Glue Logic Family Selection

Which Logic family do I need,
Why would I chose one logic family over another




What are the different glue logic families

There are four basic types of logic chip families: Bi-Polar, CMOS, BICMOS, and ECL.
Bi-Polar devices are the normal TTL series of devices. These consist of the 74xx, 74S, 74LS, 74AS, 74ALS, 74F.... type devices.
CMOS devices which consist of the 4000 series of devices, and all the parts that are identified by the 'C' in the part number.
ECL devices consist of the 10K, 100K, or 10E [EcLinPS] series of devices.
BiCMOS devices consist of the ABT, BCT and others.

The graphic below lists most Glue Logic family types. Another view of the Glue Logic family list which may be more up-to-date


Different IC Families




All of the Glue Logic parts ever designed [shown above] are still being produced. However a great many types are not recommended for new designs. Note the color code legend in the right hand corner.

Bi-Polar devices use transistors as the inputs and outputs for their devices. The output transistors are operated in the saturated region and draw a large amount of current.
ECL devices also use transistors, but are operate in the linear region.
CMOS devices use MOSFETs
BiCMOS devices use bipolar and MOSFET circuits


Which glue logic family are in full production

All types of Glue Logic families are still in mass production, but a number of families may soon be obsolete. Below is a copy of a four year old graphic from a Texas Instruments; TI CD. Based on the graphic the 4000 series CMOS family, and many of the TTL [only] families are moving to obsolescence.

IC in Production

Given the list of possible glue logic families; the first step in selecting a logic family to use is based on weather or not you are producing a new design or working on a previous design. If you have a previous design and don't want to put additional engineering effort into the product then the selection of the same logic family used is called for. A new design would want to use devices which could be procured for years to come.


I have a new design, should I use one of the TTL glue logic families

Not really. There are very few reasons to use one of the TTL glue logic families. The TTL logic families are slower and consume large amounts of power [10x more than CMOS]. How ever, there may be that one case that you don't care about power consumption and can't deal with the higher switching speeds of the newer CMOS families. In most cases the same part number is used for either type.




Which glue logic family consumes the most power

Glue logic families will normally be selected by the speed at which they switch [operate at], or by the power they consume. How ever, in some cases, by selecting a device based on speed there is a price to pay, as increased current consumption. See the graph below to determine the speed vs power for the different logic families. For a better graphic see Logic Speed x Power Chart {This Web Site}. Low power consumption is the best reason to select a CMOS logic family over selecting a TTL logic family. While not switching (changing state) or in their Quiescent state CMOS devices have basically zero power consumption. TTL device on the other hand TTL logic families consume power regardless of what they do.

IC Speed vs Power

What are the Glue Logic Switching Levels

The voltage level a device [by family] switches at determines how well the logic device will handle any noise in the system. CMOS devices will output near 5 volts, while TTL devices will only output 2.4 volts given a 5 volt power supply. This is one of the best reasons why a CMOS logic family is better than a TTL logic family. Of course the best reason to use CMOS is the near zero power consumption.

IC Switching Levels

What are the Low Voltage Glue Logic Switching Levels

Low Voltage [LV] families switch at the same voltage levels as the normal TTL glue logic families

Detailed Digital Low Voltage Logic Threshold Level per Logic Family

What are the details of the different logic families

74AC: A high speed CMOS logic family with CMOS input switching levels and buffered CMOS outputs that can drive +/- 24mA of IOH and IOL.

74ACT: A high speed CMOS logic family with the same buffered CMOS outputs that can drive +/- 24mA of IOH and IOL. This family has a TTL-to-CMOS input buffer stage. The inputs will interface with TTL outputs operating at 5 volts with VOH = 2.4 volts and VOL = 0.4 volts. The devices have the same output buffered structures as the AC family.

74LCX: These devices have a mixed 3volt-to-5 volt capability for use with applications that have both 3 volt and a 5 volt devices which interface with one another.

74LVX: This family consists of low cost devices with 5 volt tolerant inputs. The devices can receive and output 3 volts or 5 volts.

74LVQ: This family consists of low cost devices designed for 3.3 volt only applications.

74LVT: This family has both high speed and a high output drive. These devices have a +64mA / -32mA output drive currents. The chips are 5 volt tolerant and are designed to be used with applications that have both 3v and 5v devices which interface with one another.

