Frequency Counter

Thread Starter

PaulDFL

Joined Jan 1, 2020
49
Designed a counter using LS circuits.
All was going well up to the last decade.
The circuits track the frequency as designed to 5KHz.
After that the last stage output sends a pulse to the fifth once every four gate times.
Lower the frequency, the pulse stops.
I swapped out the fourth and fifth devices with different ones, moved the fifth device to a different place on the protoboard.
I would like to know what this glitch is.
 

dl324

Joined Mar 30, 2015
10,994
After that the last stage output sends a pulse to the fifth once every four gate times.
Lower the frequency, the pulse stops.
I swapped out the fourth and fifth devices with different ones, moved the fifth device to a different place on the protoboard.
We don't have the advantage of being able to see the waveforms or schematic, so your comments mean little.
  1. What are the 4th and 5th devices?
  2. What is the frequency at which "the pulse stops"
  3. How is the circuit wired? Solderless breadboards?
 

Thread Starter

PaulDFL

Joined Jan 1, 2020
49
We don't have the advantage of being able to see the waveforms or schematic, so your comments mean little.
  1. What are the 4th and 5th devices?
  2. What is the frequency at which "the pulse stops"
  3. How is the circuit wired? Solderless breadboards?
The devices are 74LS390. pulse stops at 4.999KHz.
Solderless breadboard
 

dl324

Joined Mar 30, 2015
10,994
I have a decoupling capacitor on each power distribution bus.
What has changed since the circuit last worked? Are you using a different power supply? Could any wires have been disturbed?

The devices are 74LS390. pulse stops at 4.999KHz.
Post a schematic and indicate where you're measuring the misbehavior.

You're going to end up troubleshooting using typical techniques. You work backwards in the circuit until you find the component that isn't working as expected.
 

Thread Starter

PaulDFL

Joined Jan 1, 2020
49
What has changed since the circuit last worked? Are you using a different power supply? Could any wires have been disturbed?


Post a schematic and indicate where you're measuring the misbehavior.

You're going to end up troubleshooting using typical techniques. You work backwards in the circuit until you find the component that isn't working as expected.
All ready have done that.
It’s
The fourth counter.
Replaced it with another device, same results. Used a HC version of same device, no change.
 

dl324

Joined Mar 30, 2015
10,994
All ready have done that.
If you want help, humor us and give us a schematic.
Used a HC version of same device, no change.
HC aren't guaranteed to be drop-in replacements for 74LS. Sometimes they'll work and sometimes they won't. To guarantee correct operation, you need to put pull up resistors on the HC inputs.

HCT is a drop-in replacement for 74LS as long as don't load the outputs.
 

Thread Starter

PaulDFL

Joined Jan 1, 2020
49
If you want help, humor us and give us a schematic.
HC aren't guaranteed to be drop-in replacements for 74LS. Sometimes they'll work and sometimes they won't. To guarantee correct operation, you need to put pull up resistors on the HC inputs.

HCT is a drop-in replacement for 74LS as long as don't load the outputs.
I did not know that about the HC devices.
Suggestion for pull up resistor?
 

KeithWalker

Joined Jul 10, 2017
1,058
Why only this stage and not he four preceding stages?
You state that the pulse stops at 4.999KHz, Does the mean that is only works a 5KHz and nothing lower? Can you be a little more specific please?
Do you get a signal at the input signal to counter 5 when it stops working?
If you do, the problem is in counter 5.
If not, the problem is before that.
You have to start doing some logical trouble shooting. We do not have access to your circuit so our guesses are limited by the small amount of information you have given us.
Regards,
Keith
 

Thread Starter

PaulDFL

Joined Jan 1, 2020
49
You state that the pulse stops at 4.999KHz, Does the mean that is only works a 5KHz and nothing lower? Can you be a little more specific please?
Do you get a signal at the input signal to counter 5 when it stops working?
If you do, the problem is in counter 5.
If not, the problem is before that.
You have to start doing some logical trouble shooting. We do not have access to your circuit so our guesses are limited by the small amount of information you have given us.
Regards,
Keith
The problem doesn’t appear until the frequency reaches 5KHz. It’s works as designed to that point.
No pulse at the input below 5KHz.
I read that TTL devices are inherently noisy.
Is this true?
 

KeithWalker

Joined Jul 10, 2017
1,058
The problem doesn’t appear until the frequency reaches 5KHz. It’s works as designed to that point.
No pulse at the input below 5KHz.
I read that TTL devices are inherently noisy.
Is this true?
You say it stops a 5KHz.So what is displayed when the frequency goes below 5 KHz? Does it go to zeros? Does it display 5000?

TTL is characterized by high switching speed upwards of 125 MHz and relative immunity to noise . Its principle drawback is the fact that circuits using TTL draw more current than equivalent circuits using metal oxide semiconductor logic. Low-current TTL devices reduced current demand at the expense of some operating speed.

Regards,
Keith
 

Thread Starter

PaulDFL

Joined Jan 1, 2020
49
You say it stops a 5KHz.So what is displayed when the frequency goes below 5 KHz? Does it go to zeros? Does it display 5000?

TTL is characterized by high switching speed upwards of 125 MHz and relative immunity to noise . Its principle drawback is the fact that circuits using TTL draw more current than equivalent circuits using metal oxide semiconductor logic. Low-current TTL devices reduced current demand at the expense of some operating speed.

Regards,
Keith
The display shows what the frequency is set for.
Right after the data transfer for 5000 the pulse glitch appears.
 
Last edited:

MrChips

Joined Oct 2, 2009
21,364
I read that TTL devices are inherently noisy.
Is this true?
Noise is relative.

7400 series TTL components have been and still are in operation in billions of installations since they were introduced in 1964.

CMOS logic is relatively less noisy because they switch much lower currents.
This does not mean that 7400 chips are difficult to get working properly. On the contrary, 7400 TTL circuits are very reliable.

The one rule-of-thumb that must be religiously followed is to install 0.1μF ceramic capacitors between Vcc and GND pins of the chip (not just the supply rails) regardless of chip type, TTL, CMOS, digital or analog.
 
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