Mixing 4000-series CMOS and 74HC in a 5V system - any issues?

No problem in the fast-to-slow direction. In the 4000-to-HC direction, you might check that the HC input slew rate isn't too slow; schmitts might be prudent.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Using hard logic is your desire? Your passion is to wire up a circuit and to strap down the chips? Very kinky!

One day you may find a need for the softer side of things. When that happens, don't be afraid to step out of your cable closet and explore a new path.

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Rick C

Good point. Luckily this little project looks like it's all 74HC driving 4000 or 4000 driving 4000, at least at this stage of the design. But I'll keep that in mind in case I have to drive 74HC from

4000. Schmitts are cheap enough and I normally use them anywhere I need an inverter, just because.

Whether you need to schmitt things depends on the circuit. Each logic gate has a fair bit of gain so speeds up rise & fall times. Whether that's good enough just depends. A bank vault system that calls the flying squad if an edge goes high for 0.5uS would not want the results of a sluggish input to a gate causing a brief output error. But if your circuit controls bulb brig htness, who cares if it gets 0.5uS of full brightness. If a 4000 output goe s through 2 spare HC inverters before it hits the nand gate it'll be fast b y then.

NT

AFAIK pretty much anything that's currently available in 4000-series is available in 74HC4000-series. What's not there?

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
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I'm not sure why people are making such a big deal of this issue. If logic is designed properly the speed of an edge has no impact on the logical result other than on clock lines. For purely combinatorial logic changing edges can always generate race conditions through multiple paths, so you should expect anomalous results until the input change has settled down. Designing logic to be race free is a special task which takes a lot more design effort than standard logic design, so it is seldom used.

To the OP, are you counting on combinatorial logic outputs to be race free, that is for one input edge to result in a single output edge? If not, don't sweat the slower inputs driving faster logic.

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Rick C

You cannot SEE the MOSS...

CD4026 and CD4033 don't seem to have direct 74HC equivalents, and CD4033 is perfect for what I want to do. I haven't searched for a similar 74HC function since CD4033 is still readily available.

Another poster asked about race conditions. That's not an issue, it's a synchronous design, and slow.

There are a whole lot that are not, but they're not not usually what we would strongly consider for a production situation because they're probably on the way out.

Eg. 4054,55, 56, 4062, 4068, 4089, etc. etc.

--sp

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Best regards,  
Spehro Pefhany

Mostly that's because if you are using three of any of these devices you can do better with programmable logic or software. When designing something for production it's not very often programmable logic or software are just tossed aside without consideration. This is obviously not a serious commercial design.

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Rick C

If it's a one-time project and you're running off 5 volts and don't have a super-stringent power supply budget you could just use a 74LS47 and

74HC160 synchronous BCD decade counter to drive it. At 5 volts HC outputs will drive LS inputs no problem.

If the digits in whatever display (if that's what you're using it for) are on all the time the static LS power consumption will be small fraction of the total.

First, you can get almost any function you want in the 74HC family. There are a FEW functions that have not been brought over. Excepting those, the only reason to use 4000-series is when running them on 12 V or so.

Jon

How low will these two logic families go? I don't think I've ever tested them. I know what the spec sheet says, but I seem to recall hearing that 4000 series logic will work at very low power supply voltages, so much so that it can be hard to get memory to forget the contents when you want it to.

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Rick C

Yep, with HC ~2 volts is an absolute minimum, with 4000B series 3 volts is just a "recommendation", the absolute minimum is actually listed as

-0.5V (?!)

I've run a CD4093B off 1 volt fine before...

At 5V supply 4000-series logic has the zippiness of molasses. In winter.

Mostly because they have been bitten by it.

It can become a problem in pulse counters. If the transition is too slow you might get 3-5 counts even though there was only one pulse.

In general that isn't a good thing. Aside from counters and such I have also seen EMC issues creeping up. The innards of a fat fast chip briefly sang at hundreds of MHz during each transition, that got conducted out, radiated and then there was some egg in the faces at the EMC test. Since quasi-peak rules became law at higher frequencies they won't let us mash that into the carpet anymore by averaging.

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Regards, Joerg 

http://www.analogconsultants.com/

Did you read where I excepted clock lines? If you are trying to count pulses on signals with slow edges it's not hard to design that properly.

Yes, you've mentioned before the poorly designed chip. Anyone can create crap that you need to deal with.

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Rick C

Do you really not understand what that means? With both HC and 4000 the absolute minimum means damage can occur and is -0.5 volts for both. HC has a "specified" operation from 2.0 to 6.0 or whatever limits the maker provides. That means the data is guaranteed over that range of Vcc. For 4000 series parts I find separate specs for operation at 5 10 and 15 volts Vdd.

That's pretty good. I wonder how low 74HC will run.

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Rick C

I'm not intimately familiar with the operating parameters of an antiquated glue-logic series that probably came out nearly two decades before I was born, no.

All I can go by is what the datasheets say, and they often don't explain things too good. For example this whitepaper on the HCMOS family from Phillips doesn't say anything about -0.5 volts anywhere as far as I can tell:

Nevermind, it does down in the specifics.

Whatever old datasheet for the 4000 series I was looking at seemed to give the impression that -0.5 was the minimum "recommended" operating voltage. Which wouldn't make sense as I don't see how it could function with the rails inverted. It's confusing to put that figure along with "typ" and "max" in a table, I think it should just be put in the "Absolute Maximum Ratings" section.