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TMS320LF2406 - load capacity at crystal

Martin Kumar
Prodigy 20 points

Hello,

i have to redesign an old circuit with a TMS320LF2406 MCU. Now i'm designing the crystal-circuit. There are 2 capacitors that have to be connected to the XTAL1/XTAL2 Pins. I looked in the datasheet for the TMS, but there were no recommended values for these capacitors. I use a 20Mhz crystal.

Does anyone have experience with this two capacitors and can help me with the right value of them? I know that the datasheet says, that i should use a crystal with 20pf load capacitance. This type of crystal i got. But what about these two external capacitors?

Thx in advanced,

Martin

  • 0
    Guru 192875 points

    Hi Martin,

    I use a 20Mhz crystal.

    The capacitor values are mentioned in Crystal's datasheet. It'll hamper the performance if you neglect the datasheet info. Do check that out!

    Regards,

    Gautam

  • 0 in reply to Gautam Iyer
    TI__Expert 7885 points

    Hi Martin,

    The load capacitance value for the crystal is the value you should use for the two caps.  Cb1 and Cb2 both should be 20pF.

    Hope this helps.

    Regards,

    Adam

  • 0 in reply to Adam Dunhoft
    Prodigy 20 points

    Hi Guys,

    thanks for your replies. After a few calls with different crystal manufactures i got an answer. It is not easy to choose the right values for Cb1 and Cb2...... The capacitance for Cb1 and Cb2 are not mentioned in the crystal's datasheet, because it's depends on the entire capacitance of my design for the crystal part. Just the load capacitance is mentioned and load capacitance IS the entire capacitance of this crystal design part. The load capacitance depends on the capacitance of the tracks between uC and crystal, the input-capacitance of the uC and on the two capacitors Cb1 and Cb2. The high-frequency equivalent circuit looks like this:

    _____________________________

    |                                  |                               |

    Cb1                           |                               |

    |                                 Ctracks                  CuC                              =                  Cload

    Cb2                           |                               |

    |                                  |                               |

    _____________________________

     

    The manufacture said, that a typical value for Ctracks is between 2pf to 5pf. The input capacitance of the uC could between 5 to 10pf. So in case of a load capacitance of 20pf, Adam you are nearly right with 20pf for Cb1 and Cb2.

     

    Thanks again for your replies.

  • 0 in reply to Martin Kumar
    Guru 192875 points

    That's great, Martin. Actually I do refer datasheet for this info and they do clearly specify the range.

    You can refer this link for more info: http://e2e.ti.com/support/microcontrollers/c2000/f/171/t/327313.aspx

    Goodluck & Regards,

    Gautam