74ALCX: This devices are about the same as the LVT family with out the high drive currents.

74xx: The first TTL family developed. The 74xx family offers a wide variety of logic functions. There are a number of other TTL logic families which offer either higher speed or lower power.

74Lxx: This is the Low power version of the TTL family above. The value of the internal device resistors have been increased by a factor of 10x. The family offers 1/10 the power consumption as the previous family, but operates at 1/3 the speed.

74LSxx: devices adds a Schottky diode between the Base and Collector of the transistor. The Schottky diode prevents the transistor from going into full saturation. So the 74LS family operates at the same speed as the 74 family [10nS] with only 2mW of power dissipation compared to 10mW for 74xx or 1mW for the 74L family [at 33nS].

74Sxx: gains its speed using the same Schottky diode as the 74LS family, but the value of the internal device resistors have been decreased by half the original values of the 74xx family. So the speed increases [3nS] and the power consumption also increases [20mW].

74ALSxx: The advanced LS family offers near same high speed as the 74Sxx family at 4nS at a power dissipation of only 1mW. Except for the 74Sxx family, the 74ALS family outperforms the other three TTL families listed. If the design calls for a TTL family which needs to operate at 3nS, this is the family to use.

74ASxx: If the design needs to operate faster than 3nS then the 74AS family should be use. This family is twice as fast as the 74ALS family with a propagation delay of only 1.5nS. The price is a 7mW power dissipation.


What is the pricing for Glue Logic Family devices

The table shows the cost of a standard NAND gate for several different logic families. The prices were obtained on 3/23/03 for one piece qty.


Glue Logic Family Pricing
Function 7400 74LS00 74F00 74AS00 74C00 74HC00 74AHC00 74AHCT00 74AC00 74ACTQ00 74VHC00
NAND Gate 0.88 0.48 0.47 0.80 1.49 0.56 0.52 0.52 0.42 0.92 0.50
NOR Gate 0.96 0.53 -- 0.92 -- 0.42 0.52 0.52 0.56 -- --



Additional considerations

Noise Margin Calculation
Noise Margin Output high = VOH [driving device] - VIH [receiving device]
Noise Margin Output low = VIL [receiving device] - VOL [driving device]
The higher the numbers the better, with negative numbers indicating interoperability . Use Minimum numbers for output High, and maximum numbers for Output Low.


Noise Margin
-- VOH VIH Voltage Margin VIL VOL Voltage Margin
TTL [5volt] 2.4v 2.0v 400mV 0.8v 0.5v 300mV
FCT [5volt] 2.5v 2.0v 500mV 0.8v 0.5v 300mV
BTL [5volt] 2.1v 1.62v 480mV 1.47v 1.1v 370mV
GTL [5volt] 1.5v 1.05v 450mV 0.95v 0.55v 400mV
CMOS [5volt] 4.9v 3.85v 1050mV 1.35v 0.1 1340mV
LVTTL [3volt] 2.4v 2.0v 400mV 0.8v 0.4v 400mV
LVCMOS [3volt] 2.8v 2.0v 800mV 0.8v 0.2v 600mV
CMOS [2.5v] 2.0v 1.7v 300mV 0.7v 0.4v 300mV
CMOS [1.8v] 1.35v 1.1v 250mV 0.66v 0.45v 210mV

FanOut

The number of gates a device can drive is determined by the current it can source and sink. The lower of the two numbers indicates the possible fanout.
With the output high => IOH [driving device] / IIH [receiving device] = number of possible gates driven
With the output low => IOL [driving device] / IIL [receiving device] = number of possible gates driven.
To increase fan-out some families allow the devices output [in the same package] to be paralleled, or connected together. FanOut Definition

Slew Rate




Device Slew Rate
Device F ALS ABT AC HC AHC AHC LVT ALVC LVC LV
Propagation
Delay {nS}
4 6 2.7 5 13 5.5 8.3 2.4 2.0 4.5 10
Voltage
Swing {V}
3 3 3 4.8 4.8 4.8 3 3 3 3 3
Slew
Rate {V/nS}
1.3 1.0 1.0 2.0 0.9 0.8 0.5 1.2 1.3 0.9 0.7
-
VCC = 5.0v
VCC = 3.3v

See the Device Slew Rate page for additional information and a better description

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Modified 2/26/12
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