AM625: PLL Spread Spectrum Modulation

Hi,

My customer wants to use PLL Spread Spectrum Modulation (SSMOD) on AM625x.
The customer referred to TRM section 6.4.5.4.2 and tried to apply SSMOD for PLL16 (used for LVDS source).
What he did were below steps.
1) Write 0x0005001F to address 0x00690044(pll16_SS_SPREAD)
2) Write 0x00000000 to address 0x00690040(pll16_SS_CRL)

But he could not observe any frequency changes in LVSD clock.
Could you tell me how to enable SSMOD for PLL16?

Thanks and regards,
Koichiro Tashiro