What's the difference between chip families

Question

I've been trying to follow Ben Eater's 8 bit computer design (https://eater.net/8bit) and I've manged to build something that broadly works but it's not very reliable. Sometimes modules don't work, if I reset the system and try again they might work. It's all very frustrating.

The main point where I've deviated from the original design is the choice of chips. In the original design, Ben used 74LS___ chips, but I had trouble getting hold of these, so instead I've used some 74HC___ chips.

I'm using a mixture of LS, HC and HTC.

Can somebody tell me what is the difference between

• 74ls___
• 74hc___
• 74hct___

I'm using a 5 volt bench power supply and lots of decoupling capacitors. I can see that all the chips are all getting about 5 volts. The clock is running at fractions of hertz through to 10's of hertz, so speed should not be a problem.

Answer 1

74LS is Low-power Schottky - the output High level may be as low as 2.4 volts whne Vcc is 5 volts

74HC is High-speed CMOS - its inputs expect a High level to be above 2.5 volts when Vcc is 5 volts

74HCT is TTL=compatible High-speed CMOS. Its inputs have a lower threshold for a High, to be compatible with 74LS outputs.

This information can be found in the datasheets for the various families.

Answer 2

In comments you said,

I'm using a mixture of LS, HC and HTC.

As other answers have pointed out, the main difference is that LS is a TTL family and HC is a CMOS family. TTL and CMOS have different ranges of voltages that are considered valid logical 1's or 0's. So you have to be careful when mixing them in a design.

The main thing you want to watch out for is you should not connect LS outputs to HC inputs. This is because TTL is quite weak when driving a "1", while CMOS requires a fairly strong "1" to work reliably.

This issue is exactly why HCT parts exist --- they are CMOS chips with TTL-compatible inputs, so having an LS output drive an HCT input is perfectly okay.

Having a HC or HCT output drive an LS input is also not ideal, but will usually work acceptably. If you have very large fan-outs (one output driving more than 4 or 5 input pins) you should look more carefully at whether that output can deliver enough current to drive all the inputs you have connected.

If you're seeing unreliable operation, most likely you have a TTL output driving a CMOS input somewhere.

One other thing to watch out for:

Allowing a TTL input to float is usually okay (it will be equivalent to a high input), but you should never leave a CMOS input floating, even if you aren't using the output of the gate associated with that input.

Answer 3

There are a number of important differences. One key item is that the CMOS devices (74HC and 74HCT which are high speed CMOS and high speed CMOS with TTL compatible levels) do not like slow rise and fall times.

Here is a snippet from the 74HC(T)173 quad D type flip flop:

You will notice there is a maximum transition rate.

You can read more on this subject at this TI application note. Here it is not the clock rate, but the input rise / fall rate that matters.

With a slow rising or falling input, the internal CMOS structure is in the active region for quite a while and will consume significant (for CMOS) class A current.

The fundamental internal architecture is also very different so even though functionally a 74HC74 and 74LS74 are the same (but with significant timing differences), internally they are very different indeed.

Another issue you may have is that due to the different propagation delays through the gates, you may be encountering logic race conditions that do not exist when using 74LS parts.

For illustration, here is the propagation delay for the 74HC173 (notice that clock to output high and clock to output low is symmetrical):

Now here is the same specification for the 74LS173:

You can see that not only are the propagation delays different between the two parts, for the LS device, the delay is different for high to low and low to high.

Any of these differences could easily cause problems.

Answer 4

Welcome, the difference between these component names are

1. 74ls = Low-power Schottky
2. 74hc = High-speed CMOS
3. 74hct = Best* of both worlds (*sort of), this part is a combination of characteristics of the LS & HC versions.

I found this out here at electronics club worth having a read through this.

You would expect to see very similar performance out of all the variants during general use. You should only notice a difference when comparing the power consumption or the high-speed performance.