22

What are the primary differences between 74LS series and 74HC series ICs? I know that LS series are TTL and HC are CMOS, but beyond that what are the key differences?

For example, Ben Eater uses LS series chips in his 8-bit computer. Why would he do this as opposed to using HC series chips? What is most ideal to implement his design and why?

Ben's 8-bit computer: https://eater.net/8bit/

rjm27trekkie
  • 440
  • 1
  • 5
  • 13
  • 6
    Possible duplicate of [CMOS vs TTL Logic Gates?](https://electronics.stackexchange.com/questions/51299/cmos-vs-ttl-logic-gates) – koalag Feb 27 '18 at 02:41
  • 1
    Much of the differences are covered in the other answer, but I'm also concerned with specific applications that are best for each series of device. Should I edit to focus on this aspect? – rjm27trekkie Feb 27 '18 at 02:45
  • 1
    The slower logic families may permit less use of bypass caps strapped between pins 7/14 or 8/16 of the TTL ICs. And less need for a grid-of-wires to provide low-inductance return paths. – analogsystemsrf Feb 27 '18 at 03:01
  • If I were trying to replicate Ben's circuitry, I might try to use LVC (if I'm able to get all the functions I'd need.) It seems pretty nice, though I'm not sure it is well-populated. I have not yet tried it, though. – jonk Feb 27 '18 at 03:06
  • I think that would be too broad to answer. There are so many parameters that will influence your decision(an audio guy will look for low noise mostly, while someone designing a mobile device will look for low power consumption, while someone else again might be in need of a very small propagation delay...the list goes on); maybe you could link the example you mention or describe what you are trying to achieve. – koalag Feb 27 '18 at 03:06
  • If you are going to reproduce Ben Eater's design, _please_ put some bypass capacitors across the chips! HCT is all I use now in retro computer projects (I threw away my vast collection of LS TTL chips many years ago...) – Bruce Abbott Feb 27 '18 at 03:18
  • 9
    Unused inputs of LS chips float to logic high. Unused inputs of HC chips float anywhere (high, low, and in-between). No proper LS design lets inputs float, but some do. *Never let* HC inputs float, ever. – glen_geek Feb 27 '18 at 14:35
  • 2
    We built a full CPU out of those 74 chips as a hobby. The most important takeaway concerning LS/HC is that HC can work at almost any voltage, while some LS chip start introducing errors when voltage is drops to 4v. When project spans to ~10 breadboards, it is hard to track the exact voltage on each. and analog problems is something to avoid, so we used only HC. – Ron May 01 '22 at 22:50

3 Answers3

32

There are 5 primary differences between the 74LS and the 74HC series. Speed is not one of them, as their upper limit is about 20 MHZ in terms of clock speed. They have the same pin outs for both DIP and SOIC packages, so that is not an issue. They are both considered 5 volt devices.

Many issues do stand out.

  • Logic threshold voltage.

  • Power consumption.

  • Input impedance.

  • Output drive current.

  • Output drive voltage.

INPUT VOLTAGE

The input voltage for the LS and HC series is close, but the HCT series mimics the LS series in terms of a '1' input and a '0' input. 74HC needs more voltage to be considered a logic '1'. Both types have a no-mans land of about 2.5 VDC where it is in a linear mode and may oscillate.

INPUT IMPEDANCE

The 74HC series has a very much higher input impedance than the 74LS, which actually 'leaks' a small positive current out of an input pin. It is in the uA range so normally is not an issue.

OUTPUT DRIVE CURRENT

The 74HC series has an output drive current of +/- 25 mA, so with a resistor it can drive LED's directly. A 330 ohm to 1 K resistor will cover most LED's except the "high-powered' types. The 74LS can sink a couple of mA and source about 1 mA at most.

OUTPUT DRIVE VOLTAGE

The 74HC outputs swing rail to rail, even with a 15 mA load to either rail. The 74LS series can sink an output close to ground voltage, but can only source Vcc-1.2 volts due to its bjt outputs. It is very common to see pullup resistors on both the 74HC and 74LS data and address lines to prevent floating if nothing is driving the bus, but the 74LS series used them to help make logic '1's closer to the Vcc supply rail.

CURRENT CONSUMPTION

The 74HC/74HCT series also consumes MUCH less current, just a few uA in a static mode. The 74LS consumes 1,000 times as much idle current.

