AnsweredAssumed Answered

Spread Spectrum i.MX7 VIDEO_PLL

Question asked by marcfigliuolo on Feb 26, 2019
Latest reply on Mar 18, 2019 by Artur Petukhov

Hello There

 

I try to use the spread spectrum function of the i.MX7 to modify the frequency of the VIDEO_PLL. 

First of all:

Q1: Are the formulas noted in the i.MX7 Reference-Manual correct?

 

 

 

My measurements on the output-clock have shown, that changes on the parameters (stop, step, B) behave more like the formulas noted in I.mx6 app. manual:
1) Modulation frequency = Fref * STEP/2*STOP

2) Max frequency change = STOP/DENOM * Fref

 

Q2: Which formulas should I take for my calculations i.MX7 or i.MX6?

 

Further, one can see that values with the same results for equation 1) and 2) behave different when measured:

Let’s say one want to spread the clock 1% of its original value.

The clock is set to 650 MHz -> fRange = 6.5 MHz and fmod = 500kHz.

So, when for example the parameters were set up like:

 

Setup

DEN

STEP

STOP

fmod

fRange

A

12.000E+3

136

3250

502.2E+3

6.500E+6

B

15.000E+3

170

4063

502.1E+3

6.500E+6

 

One can see that the center frequency of the measured clock-output is not the same as defined without frequency spreading.

And even worse the center frequency of parameter setup A differs from setup B.

 

Q3: Are there any constrains on how the parameters can be set?

 

Q4: Why does the center frequency of the modulated output-clock changes?

 

Best regards Marc

Outcomes