Digital Clock: Oscillator changing its frequency :(

Discussion in 'The Projects Forum' started by Zasmussen, Dec 9, 2015.

  1. Zasmussen

    Thread Starter New Member

    Nov 29, 2015

    In my electronics class we have been tasked with creating a digital clock using only TTL logic. The attached file is my final design except instead of a waveform generator, we used a 74HCT4060 chip that has an oscillating function. Pin 12 (Master Reset) is grounded, 16 powered, 8 grounded, and pin 3 is used for our output for the second pulses which goes into every counter chips' clock input. Right now I have a .1uF capacitor (Code 104), a 3.3 Ohm resistor for RS, and a 2K Trim-Pot resistor for Rtc. I don't exactly understand how this oscillator chip works.
    Basically when my real solder board design is turned on, the oscillator randomly speeds up and slows down especially when I press any of the buttons. I had made those resistor and capacitor combination to make pin 7 have 1024 Hz (which means pin 3 has a seconds pulse). It worked until I pressed the buttons and now it's doing it more often.

    Any ideas or suggestions anyone has will be greatly appreciated!

    Thanks everybody!

    Clock Design.jpg Oscillator.jpg
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    This is a CMOS part. Even a normal output cannot drive a 3.3 ohm resistor, and the clock pins are not normal. In this application, 3.3 ohms for Rs is basically a dead short. As a rule of thumb for this part, the CD4521, etc., no timing resistor should be less than 1K, and 10K is better. Rs should be at least 10x Rtc unless that pushes it up above 5 megohm (too much noise pickup). The larger Rs is, the less it affects the oscillator frequency.

    Attached is a National Semiconductor app note on CMOS oscillators. This note assumes standard CMOS gates, and as mentioned the 4060 oscillator pins are not standard. But this should get you close enough.

    I suggest you add a 10 K fixed resistor in series with the trimpot, increase Rs to 220K, calculate a new value for Ctc based on Rtc=11K, and go from there. Note that your clock frequency will drift with changes in room temperature because CMOS transition levels are temperature-dependent. The only serious shortcoming of the 4060 is that it cannot make 1 Hz with a 32.768 kHz crystal without another chip.

    Last edited: Dec 10, 2015
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    an rc oscilator is inherently unstable. either use the line frequency (60 or 50 Hz) and devide down to 1 pps, or a crystal oscilator devided down.
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Last edited: Dec 10, 2015
  5. crutschow


    Mar 14, 2008
    What accuracy do you need for the clock?