OPTIONS

There is no reason to use the 74LS series except for legacy reasons. Even so, replacing some with a 74HC/74HCT series will reduce power consumption a great deal. The 74HCT is a better match to existing 74LS logic in terms of input threshold. For faster logic the 74AC series is good to 120 MHZ, the limit of the 5 volt 74ACxx, 74HCxx, 74HCTxx logic series. The 74HC/74HCT/74AC series is still in production by several manufactures, while the 74LS is going the way of the Edsel. You may find them on Ebay. There are 5 volt ECL/PECL logic IC's but that is not what this post is about.

LINKS

The data sheets are too long to include all of them here, so I picked the 74xx74 flip-flop as a common example. There is no guarantee these links will last for decades.

http://www.ti.com/lit/ds/symlink/sn74ls74a.pdf

http://www.ti.com/lit/ds/symlink/sn74hc74.pdf

http://www.ti.com/lit/ds/symlink/sn74hct74.pdf

http://www.ti.com/lit/ds/symlink/sn54ac74.pdf

  • You are right about the speed. I had been comparing LS with AHC in my mind for some reason. I think I'll remove my answer, rather than try and fix it. Thanks!! – jonk Feb 27 '18 at 03:05
  • My hobbies were limited to about 9-10 MHz. (I can recall doing an 8.5 MHz wire-wrapped board, for example.) I also worked on PCI at 33 MHz and 66 MHz. Clock skew relative to data had to be 2ns for 33 MHz and 1ns for 66 MHz. But that's a reflection mode bus and it requires very expensive equipment to test and verify. And it was at Intel as part of a team. At home, I wouldn't even consider the idea of a system-wide clock at 100 MHz. I have no experience at all with LVC, but it looks interesting. If I get time and something I care about doing, might try and see. – jonk Feb 27 '18 at 03:18
  • Oh. You might add something about HCT, since you mentioned "replacing some" to "reduce power consumption" and this implies to me the idea of keeping some of the LS. In that case, the "T" input thresholds might be nice. – jonk Feb 27 '18 at 03:21
  • @jonk. In 1988 I built a counter/timer with all 74AC logic and a 74F190 pre-scaler so I had a resolution of 10 nS with a 100 MHZ xtal. –  Feb 27 '18 at 03:21
  • 3
    74AC is very fast and high drive, 1ns rise times, not suitable for breadboarding/prototyping, needs good layout otherwise signals will overshoot and ring, and the thing will turn into a wideband radio jammer! 74HC/HCT is a lot more civilized. – bobflux Feb 27 '18 at 20:00
  • I only listed 74ACxx for comparison. I have built with 74ACxx and know it cannot be bread-boarded. It needs a ground plane. –  Feb 27 '18 at 20:35
  • Driving 74HC from 74LS is straightforward - use pullup resistors. Of course, then you start losing the power advantage you get with HC, but that's a tradeoff you can evaluate for yourself. – WhatRoughBeast Jan 14 '19 at 03:43
  • I would also mention the 74AHC series, which has most of the speed of 74AC but with the drive strength more compatible with 74HC-based designs. Another advantage of CMOS gates in general (but not the ones with TTL compatible inputs) is that they will work on lower supply voltages than 5V, usually down to 1.8V, though a bit more slowly. – Chromatix Jun 27 '19 at 18:43
  • @Chromatix I *could* have ranted on about the 74Axx series but the OP's question was to compare 74LSxx to 74HCxx. I stepped out-of-bounds by even mentioning the 74ACxx series, so it is best that I say no more about it. –  Jun 27 '19 at 19:44
  • Thinking about it, I would be not surprised if the fabs doing 74LS/ALS tends to be older, as 74HC is CMOS. – Yuhong Bao Jun 23 '22 at 07:13
10

Ben uses 74LS series mainly because their TTL output driver include serial resisters which make it much easier to light up the LEDs. You can use 74HC or even 74LVC to replace them, but, since the CMOS type use Push-Pull output driver, then you must include serial resisters to limit LED current.

Jony
  • 216
  • 2
  • 4
-1

Another thing to add is that TTL chips will default open inputs to HIGH, whereas CMOS will oscillate or exhibit unpredictable behavior.

Anonymous
  • 1
  • More like, open inputs in TTL mostly tend to float high by default, but saying that they will is awfully optimistic. They have been seen to exhibit unpredictable behaviour too. – Justme Jul 21 '23 at 21:54