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SN74HC4060: RC oscillator, frequency lower than expected

Adrian Torregrosa
Prodigy 90 points
Part Number: SN74HC4060
Other Parts Discussed in Thread: SN74LVC3GU04, SN74LVC1G14

Hello.

I recently acquired an SN74HC4060N I intend to use it as a time counter. Following the scheme and formulas of page 6 of the IC's datasheet, I attached a capacitor of C=120nF to pin 9, a resistor or R1=5.6KOhm to pin 10 and another resistor R2=470Ohm to pin11, connecting their three ends. I was expecting a frequency of around 1.2KHz and thus that the Q14/pin 3 would go up after 8 seconds, however surprisingly enough (for me) it did go up but only after 10.5 seconds. I have also tried replacing the capacitor with another one of 100nF, which should have resulted in a higher frequency but still it took 10.5 seconds for pin 3 to go up. I'm using Vcc=5V.

Is this a normal behaviour? Is the IC prepared maybe for a different ratio of R1 and R2, with higher frequencies? Please comment.

Best regards.

  • 0
    Prodigy 90 points
    Correction: with the 120nF capacitor the frequency should be 1.0KHz; with the 100nF capacitor, according to the formula, 1.2KHz.

    BR
  • 0 in reply to Adrian Torregrosa
    TI__Guru* 96591 points
    Hi Adrian,
    Do you have pin 12 tied to ground? This is a common oversight that can lead to strange problems and even damage to the device.
  • 0 in reply to Emrys Maier
    Prodigy 90 points
    Hello.

    Yes, sorry I didn't mention it earlier, pin 12 is indeed tied to ground.

    BR
  • 0 in reply to Adrian Torregrosa
    TI__Guru* 96591 points
    Hey Adrian,
    Another possibility is the supply ramp rate -- how fast is your supply starting up?

    Two inverter oscillators, like the one used in the HC4060, are not the most reliable architecture out there, and don't necessarily always oscillate. They essentially count on a noisy startup of the device to 'kick start' the oscillator.

    You could build an external oscillator with a Schmitt-trigger inverter (SN74LVC1G14) or triple inverter like SN74LVC3GU04 to have a better and more reliable oscillator for the system.
  • 0 in reply to Emrys Maier
    Prodigy 90 points
    Thank you Emrys for your helpful reply.

    I'm afraid those suggestions are way beyond my current capabilities. I have downloaded and read the specs for those ICs, however for the time being I think I'm going to stick with my current design.

    I am though a bit disappointed with the HC4060 itself: if the IC had some limitations in its behaviour, such as the one described here, I believe those should be described in the IC specifications.

    Anyway, thanks a lot for your support and suggestions.

    BR