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TMS320F2800152-Q1: Damping resistor calculation

Charan Bandikatla
Prodigy 50 points
Part Number: TMS320F2800152-Q1

Tool/software:

Iam using  20.000MHZ 30Ohm 8pf 500uW crystal for my automotive rear lighting application.
But in the datasheet of MCU it is mentioned to use a 1mW crstal.
Unfortunately i cannot find a crystal of 1mW,20 Mhz crystal.
i decided to use a damping resistor. 

can u let me know how to calculate the value of damping resistor.


i can see that there is a 20Mhz crystal used in eval board without damping resistor. but there is no BOM provided in that.
can u provide anty recommended crystal for my application.


  • 0
    TI__Mastermind 34785 points

    Hi Charan,

    The F280015x crystal oscillator circuit is capable of driving 1mW to an external crystal.  It does not necessarily mean that it is driving 1mW power every time.  It will only provide the power that the crystal requires.  If crystal consumes 1.2mW of power and crystal is only spec'd at 500uW, then this poses a reliability issue to both the external crystal and the crystal oscillator circuit.  This is the case where you will need a dampening resistor.

    You would have to characterize the external crystal performance against the crystal oscillator of the f280015x to see how much power is the crystal consuming.  One way to do this is to have a very simple code running on the f280015x which does nothing but configure the clock source to be from X1/X2.  No other IPs are required to run so that current consumption from other modules will not influence your measurement results.  Prior to configuring X1/X2 as clock source, power on the device and observe the current flowing to the 3.3V rail.  You need to supply the 3.3V rail with a source that has a precise ammeter to allow you to observe the slight increase in current once X1/X2 clock is configured.  Soon as X1/X2 is configured in your code, the current in 3.3v rail will increase.  Record this current change.  You also need a vey low capacitance active probe to scope out the waveform on X1.  You can use the scope's measurement capability to determine the sine wave RMS value of the waveform in X1.  Multiply this RMS value with the current change measured in the previous step to derive the power consumed by the external crystal to calculate the power consumed.  If power consumed is greater than the specified max power consumption of the crystal, add Rd in series, so that excess power will be absorbed by the dampening resistor.  Repeat the measurement steps described for every value of Rd that you try, ensuring that power consumed by the crystal is not exceeding its limit.

    Regards,

    Joseph 

  • 0 in reply to Joseph Casuga
    Prodigy 50 points

    Okay . But here i'm only responsible for the schematic design. So, can you please let me know what resistor value to use for damping in design phase.
    Crystal details:
    Supplier : Mouser , Sup Pt No 449-LFXTAL071671REEL , Mfg : IQD, Mfg Pt No : LFXTAL071671Reel.

    And what crstal are they using in the eval board without damping resistor.

  • 0 in reply to Charan Bandikatla
    TI__Mastermind 34785 points

    Charan,

    You need to have someone from your team to characterize if the XTAL of your choice will work with the F280015x device.  You also need to ensure that the crystal parameter match the datasheet spec for crystals under section 6.13.3.4.7 of the datasheet.  You can refer to the LAUNCHXL-F2800157.for the crystal circuit.

    Regards,

    Joseph