When I started working with electronic circuits, and IC’s in particular, a few years ago, I scarcely knew what TTL stood for. Now the proliferation and expansion of various logic families has provided an even more bewildering array of IC’s from which to choose. This article is a collection of data from various data books and specification sheets and is intended to provide a useful source of information when you have to choose what kind of IC to use for your projects and what, if any, interface is required between the various families. Some of the earlier and some of the less common varieties have been omitted.
FAMILIES
TTL (7400 series) regular transistor-transistor logic LSTTL (74LS series) low power schottky (fast switching) TTL ALSTTL (74ALS series) advanced LSTTL (faster and more economical in power) CMOS (4000 series) complementary metal oxide semiconductor (slow) HCMOS (74HC series) high speed CMOS (as fast as LSTTL) HCTMOS (74HCT series) high speed TTL compatible CMOS
Principal characteristics of the logic families are listed below:
TTL CMOS
7400 74ALS 74LS 74HCT 74HC* 4000*
Supply voltage 5V 5V 5V 5V 2-6V 3-18V
Power dissipation/gate 10mW 1mW 2mW 2.5nW 2.5nW 2.5nW
Speed (delay/gate) 10ns 4ns 9ns 8ns 8ns 40ns
Logic 1 min high input 2.0V 2.0V 2.0V 2.0V 3.15V 3.5V
Logic 0 max low input 0.8V 0.8V 0.8V 0.8V 0.9V 1.5V
Logic 1 min high output 2.4V 2.7V 2.7V 4.0V 4.0V 4.5V
Logic 0 max low output 0.5V 0.4V 0.5V 0.26V 0.26V 0.4V
* assuming 5V operation
Notice that the newer 74HCT devices are direct replacements for the 74LS — with better output voltage characteristics (wider noise margins) and much lower power requirements.
FANOUT
The ability of one device to drive another can be calculated by dividing the maximum output current of the driver by the required input current of the device being driven. The fanout at both logic levels should be calculated and the lower of the two taken as the limit. The results can be summarized as follows:
TO THIS DEVICE
TTL CMOS
7400 74ALS 74LS 74HCT 74HC* 4000*
7400 10 20 20 * ** **
74ALS 5 20 20 * ** **
FROM THIS 74LS 2 20 10 * ** **
DEVICE 74HCT 2 20 10 * * *
74HC 2 20 10 * * *
4000 0*** 2 1 * * *
* Unlimited but delays due to input and lead capacitance should be considered.
** Logic levels are incompatible but 74HC levels can be made compatible if the supply voltage to the 74HC is reduced to 3V. Pull-up resistors (1 to 10K) can raise TTL outputs to required CMOS levels (small value for speed but a larger value for less power required).
*** Buffered CMOS outputs can sink up to 1.6mA and can drive one or two TTL devices.
Different supply voltages can be accommodated. A CMOS buffer (eg. 4049/50 etc.) can interface 15V CMOS to 5V TTL. A 7406/7 (30V max) or a 7416/7 (15V max) can raise TTL to CMOS voltage levels. Discrete transistors can also be used to interface the different logic families.
If you can find them, and afford them (actually they’re quite cheap), start using the 74HCT devices in place of 74LS.