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CLKMODE for External clock or internal OSC at power-up

Nori Shinozaki
Intellectual 2430 points
Other Parts Discussed in Thread: AM1808

Hello,

8.2 PLL Controllers from AM1808 Reference Guide(sprugm9a) says that "the system operates in bypass mode by default and the system clock (OSCIN) is provided directly from an input reference clock (square wave or internal oscillator) selected by the CLKMODE bit in PLLCTL".

However CLKMODE is default to internal oscillator.

If we run an external clock, is it Ok leaving CLKMODE=0?

Regards,

N.Shinozaki

  • 0
    TI__Guru* 83855 points

    Shinozaki-san

    Nori Shinozaki said:
    If we run an external clock, is it Ok leaving CLKMODE=0?

    I think this is ok. In general it is recommended to set CLKMODE=1 when using external clock.

    Setting CLKMODE=1 when using external clock will ensure that internal oscillator is shut down, so it is essentially going to save some power for oscillator shut down,  no other functional issue (I will follow up with design team on this)

    Regards

    Mukul

  • 0 in reply to Mukul Bhatnagar
    TI__Intellectual 2430 points

    Mukul,

    Thank you for your follow up.

    I think so. Lot's of other DSPs have the same design and CLKMODE=0 and we don't hear any issue with using external clock.

    However from the Reference Guide, it looks like we should select one of them.

    "OSCIN is provided directly from an input reference clock (square wave or internal oscillator) selected by the CLKMODE bit in PLLCTL"

    Regards,

    N.Shinozaki

  • 0 in reply to Nori Shinozaki
    TI__Guru* 83855 points

    Shinozaki-san

    Nori Shinozaki said:

    However from the Reference Guide, it looks like we should select one of them.

    "OSCIN is provided directly from an input reference clock (square wave or internal oscillator) selected by the CLKMODE bit in PLLCTL"

     

    I think the documentation is a bit misleading in this respect. Setting CLKMODE to 0 or 1 has  no impact on the PLL controller functionality  except for the negligible power wasted keeping the internal oscillator enabled. I reconfirmed this from the design team.It is still a good practice to set CLKMODE to 1 to disable internal oscillator when sourcing an external square wave.

    Regards

    Mukul