Spread spectrum of i.MX6Dual.

[takashitakahash]

Hi community.

Our customer has question below.

They are  use the Spread Spectrum in the System PLL.

Setting are below.

CCM_ANALOG_PLL_SYS_SS =0x00DC8001 (stop=220,step=1)

CCM_ANALOG_PLL_SYS_DENOM=0x000003E8 (DENOMB=1000)

Spread spectrum range=5.28MHz (1% of 528MHz )

Frequency change step=24KHz

Modulation frequency=54.55KHz

Was set as described above, by using a timer of  GPT, the period of output pulse from the GPIO becomes than 2.5% longer before setting.

This 2.5% became longer is correct  ?

  Please tell me the reason and calculation method to be 2.5%, also what kind of rounding in this case ?

Thnks,

Best.

[igorpadykov · Accepted solution]

Hi Takashi

seems GPT uses Peripheral Clock or High Frequency Reference Clock

as sources please check Figure 18-2 i.MX6DQ RM, they are produced from

system pll. It is not possible to give formula for error, as this depends on

many factors. Please try to use as GPT source clock another frequency

(not affected by system pll), for example Low Frequency Reference Clock

or Crystal oscillator as Reference Clock, selected by field CLKSRC

in register GPT_CR.

Best regards

igor

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[takashitakahash]

Hi igor.

Thank you for your replay.

The following comments came from customers.

GPT clock use from ipg_clk.

In this case,  when Spread Spectrum is Disable comparison with the Enable, Then Enable changes the 2.5% .

The cycle of the pulse output from the GPIO was not changed by the Spread Spectrum if use a low-frequency clock.

For example Spread Spectrum Range change to 1%,2%,,,,,, of 528MHz,

In this case ,do you know easy calculation method  the period of the timer is how to change.