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TMS570 PLL modulation

Yang ShunFan
Intellectual 570 points

Dear Sir,

I am studying Technical Reference Manual(SPNS192A). As my known, The PLL1 include modulation feature. Could you explain what benefit is in this function?

Typically, what application will request this function? Thanks a lot.

Best Regards,

Fan

  • 0
    TI__Guru**** 197126 points

    Hi Fan,

    I will forward your question to our PLL expert, and he will respond to your question soon.

    Regards,

    QJ

  • 0
    TI__Mastermind 20080 points

    The internal clock of the device switches many transistors in a tight window around each clock edge. Other gates typically see signals changing near the clock edge as well. The result is that any active device emits power at clock harmonics. In systems where the electromagnetic emissions from a device are critical, the spread-spectrum clock can be used to smear the power over a wider bandwidth so that a receiver does not see the emissions within a tight bandwidth.

    A web search for EMC and Spread Spectrum PLL will yield a lot of references.

     

    Best Regards,

    Kevin

     

  • 0 in reply to Kevin Lavery
    Intellectual 570 points
    Dear Kevin,

    Thanks a lot for your reply. I search some material for EMC and spread spectrum PLL.
    I have one question. The PLL clock is square waveform to send to other peripherals.
    If we modulate it by triangle waveform, why do the peripherals can work normally?
    Each peripherals include demodulated unit?

    Best Regards,
    Fan
  • 0 in reply to Yang ShunFan
    TI__Mastermind 20080 points

    Fan,

    You might not be picturing this right. The clock is square and the modulation is triangular but these shapes don't really matter. The modulation is the systematic way in which the frequency is changed; the output clock is always square.

    The triangular modulation scheme means that the frequency steps up (or down) a prescribed amount per unit time. Further, when averaged over a modulation period, the clock frequency is identical to the non-modulated average frequency.

    As you change the frequency, it is possible to run the device too fast so you must still observe maximum allowable frequency for the device. Most peripherals respond just fine to the modulated clock:

    as an example of a peripheral that responds well, consider the SPI. Both SPICLK and SPISIMO dither but they do so together. As long as the SPI-slave can work across the frequency range, this works perfectly because it is a synchronous protocol.

    SCI/LIN are asynchronous but slow enough that they typically also work fine. These peripherals are typically fine because the duration of the bit is often longer than the modulation period (which means that the bits see effectively very little modulation).

    CAN can be made to work with a modulated clock but it is easier in a real system to use a non-modulated clock for the CAN. This is why the device gives you this option...

    Kevin

  • 0 in reply to Kevin Lavery
    Intellectual 570 points
    Kevin

    Thanks your reply, it totally clear my confuse.

    Fan