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AM5716: How to change the frequency of A15 arch timer

Zhengxin Rui
Intellectual 290 points
Part Number: AM5716


Hi all,

The A15 core has a arch timer,

Linux boot message shows the frequency of this timer is 6.14MHz, the precise value read from cp15 CNTFRQ register is 6147541. 

In our application, the frequency must set to the multiple of 1KHz.

omap_smc1(0x109, 6140000) is added in init_cpu_configuration function of u-boot code according the AM571x TRM 33.4.5.

The read result of cp15 CNTFRQ register is the new value 6140000.

We measured the real frequency of the arch timer, it was not channged to 6140000 actually.

How to change the frequency of A15 arch timer?

Can anyone give me help, thanks.

Best Regards.

  • 0
    TI__Guru**** 164670 points

    Hi Zhengxin,

    I will check with the experts and get back to you on this.

    Best Regards,
    Keerthy

  • 0 in reply to Keerthy J
    TI__Guru**** 164670 points

    Hi Zhengxin,

    Here is the response from our expert:

    Yes from a spec perspective its possible to change the frequency but not in the way tried.  The CNTFRQ is informational only for things like ROM code it doesn’t affect the clocks.  The change would have some system wide effects which may or may not be an issue.

    • ·        There is an issue here in that if you target a value which doesn’t mesh well with clock correction circuit (based on FUNC_32_CLK rate) then time will drift and this might cause more issues than whatever % you are off of their 1KHz using MPU based dividers.

    o   Why can’t we just use a divided value of 6.14MHz?

    • ·        The A15 timer is derived from either the SYS_CLK or the ABE_LP_CLK (Aka: WKUPAON_ICLK).  If you select the ABE DPLL for the source of the WKUPAON_ICLK then the MPU timer should then tick a new rate.

    o   If you do this, the L4 will now run with different timings causing some possible fan out effects

    o   If they do this they must choose careful values for the master counter to avoid drift.

    Best Regards,
    Keerthy

  • 0 in reply to Keerthy J
    Intellectual 290 points

    Hi Keerthy,

    Thanks for your help.

    We'll try it or change the system timer to TIM module of SOC.

    Best Regards,

    Zhengxin