Logic Family Choices

With about 20 logic families now extant, some comments are in order about which to use for teaching lab excercises and projects. You are aware, I sincerely hope, that Morse Teaching Center policy is to simplify ordering and to lessen confusion among TAs and students by insisting on close adherence to a set of Recommended Parts. To be sure, such a list will never be complete, so we are willing to expand it as your needs evolve, but the process should be governed by informed choice. We don't want our lab activities to become historic restoration projects.

This first figure comes from a Texas Instruments promotional brochure but is a convenient way of showing most (not all) logic families and where they fit generationally. The original TTL family (e.g. 7400, 7493) had its heyday 20 years ago and is thoroughly obsolete. One of its prominent successors, the 74ALSxxx family, is still quite current and readily available.

But with CMOS becoming the hugely dominant force in logic integration, this is what we should expose students to. The best choice for general logic experiments and design is the 74HCxxx family. So the modern 74HC76 will be a supply-voltage (+5V) and pin-compatible dual flip-flop like the old 7476. For those interested in more details, the electrical characteristics of the major families are in Table 9.1 (pictured below) from Horowitz & Hill "The Art of Electronics," with much additional information in the rest of chapt. 9

Not all logic functions you might will be available in the modern HC and HCT families, especially when it comes to specialized interface functions like display drivers. In those cases we need to resort to ALS or even LS. But always please consider if redesign can bring your logic needs into the HC family. When a design has to mix logic families, some care may be indicated as per the interconnection guidelines in Horowitz & Hill Table 9.2. (pictured below)

Lou Berman and I are ready to work with you on clarifying any particulars of logic IC (and other component) choices, to bring them into harmony with, and constructively evolve, our " Recommended Parts" library.


TABLE 9.1. -- LOGIC FAMILIES
  tpd(’00)
(CL=50pF)
  Pdiss   Vsupply  
 
fclk(’74) (CL=0) IOL IIL Vth
Date
Family typ
(ns)
max
(ns)
max
(MHz)
@1MHz
(mW/gate)
@0.5V max
(mA)
max
(mA)
typ
(V)
min
(V)
nom
(V)
max
(V)
of intro

CMOS
   AC
   ACT
3 5.1 125 0.5 24 0 V+/2
1.4
2
4.5
5 or 3.3
5
6
5.5
1985
   HC
   HCT
9 18 30 0.5 8 0 V+/2
1.4
2
4.5
5
5
6
5.5
1982
4000B/74C@10V
@5V
30
50
60
90
5
2
1.2
0.3
1.3
0.5
0
0
V+/2 3 5-15 18 1970

TTL
   AS 2 4.5 105 8 20 0.5 1.5 4.5 5 5.5 1980
   F 3.5 5 100 5.4 20 0.6 1.6 4.75 5 5.25 1979
   ALS 4 11 34 1.3 8 0.1 1.4 4.5 5 5.5 1980
   LS 10 15 25 2 8 0.4 1.1 4.75 5 5.25 1976

TABLE 9.2. -- LOGIC FAMILIES CONNECTIONS
TO
FROM
TTL HCT
ACT
HC
AC
HC, AC
@3.3V
NMOS
LSI
4000B,
74C
@5V
4000B,
74C
@10V
TTL OK OK A OK OK A B
HCT
ACT
OK OK OK NO OK OK B
HC
AC
OK OK OK NO OK OK B
HC, AC
@3.3V
OK OK NO OK OK B B
NMOS
LSI
OK OK A OK OK A B
4000B,
74C
@5V
OKa OK OK NO OK OK B
4000B,
74C
@10V
C C C C C C OK
(a) with limited fanout.
A - pullup to +5V, or use HCT as interface.
B - use i)OC pullup to +10V, or ii)40109, 14504, or LTC1045 level translator.
C - use 74C901/2, 4049/50, 14504, or LTC1045 level translator.