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[MSP430F5247] xtal requirement for RTC

da
Expert 2780 points
Other Parts Discussed in Thread: MSP430F5247

Hi,

My customer has some questions about the requirement for RTC xtal on MSP430F5247.

1. Do you have any requirements for xtal load capacitance (so called CL) ? It seems there is no description in datasheet. Please note they are considering to use a xtal with 22pF CL. I believe this value is so generic and there is no problem/concerns for this. Correct, right ?

2. According to the datasheet, external capacitors (C1/C2) are required to be introduced into the circuit between P5.4/XIN/P5.5/XOUTS and GND. Do you have any recommended value for them ?

Best Regards,

da

  • 0
    Guru 74080 points

    Hi!

    da said:
    Do you have any requirements for xtal load capacitance (so called CL) ? It seems there is no description in datasheet.

    The required load capacitance is given by the crystal manufacturer and is therefore not listed in the datasheet of the MSP430.

    da said:
    Please note they are considering to use a xtal with 22pF

    I assume you found this value in the datasheet of the crystal.

    da said:
    According to the datasheet, external capacitors (C1/C2) are required to be introduced into the circuit between P5.4/XIN/P5.5/XOUTS and GND. Do you have any recommended value for them ?

    This is the common circuit topology for a crystal oscillator. XIN to one pin of the crystal and one of the load capacitors to GND. Same for XOUT. The value has to be calculated. It is

    C = 2 x (C_load - C_stray)

    where C_stray is the stray capacitance of the board and the input pins - you can take 2pF fo this. C_load is the desired load capacitance for the crystal. This means in your case:

    C = 2 x (22pF - 2pF) = 2 x 20pF = 40pF

    This means you should add two capacitors with about 40pF (nearest possible value). A lower capacitance will increase the frequency and vice versa.

    Dennis

  • 0
    Mastermind 9125 points
    Hi da,

    I would recommend your customer to use a crystal with 7pF CL. 7pF is very common in RTC crystal. And the most of important, you can compensate it fully by MSP430 internal capacitors using firmware. And hence, no external capacitor is required.

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