Author Topic: 74HCT74 NXP vs Texas  (Read 2652 times)

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Offline Jan Audio

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74HCT74 NXP vs Texas
« on: December 26, 2018, 03:15:23 am »
Hi, merry cristmas.
I have another problem :

When i change my 74HCT74 from NXP to Texas instruments it dont works anymore.

I have a square wave to 8K2 resistor going into a 2n2 capacitor,
the capacitor out goes with a 10K and diode to ground and to the CLK input of the 74HCT74.

Why is it different ?
thanks

btw : it does work somehow only it toggles twice really fast so the output looks like always on state.
« Last Edit: December 26, 2018, 03:28:39 am by Jan Audio »
 

Online rhb

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Re: 74HCT74 NXP vs Texas
« Reply #1 on: December 26, 2018, 03:47:39 am »
Perhaps you should post a schematic?  That way readers would know what you're trying to do.
 

Offline DaJMasta

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Re: 74HCT74 NXP vs Texas
« Reply #2 on: December 26, 2018, 03:53:20 am »
Is the TI part also an HCT series chip?  Some different families require different voltages or have different response characteristics (or are open collector outputs, like the F series).

Hard to tell exactly from your description, but is your clock input signal AC couple but not biased to stay above negative voltages (can cause erratic behavior)?  Also could be that the TI part is configured to use any old edge trigger whereas the NXP part is only rising or falling edge - giving you 2x the switching rate - the datasheet should be able to tell you if that's the case, though usually these are different part numbers on the package (the 74 model should be positive edge triggered only).
 
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Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #3 on: December 26, 2018, 04:00:08 am »
Yes the Texas is the SN74HCT74N.
« Last Edit: December 26, 2018, 04:02:23 am by Jan Audio »
 

Offline podericof

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Re: 74HCT74 NXP vs Texas
« Reply #4 on: December 26, 2018, 04:01:16 am »
Hi there,
can you also tell us the complete PN for the 2 ICs?
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #5 on: December 26, 2018, 04:02:40 am »
Hi, sorry what is a PN ?

Texas is : SN74HCT74N
NXP : 74HCT74, i have to look.
 

Online blueskull

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Re: 74HCT74 NXP vs Texas
« Reply #6 on: December 26, 2018, 04:03:30 am »
Probe the input voltage to the logic chip. Different company chips have different design. They both conform to a single standard (VIL range, VIH range, etc.), but the exact value is not guaranteed.

Hi, sorry what is a PN ?

Part number.
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #7 on: December 26, 2018, 04:15:02 am »
The NXP is : 74HCT74N
 

Online wraper

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Re: 74HCT74 NXP vs Texas
« Reply #8 on: December 26, 2018, 04:34:54 am »
If it does not work and IC is not faulty, there is something wrong with your circuit. Just happens to be that one of them marginally operates.
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #9 on: December 26, 2018, 04:38:47 am »
Thanks, i will re-check my circuit again, hope the Texas works since i have 100 piece.
 

Online wraper

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Re: 74HCT74 NXP vs Texas
« Reply #10 on: December 26, 2018, 04:39:42 am »
Quote
I have a square wave to 8K2 resistor going into a 2n2 capacitor,
the capacitor out goes with a 10K and diode to ground and to the CLK input of the 74HCT74.
You shall not input signals with slow rise/fall time into CMOS ICs to begin with. The only exception are ICs with schmitt trigger inputs. You have not described diode orientation as well. As people already said, post the schematic.
 

Offline Zero999

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Re: 74HCT74 NXP vs Texas
« Reply #11 on: December 26, 2018, 06:35:03 am »
What are you trying to achieve the the resistors and capacitor.
 

Offline Bassman59

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Re: 74HCT74 NXP vs Texas
« Reply #12 on: December 26, 2018, 11:34:29 am »
Thanks, i will re-check my circuit again, hope the Texas works since i have 100 piece.
Post a schematic.
 

Online David Hess

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Re: 74HCT74 NXP vs Texas
« Reply #13 on: December 26, 2018, 11:49:53 am »
For the same part from different manufacturers, some clock inputs are implemented as schmitt triggers and some are not but this is not always reflected accurately in the datasheets.  This may be one of the later cases and it is exactly what I would expect of the 74HCT74.

The only way to be sure is to either qualify the parts or include your own schmitt trigger before the clock input if you are going to use different manufacturers.
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #14 on: December 27, 2018, 12:26:04 am »
Here was the NXP schematic :

                                              2N2
[ 12v waveform ]-----[8K2]-----||------+----[10K]---ground;
                                                          +----|<4148]---ground;
                                                          +----[CLK]

I fixed it with this for Texas :

                                              2N2
[ 12v waveform ]-----------------||------+----[1K]---ground;
                                                          +----|<4148]---ground;
                                                          +----[CLK]

Maybe it is a bit short circuit like this ?

You shall not input signals with slow rise/fall time into CMOS ICs to begin with. The only exception are ICs with schmitt trigger inputs.

