Author Topic: 74HCT74 NXP vs Texas  (Read 2660 times)

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Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #25 on: December 28, 2018, 03:14:09 am »
The zeners might be usefull, i go look for those.
I think the chip wont break since the difference is about 5 volt.
+ the saw is also a slow ramp.

As long it works im happy for now, going to solder a new circuit.
 

Offline IDEngineer

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Re: 74HCT74 NXP vs Texas
« Reply #26 on: December 28, 2018, 05:30:11 am »
the saw is also a slow ramp.
I have a remarkably similar situation in one of my existing designs, and I specified the 74AHC74 specifically because it has a Schmitt Trigger on its CLK input. There is also a 74AHCT74 version if you need the TTL compatible logic levels. A quick glance at DigiKey shows this part stocked in many thousands of pieces and available from at least two manufacturers. Cost starts under $0.25/100's.
My political litmus test: I will vote for any candidate, regardless of party affiliation or positions on other issues, who promises to abolish the TSA.
 
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Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #27 on: December 29, 2018, 02:25:44 am »
Thank you Engineer, usefull information.
 

Offline jmelson

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Re: 74HCT74 NXP vs Texas
« Reply #28 on: December 29, 2018, 07:17:18 am »
If you really need to work with 12 V signals, maybe you should use a 4000 series part.  I think the equivalent is a 4013, but the pinout will not be identical to the 74xx74.
These are much slower parts, and might work without additional components.  But, if a slow transition on the clock is the problem, then a Schmitt trigger on the clock is the very best solution.

Jon
 

Offline David Hess

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Re: 74HCT74 NXP vs Texas
« Reply #29 on: December 29, 2018, 07:36:49 am »
Or use 74Cxx which is high voltage CMOS like the 4000 series but with 74xx pinouts and functions.
 

Offline IDEngineer

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Re: 74HCT74 NXP vs Texas
« Reply #30 on: December 29, 2018, 08:09:14 am »
Or use 74Cxx which is high voltage CMOS like the 4000 series but with 74xx pinouts and functions.
I had the exact same idea, but then I looked at specs and availability... the 74C74 is listed as "obsolete" from many manufacturers and the only one in stock (at DigiKey anyway) wants $7 each in low volumes. Also, the spec sheets I checked do not mention a Schmitt Trigger input on the clock, nor any other input.

There were other CMOS-based 74-series parts back in the day, so perhaps one of those is available and would work, but I'll leave such research as a worthwhile exercise for the OP. If he's got a $7 budget to solve this problem, my 25 cent recommendation and a couple of FET's would allow him to use 3.3 or 5V logic to control a 12V environment and still leave enough money for lunch.  :)
My political litmus test: I will vote for any candidate, regardless of party affiliation or positions on other issues, who promises to abolish the TSA.
 

Offline David Hess

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Re: 74HCT74 NXP vs Texas
« Reply #31 on: December 29, 2018, 10:01:38 am »
Or use 74Cxx which is high voltage CMOS like the 4000 series but with 74xx pinouts and functions.
I had the exact same idea, but then I looked at specs and availability... the 74C74 is listed as "obsolete" from many manufacturers and the only one in stock (at DigiKey anyway) wants $7 each in low volumes. Also, the spec sheets I checked do not mention a Schmitt Trigger input on the clock, nor any other input.

Clock inputs often use Schmitt trigger operation to avoid the exact problem you had but like I said, this is not always reflected accurately in the datasheet and it varies between manufacturers for the same part.

Quote
There were other CMOS-based 74-series parts back in the day, so perhaps one of those is available and would work, but I'll leave such research as a worthwhile exercise for the OP. If he's got a $7 budget to solve this problem, my 25 cent recommendation and a couple of FET's would allow him to use 3.3 or 5V logic to control a 12V environment and still leave enough money for lunch.  :)

Yea, availability and pricing will be a problem for such an old part and technology.  I do not know offhand what is still considered current.

It is too bad because high voltage 4000 and 74C series CMOS are very useful in mixed-signal applications which now have to be redesigned to use a lower voltage logic family and level shifting.
 

Online vk6zgo

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Re: 74HCT74 NXP vs Texas
« Reply #32 on: December 29, 2018, 02:02:48 pm »
IC manufacturers are not always innocent when it comes to strange things happening when using supposedly identical devices only separated by a fair few years between manufacture.

