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TMS320F280025C: Clocking selection

Johnny Chen
Expert 5220 points

Part Number: TMS320F280025C

Hi Champ,

I am asking for my customer. They are using SCI with baud rate 115200 bits per second.

In general, adding external crystal may be suggested for using fast asynchronous transmission (UART).

Would the internal oscillators accuracy be satisfied with the customer's current baud rate (115200 bps) ?

Also, is there a principle or app note to clarify how fast the baud rate that it is suggested to add the external crystal ?

Thanks and regards,

Johnny

  • 0
    TI__Mastermind 19795 points

    Hi Johnny,

    Thanks for your question. There is no specific frequency that requires external crystal. General recommendations are that a crystal provides better clock accuracy, and so also provides better SCI baud rate accuracy. Our SCI module does not have good granularity at high baud rates (baud rate steps are very large at high baud rate, meaning you can't make small changes to baud rate). That's why we typically recommend crystal for higher baud rates.

    The best way to check whether you need it is to see what baud rates are available around your chosen one (move the bits up and down by 1). If those values are too far away from 115200, then you could have communications issues.

    Regards,

    Vince

  • 0 in reply to Vince
    TI__Expert 5220 points

    Hi Vince,

    Thanks for your clarification. For sure, I would check the way as your suggestion.

    As a reference, may I know from your past experience, would you add an external crystal in the case that baud rate is chosen at 115200 bps ?

    Thanks and regards,

    Johnny

  • 0 in reply to Johnny Chen
    TI__Mastermind 19795 points

    Hi Johnny,

    With a SYSCLK of 200 and LSPCLK of 100, the following are the closest baud rates possible:

    BRR(LSB) Baudrate  Actual Error
    111 115200 111607 3.12%
    110 115200 112613 2.25%
    109 115200 113636 1.36%
    108 115200 114679 0.45%
    107 115200 115741 -0.47%
    106 115200 116822 -1.41%
    105 115200 117925 -2.37%

    As you can see, the granularity is low here, but not too bad.

    F28002x has an INTOSC accuracy of -2% to 1.5%

    Worst case scenario, -2% INTOSC on one chip to another F28002x device with +1.5% means a total worst-case range of 3.5%.

     

    So if the INTOSC is off by 3.5% from another device, you can still correct the baud rate with enough granularity to get within 1% error, without an external crystal.

    If you go with an external crystal though, you can avoid having to change the baud rate in order to match frequency. There's other benefits too, but this is the main one for our discussion.

    Regards,

    Vince