables. DTLL comes under the category HVDS (high-voltage differential signalling), and is the most popular choice in this category.

LVTTL

LVTTL is the low-voltage version of TTL.

Common drive-strengths are 24mA.

Converters

TI SN65LVELT23 converts LVPECL and LVDS to LVTTL.

LVDS

Stats

Because the current is kept constant (3.5mA), it doesn’t put as much pressure on the decoupling capacitors to provide the energy during switching states. The low common-mode voltage (1.20V), allows this signalling standard to be used with a wide variety of ICs with power supplies down to 2.5V or lower.

LVDS consumes very little power compared to other differential signalling techniques. At a 2.5V supply, the power to drive a line with LVDS is 8.75mW

Converters

TI SN65LVELT23 converts LVPECL and LVDS to LVTTL.

LVPECL (Low-Voltage Emitter-Coupled Logic)

LVPECL is the low-voltage version of PECL.

Converters

TI SN65LVELT23 converts LVPECL and LVDS to LVTTL.

HSTL

HSTL compares the input voltage with a reference voltage.

PECL (Emitter-Coupled Logic)

PECL is also called CSL (current-steering logic), CML (current-mode logic) or CSEF (current-switch emitter-follower logic).

The MOSFET-based equivalent of PECL is SCFL (source-coupled logic).

SCFL (Source-Coupled Logic)

The transistor-based equivalent of SCFL is PECL (emitter-coupled logic).

TTL

TTL (transistor-transistor logic) is a very common voltage level signal used by many embedded devices today. Even though the standard “high” is 5.0V, many systems transmit logic highs at 3.3V.

Low: 0-0.8V High: 2-5.0V

Power dissipation: 10mW per gate[^ni-differences-between-cmos-ttl] Propagation delay: 10ns when driving a 15pF/400Ohm load[^ni-differences-between-cmos-ttl]

References