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AM335x Pin Capacitance for RTC_XTALIN and RTC_XTALOUT pins (ZCZ Package)

In order to do a proper component selection, I need to know what the input and output capacitance is for the RTC pins (RTC_XTALIN and RTC_XTALOUT respectively).

I have looked at the datasheet and online, but can't seem to find a published value.

  • Are you trying to select the components of a crystal circuit. If so, the information you need is provided in the OSC1 Internal Oscillator Clock Source section of the AM335x data sheet.

    Regards,
    Paul
  • Thanks for the input Paul.

    Yes I did look at that page, but it only states the minimum and maximum load requirements on the RTC pins, not what the actual input/output capacitance for the device is.

    In order to calculate the capacitor values for the RTC circuit, I need to know the total capacitance of each pin which consists of the capacitor I choose for each pin + the input/output capacitance of the RTC pins of the AM335X device. These values, along with stray PCB capacitance will help me match the C(load) of the crystal.

    This should have been spec'd in the datasheet.
  • We provide Cpkg which is the AM335x portion of shunt capacitiance. Please read note B in the Crystal Circuit Schematic figures. A formula is provided in this note that should be used when selecting crystal component values.
  • Cpkg is the capacitance across the pins (max 1.5pF).

    Per Note B:  "...plus any mutual capacitance (Cpkg + CPCB) seen across the AM335x RTC_XTALIN and RTC_XTALOUT signals."

    This capacitance should not be confused with the input and output capacitance of the pins.

    I have included a picture to show this. You'll notice the buffer inside the AM335X will have some capacitance associated with its input (Cin) and output (Cout).

    This capacitance is in parallel with the external capacitors C1 and C2. Therefore the combined capacitance needs to be taken into account.

    So.....I'm looking for the input capacitance of this buffer (Cin) and the output capacitance of this buffer (Cout).