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CD74HC4024E - 7stage binary counter - strange

user4813767
Prodigy 20 points

Other Parts Discussed in Thread: CD74HC4024, SN74LVC1G17

Hi,  I had to make some circuit in which I used this CD74HC4024E chip (bought 5 pcs from RS Components - RS Stock No. 663-0329 ) ,

and I noticed some behavior which I did not expected from such quite simple logic chip.

Chip has 7-stage counter, with outputs from Q1 to Q7, and I expect that Q1 divide input frequency with 2 , Q2 with 4 ..... and last Q7 with 128

I record output with logic analyzer and check what I got (Channel 0 is input signal - frequency less than 1kHz, Channel 1 = Q1 , ... , Channel 7 = Q7.

Now from this point nothing make sense, Q1 seems that does not divide input frequency, it i same as input frequency but from time to time some pulses are missing. Q2 divide frequency with 2, which I expect from Q1. And at the end Q7 divide with 64, I expect 128.

I don't get it ?!

Just to be sure that something is not wrong with my cheap logic analyzer, I checked also this Q1 output with oscilloscope,  and there it can be also seen that some pulses are drop.

All 5 IC-s behave in a same way, don't have normal Q1 output, and Q7 divide with 64 , like it is 6-stage counter instead 7-stage ?!

  • 0
    TI__Guru* 97486 points

    It is likely that the slow edges on your input waveform are causing an issue in the CD74HC4024E.  That device has a maximum transition time on the input of 500ns, but the scope shows input transition rates  measured in ms.  Can you add a Schmitt-trigger device prior to the clock input to clean up the signal?

    If you would like to learn more about the effects of using slow signals on the inputs of CMOS devices, there's a great application note written on the subject here.

  • 0 in reply to Emrys Maier
    Prodigy 20 points

    Thank you, I just make tests again and that was a problem, signal by itself is fast enough, but it pass through optocoupler which reduce transfer speed significantly, especially darlington optocoupler which I use (ISP845X).

    So this means that actually 1st stage behave like some kind of schmitt trigger.

    I checked CD74HC4024E datasheet and I see on CP input that there are some NOT gates, I hoped that this gate could produce some schmitt trigger effect, but obviously it is not working in that way.

    Do you know (or anybody) is there compatible chip to CD74HC4024 that have built in schmitt trigger on CP input?

  • 0 in reply to user4813767
    TI__Guru* 97486 points

    I'm afraid I don't know of any counters that have Schmitt-trigger inputs.

    The SN74LVC1G17 is a single Schmitt-trigger buffer that would work with this device.