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Original question:

SN74HC4060: Output State During Startup

SN74HC4060: Issue about the SN74HC4060 RC value Design for the target frequency output

Steven Liu1
Expert 4440 points
Part Number: SN74HC4060

Hi Team, 

My customer is using the SN74HC4060 to get a fixed frequency output. 

The target frequency of the PIN3 output is 400ms (f~=41kHz), so they made the design as below:

according to above SCH, the output frequency should be 47.44kHz. 

However, the actual RC output is 41.58KHz, measured by the OSC. 

PIN3's output is 2.54kHz (394ms)

So here comes the problem:

why is there huge difference between the theoretical frequency value (47.44KHz) and the real test one (41.58KHz)?

  • 0
    TI__Guru* 96862 points
    Hi Steven,
    The type of oscillator used in this device isn't very easy to accurately represent mathematically. The actual equations required to get an accurate value require a lot more inputs and would take up an entire page of the datasheet by themselves -- which is probably why the original designers of this part provided the equations that are on the datasheet. These will provide an estimation of the final frequency, however the design must be prototyped to fine tune the frequency.

    We _always_ recommend prototyping with this type of device for this reason.
  • 0 in reply to Emrys Maier
    TI__Expert 4440 points
    Hi Maiser,
    Thanks for you reply!
    Understood! Customer has already designed the prototype and fine tune the frequency.
    The following question is that they want to learn when this fine tune design result is used in mass production, whether the same PCB layout board can get the same frequency result?
    Or It depends on each board as each may have tiny differences even PCB layout is the same.
    If there would be some difference on the frequency result, what would be the frequency change rate?
  • 0 in reply to Steven Liu1
    TI__Guru* 96862 points
    Hi Steven,
    As long as your operating period (1/F) and R/C values are at least 2 orders of magnitude larger than the device's propagation delay and internal capacitance/resistance, then you shouldn't see a shift due to process variation from the SN74HC4060. For ~40kHz, 10k, 82k, and 1nF -- all of those values are very safe. As the frequency increases and component values decrease, the SN74HC4060's internal values would become a larger factor in the final frequency.

    Once the prototype has been completed and they have dialed in all the values, they will then need to do an analysis on their component variation. For example, if they are using 2 resistors at 1%, and 1 capacitor at 10%, then their final frequency variation could be up to ~12% (technically a little more than 12%).

    If this resolves your question, please click the green "This resolved my issue" button; otherwise please respond and I will continue to try to help.