Binary Counter Gives Wrong Count Only on Real Hardware

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1

Hello,
I simulated a pulse counting circuit using a binary counter IC and everything matched expected timing. On the real board, the count occasionally skips values when pulses arrive quickly.

Input pulses come from a Hall sensor.

Questions:

  1. Should I suspect slow edges from the sensor rather than the counter IC itself?

  2. Would a comparator or Schmitt trigger cleanup stage solve this?

  3. On the PCB, should sensor inputs be routed away from clock nets and power switching traces?

2

Hi Eva
Exactly what sensor are you using. Some already have Schmidt triggers built in.
If not it would not do any harm to try this clean up solution.
Cheers Bob

3

Yes … it’s the most common cause … Hall sensors often have:

Slow rise/fall times (especially with weak pull-ups)
Open-drain/collector outputs
Noise around threshold at higher speeds

At high pulse rates, slow edges can:

Cross the logic threshold multiple times
Violate setup/hold timing of the counter … Result: missed or extra counts

Yeah, it will fix things:

Schmitt trigger buffer: simplest and very effective
Comparator with hysteresis: best if signal is analog/noisy

Read this discussin on comparison b/w the two: https://electronics.stackexchange.com/questions/213461/how-to-explain-the-advantage-of-schmitt-trigger-over-comparator-for-pulse-condit

Yes and its a hetric … Keep sensor lines away from clock traces, switching regulators etc

Route over a solid ground reference … read this guide: Routing Layers and Ground Planes & Power Planes - Engineering Technical - PCBway

Keep traces short and clean
Optional: small series resistor (100 ohm around) for damping

Your issue is almost certainly input signal quality, not the counter IC. Add a Schmitt trigger (or comparator with hysteresis) and improve routing … that normally eliminates skipped counts at higher speeds.

4

Hi ahsrab

That IS a Schmidt Trigger.
Cheers Bob