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Using an external clock source

Dave Saggs
Intellectual 720 points

Hi,

I am using the TMS570 Microcontroller Development Stick, with a TMS570LS20216 MCU (ASPGEQQ1).

Is it possible to use an external crystal for the NHET clock and the internal oscillator as an input to the PLL for a faster clock source for the main CPU?

I have used the F035 MHzPLL calculator, which gives me a PLL output of 160MHz. My program appears to run quite happily at this speed. Is it safe to run at this speed, or should I drop the speed to 100MHz? I have also found that when using an OSCIN of 16MHz, the minimum NR value seems to be 3, dispite the fact that the Technical Reference Manual (spnu489b), suggests that the minimum value for NR should be 1 (in section 7.11 example note 1). Is the calculator wrong, or is the technical referrence manual out of date?

Regards,
Dave

  • 0
    TI__Genius 13810 points

    1. The limit of frequency for the USB stick is due to the current from PC USB, which is limited to 500mA total. If you run higher than 100MHz and your PC can still supply the current via USB, it is fine.  Make sure not to run LBIST or PBIST at speed because your PC can not supply so much current.

    2. The limit of frequency for the 144 pin package is 140MHz, but we test it with margin. That is why you can run it at higher frequency.

    3.The NHET clock is equal or less than the CPU clock. NHET clock is divided from CPU clock (HCLK). YOu can configure the FMzPLL to be HCLK. You can also select other clock source like FPLL (clock 6), oscillator as your HCLK.

    4. section 7.11 is for FPLL, FlexRay PLL, you can configure it as the HCLK. The PLL calculator you played with is FMzPLL, clock source 1, usually, we configure this clock as the HCLK.

  • 0 in reply to Haixiao Weng
    Intellectual 720 points

    Thank you for your replies Haixiao.

    If I were to use the TMS570LS20216SZWT 337 pin package rather than the USB development stick, would the 140MHz frequency limit also apply to this?

    Is it true to say that I can only use the FMzPLL (to give me different clock speeds to NHET and CPU) with the OSCIN clock input?
    If I were to use the external clock (ECLK) I guess that I could not use this as the source input for the PMzPLL, so the CPU would not be able to run any faster than the external clock speed?

    Regards,
    Dave

     

  • 0 in reply to Dave Saggs
    TI__Genius 13810 points

    Dave Saggs said:

    If I were to use the TMS570LS20216SZWT 337 pin package rather than the USB development stick, would the 140MHz frequency limit also apply to this?

    Then the limit is 160MHz.

    Is it true to say that I can only use the FMzPLL (to give me different clock speeds to NHET and CPU) with the OSCIN clock input?

    FPLL can also give different clock speed. However, if you use FlexRay, then the FPLL has to configured as 80MHz.

    If I were to use the external clock (ECLK) I guess that I could not use this as the source input for the PMzPLL, so the CPU would not be able to run any faster than the external clock speed?

    The purpose of ECLK is to provide a output clock to other device, so that, you can save a crystal or oscillator. So, in application, usually, it is a output on not-used. (of cause you can use it as a GIO input). OSC_IN is the clock input for the  device. The clock on OSC_IN has to be slower than 20MHz.

    Regards,
    Dave