I had to work on a piece of equipment which used a mono stable IC in the 4000 series, where it was used to provide a quite long "on" time.

Unfortunately, one of these devices became lost.
"No worries, there are plenty in the Store!"
When I tried to use the new one, it kept timing out early, so the equipment didn't work.
The same result with all the new ones in the Store.

It turns out that the manufacturer originally specified the mono for a very large range of "on" times, but some users had problems with them with short
"on" times.

The answer was to change the design, so that the original part number was good for fairly short durations, & introduce a new part number for people who want long durations.

We had to specially order the "long duration" version, replace the originals with the new type in both the units using them, generate an internal Tech note, & change the original schematic.......Aaaarrrrgggghhhh!
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #33 on: January 03, 2019, 01:48:04 am »
So i build the new circuit, now i have something weird that wont happen with the NXP :
If i touch with the scope or even a loose wire the output of the SN74HCT74N, all working good, soon as i release the wire, the thing stops working.

In my circuit i attatched the output signal to the base of a transistor with a 470K resistor.

I hope the SN74AHCT74N dont have that problem.
Can someone explain ?
thanks
 

Offline IDEngineer

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Re: 74HCT74 NXP vs Texas
« Reply #34 on: January 03, 2019, 04:07:52 am »
If i touch with the scope or even a loose wire the output of the SN74HCT74N, all working good, soon as i release the wire, the thing stops working.
Classic behavior of a very high impedance node. Try lowering that 470K value, paying attention to the output current capacity of the '74.
My political litmus test: I will vote for any candidate, regardless of party affiliation or positions on other issues, who promises to abolish the TSA.
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #35 on: January 04, 2019, 03:15:32 am »
Thanks, i go read some datasheets.

With a 10K pulldown resistor also did not work.
 

Offline IDEngineer

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Re: 74HCT74 NXP vs Texas
« Reply #36 on: January 04, 2019, 12:17:00 pm »
Wow, do I wish we could see a non-ASCII schematic. Even pencil-on-napkin would be helpful.
My political litmus test: I will vote for any candidate, regardless of party affiliation or positions on other issues, who promises to abolish the TSA.
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #37 on: January 05, 2019, 02:20:45 am »
Ok, i will download DaveCAD somehow.

I cant find any software on this link :
https://github.com/EEVblog/DaveCAD

Where do i download ?
 

Offline SiliconWizard

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Re: 74HCT74 NXP vs Texas
« Reply #38 on: January 05, 2019, 03:00:35 am »
Ok, i will download DaveCAD somehow.

I cant find any software on this link :
https://github.com/EEVblog/DaveCAD

Where do i download ?

 ;D
 

Offline IDEngineer

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Re: 74HCT74 NXP vs Texas
« Reply #39 on: January 05, 2019, 03:44:02 am »
Wow, do I wish we could see a non-ASCII schematic. Even pencil-on-napkin would be helpful.
Suggestion: Just sketch out your schematic on a piece of paper, scan it, and post it here. Stay focused on your actual project for now. Learning any CAD-style application will take non-zero time that you could instead be using to answer your circuit questions.
My political litmus test: I will vote for any candidate, regardless of party affiliation or positions on other issues, who promises to abolish the TSA.
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #40 on: April 05, 2019, 12:38:29 am »
Thank you again IDEngineer, the AHCT works perfect.
 

Offline IDEngineer

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Re: 74HCT74 NXP vs Texas
« Reply #41 on: April 05, 2019, 04:03:46 am »
Thank you again IDEngineer, the AHCT works perfect.
Glad to hear things are going better! Sometimes "older" parts are still good solutions for certain kinds of problems.
My political litmus test: I will vote for any candidate, regardless of party affiliation or positions on other issues, who promises to abolish the TSA.
 

Offline Gyro

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Re: 74HCT74 NXP vs Texas
« Reply #42 on: April 05, 2019, 04:10:03 am »
...and sometimes, understanding exactly what is going on, and not relying on marginal factors, makes reliable products!
Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #43 on: April 10, 2019, 10:05:16 pm »
Oh wait, i had to change the 8K2 to 5K6, i take 4K7 to be sure.
So there still is a difference between Texas and NXP.
The driving of the 470K resistor was ok.
Just to make this story more complete.
 


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