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Spread spectrum (RF Desense) on i.MX53

Question asked by Martin Fuzzey on Aug 8, 2014
Latest reply on Aug 10, 2014 by igorpadykov

Hi,

 

I have enabled spread spectrum on the i.MX53 for DPLL2  for EMC reasons.

This does reduce the RF emissions but I am now observing clock drift (9 seconds in 10 minutes)

 

Running mainline linux 3.13 or 3.16 with custom patch to configure spread spectrum.

 

The nominal PLL frequency is 400 MHz

The spread spectrum parameters being used are:

 

MFD=383

MFI=8

MFN_MIN=352

MFN_MAX=-96

PDF=2

fREF=24MHz

Toggle count = 10

 

According to the calculations that should give a frequency switching between 372 MHz and 428 MHz (the average being exactly 400 MHz)

 

So,  I would not expect this to affect the system timer stability but it does:

 

Set time and Synchronize system and RTC:

# date -s "2014-07-31 14:11:00"; hwclock -w

Thu Jul 31 14:11:00 UTC 2014

 

Display initial time

# date;hwclock -r

Thu Jul 31 14:11:05 UTC 2014

Thu Jul 31 14:11:06 2014  -0.382017 seconds


Wait 10 minutes, measured from time set (so should be 14:21:00)

 

Re read times

 

# date;hwclock -r

Thu Jul 31 14:20:51 UTC 2014

Thu Jul 31 14:21:01 2014  -0.807810 seconds

 

So RTC has advanced 10 minutes as expected but system clock (based on i.MX53 GPT) only 9mins 51 seconds

 

The problem no longer occurs without spread spectrum activated.

 

Any ideas?

Has anyone else tried this?

 

Regards,

Martin

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