Hi,

I'm, using an iMX7D SoC with 30Mhz clock generated by the video PLL and attempt to activate the spread spectrum mechanism in the video PLL.

I rely on the 2 formulas presented in the iMX6 reference manual as well as in posts here. The 2 are:

1) Spectrum spread range = STOP/DENOM * Fref

2) Modulation frequency = Fref * STEP/2*STOP

I use the following registers:

CCM_ANALOG_PLL_VIDEO_DENUM 0x30360160

CCM_ANALOG_PLL_VIDEO_SS 0x30360140

and perform the following writes:

write 0x00000000 to Bit 15..15 of 0x30360140 //ENABLE=0x0

write 0x00001770 to Bit 0..29 of 0x30360160 //DENUM=0x1770

write 0x000061A8 to Bit 16..31 of 0x30360140 //STOP=0x61A8

write 0x00000006 to Bit 0..14 of 0x30360140 //STEP=0x6

write 0x00000000 to Bit 15..15 of 0x30360140 //ENABLE=0x1

From testing I've done I do not see a correlation between formula #1 and actual measured results.

Using above values I measure apx. 5MHz spectrum spectrum while using the formula I receive an unreasonable figure for a 30Mhz clock. (using Fref=24Mhz, Spectrum spread range = (25000/6000) * 24Mhz = 96Mhz)

I have played around with different values and clock frequencies and see that increasing STOP or decreasing DENUM does in fact increase the spread spectrum range but do not seem to find a correlation to the formula.

Any help on this issue or explanation of the relation between these values (DENUM/STOP/STEP) will be appreciated.

Thanks

Since you use unreasonable STOP value, you see unreasonable result on the PLL output. In your case, the formally calculated spread spectrum range exceeds the PLL output frequency, that has no meaning. For normal operation, the spread spectrum range must be no more than some units of percents of target output frequency. So, try to use some more realistic STOP value that meets the formal calculation criteria.

Have a great day,

Artur

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