Ok, thank you i did not know.
« Last Edit: December 27, 2018, 12:27:47 am by Jan Audio »
 

Online wraper

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Re: 74HCT74 NXP vs Texas
« Reply #15 on: December 27, 2018, 12:33:16 am »
[ 12v waveform ]-----[8K2]-----||------+----[10K]---ground;
                                                          +----|<4148]---ground;
                                                          +----[CLK]

I fixed it with this for Texas :

                                              2N2
[ 12v waveform ]-----------------||------+----[1K]---ground;
                                                          +----|<4148]---ground;
                                                          +----[CLK]
Some piece of nonsense you made there. If you need to reduce voltage from 12V, why not use resistive divider of resistor + zener diode? And why don't you draw normal circuit?
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #16 on: December 27, 2018, 12:55:08 am »
I have no zeners in house, another parts series to buy ?, do i really need them...
If the NXP was still available i would buy them.

I dont have software to render schematics, got any free ones ?, they should build it in this website one that locally generates on simple info.
« Last Edit: December 27, 2018, 12:58:24 am by Jan Audio »
 

Online NiHaoMike

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Re: 74HCT74 NXP vs Texas
« Reply #17 on: December 27, 2018, 01:10:35 am »
I dont have software to render schematics, got any free ones ?
DaveCAD might be really limited in features, but is definitely the easiest to use.
Fork: https://github.com/scanlime/davecad
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Online wraper

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Re: 74HCT74 NXP vs Texas
« Reply #18 on: December 27, 2018, 03:42:39 am »
I have no zeners in house, another parts series to buy ?, do i really need them...
2 resistors for divider sounds too difficult? What's the point using capacitor to begin with?
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #19 on: December 27, 2018, 03:43:47 am »
The saw wave i put in now is from 6,6v to 11,5v.
 

Online SiliconWizard

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Re: 74HCT74 NXP vs Texas
« Reply #20 on: December 27, 2018, 03:56:36 am »
Hi, sorry what is a PN ?

Texas is : SN74HCT74N
NXP : 74HCT74, i have to look.

Just take a look at the datasheets. The NXP one has a schmitt trigger in the clock input (actually most NXP logic parts have schmitt trigger inputs AFAIK), wheras the TI most likely doesn't (the DS doesn't make any mention of it).
NXP: https://assets.nexperia.com/documents/data-sheet/74HC_HCT74.pdf
TI: http://www.ti.com/lit/ds/symlink/sn74hct74.pdf

The lack of a schmitt trigger in the TI part makes the internal clock signal bounce in your case (it's not a digital signal with "clean" edges but an high-pass filtered signal.)

Don't remove the series resistor. You're going to kill the 74HCT74 eventually. If you didn't kill it yet, it's thanks to the high-pass filter which limits the duration of the 12V pulse and the internal clamping diodes. You may end up frying one.

With this kind of circuit, your best bet is to use a 74HCT74 with a schmitt trigger clock input such as the NXP one (I'm sure there are other vendors with this).

You could also add a schmitt trigger buffer in front of it, such as the SN74LVC1G17 (it's a single schmitt trigger buffer, the LVC series should be ok for your requirements and it's easily available whereas the 74HCT1G17 is not.)

If this was meant to be in a product, the second option would probably be better, because using a "standard" part (a reference available from many vendors) and requiring specific features in it from a specific vendor is a recipe for potential supply disaster later on.
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #21 on: December 27, 2018, 04:40:17 am »
Ok, many thanks.
Why did NXP quit ?, and why dont Texas has schmitt-trigger ?

I will take a look for other vendors, its for personal use only, thanks for the advice.
I also dont like to use more space on my circuit for a schmitt trigger.

Maybe Texas themself has also with schmitt trigger, i,m gonna find out.

 

Offline Zero999

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Re: 74HCT74 NXP vs Texas
« Reply #22 on: December 27, 2018, 05:39:24 am »
Use a potential divider:


R1 = 4k7
R2 = 3k3

I used the site linked below to calculate the resistor values, which I divided by 10, to give more speed and noise immunity.
https://www.random-science-tools.com/electronics/divider.htm
 

Offline Jan Audio

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Re: 74HCT74 NXP vs Texas
« Reply #23 on: December 27, 2018, 05:41:08 am »
You cant divide 6,6volt off state and 11,5volt on state, the zener is needed also.
 

Offline Zero999

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Re: 74HCT74 NXP vs Texas
« Reply #24 on: December 27, 2018, 06:10:47 am »
You cant divide 6,6volt off state and 11,5volt on state, the zener is needed also.
You're right, I missed the 6.6V and 11.5V part. The resistor values could be changed, but it would be marginal.

If you knew that, then why didn't you mention the zener diode before?

Change R2 for a 4.7V or 5.1V zener diode and keep R1 the same.

I have no zeners in house, another parts series to buy ?, do i really need them...
Zener diodes are not expensive.
« Last Edit: December 27, 2018, 06:40:44 am by Zero999 »
 


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