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:
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