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MSP430F1121A: formula for ROSC to control DCO?

AnBer
Mastermind 31375 points

Part Number: MSP430F1121A

Team,

Customer is looking for a formula in order to dimension the ROSC.
Since ROSC seems to have influence on the frequency and temp drift of the DCO it would make sense to first obtain a specific theoretical value of ROSC before experimenting.

Is there a formula we can share? Or some explanation to choose an according ROSC value?

I found the below App note speaking about MSP430 oscillator, but no specific info about ROSC vs DCO:
-MSP430 LFXT1 Oscillator Accuracy - https://www.ti.com/lit/pdf/slaa225  
-MSP430 32-kHz Crystal Oscillators - https://www.ti.com/lit/pdf/SLAA322 
-Controlling the DCO Frequency of the MSP430F11x - https://www.ti.com/lit/pdf/SLAA074 

( FYI - It seems that MSP430x2xx (compared to MSP430x1xx) has factory calibrated DCO so the voltage and temp drifts are minimized, but here the question is about MSP430Fx1xx).

Thanks in advance

Anthony

  • 0
    TI__Genius 9611 points

    Hi Anthony,

    Per the device specific datasheet:

    This section of the MSP430x1xx Family User's guide may be helpful:

    As for the reasoning behind the ROSC value, it is essentially a middle ground balancing speed and power. The resistor acts like a steady reference to keep the device consistent with change in temperature.

    Best,

    Owen

  • 0 in reply to Owen Li
    Prodigy 30 points

    Hi,

    I'd like to know the impact of resistor variation on frequency. For example, I'd like to know the frequency with a 200kohm resistor.

    Do you have a simulation file for this function ?

    I thank you for your attention.

    Regards,

    Jeremy

  • 0 in reply to Jeremy Guillonneau
    Guru 26425 points

    The actual frequency depends on the DCO settings and isn't particularly accurate. Notice the 15% tolerance.

    With the same settings, a 100K external resistor runs about 1.95MHz and internal 300K about 0.75MHz.

  • 0 in reply to Jeremy Guillonneau
    TI__Genius 9611 points

    Hi Jeremy,

    Resistor variation will change what frequencies can be achieved. We do not have any simulation files for this function.

    Our datasheet and testing only characterizes and guarantees the +/-15% tolerance with the specified ROSC value listed in the device-specific datasheet. I would suggest using the ROSC value specified in the device datasheet.

    Best,

    Owen

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