MPC55xx,FMPLL,Loss-of-clock

[ypchen]

Hello NXP:

MPC55xx,External Reference Mode-- 16MHz , and the system clock is 132MHz.

The other SYNCR's configuration is as follows:

FMPLL_SYNCR[LOCEN]  = 0;

FMPLL_SYNCR[LOLRE]  = 0;

FMPLL_SYNCR[LOCRE]  = 0

If the external  clocks fall below the threshold frequency, what the system will happen ?  stop ,or enter self-clock mode ?

[davidtosenovjan]

Hi, Regarding loss-of-lock (frequency changes out of specified range) :

While the FMPLL is in an unlocked condition, the system clocks continue to be sourced from the FMPLL as the FMPLL attempts to re-lock. Consequently, during the re-locking process, the system clock frequency is not well defined and may exceed the maximum system frequency thereby violating the system clock timing specifications. Thus it is potentially dangerous configuration and it is recommended to have enabled reset reaction.

Regarding loss-of-clock (clock dissapears):

Loss of reference frequency is defined as the reference frequency detected internally, which transitions the PLL into self-clocked mode. Again it is recommended to have enabled reset for this event.

[ypchen]

Hi, 

In the  Reference Manual : 11.4.2.6.1 Alternate/Backup Clock Selection
"If the user enables loss of clock by setting FMPLL.SYNCR[LOCEN] =1, then the FMPLL will transition
system clocks to a backup clock source in the event of a clock failure as per Table 11-9."

My question is:  If Loss-of-clock disabled ( FMPLL.SYNCR[LOCEN] =0),and then the external clock (16MHz) dissapear, the FMPLL will also enter self-clock mode (SCM) ? or what will happen? It seems that the content is not explained in the manual.

[davidtosenovjan]

If FMPLL.SYNCR[LOCEN] =0, than lock-of-clock is not functional thus it does not detect any issue and MCU does not run.

[ypchen]

Hi，David :

External Reference Mode(16MHz),  system clock is 132MHz , and  FMPLL.SYNCR[LOCEN] =0 .

First ,  power-up the board , the MPC5554 run @ 132MHz , and then I disabled the OSC (external clock  dissapear)   , but the  MCU  is still running ,the system clock is lower (~62MHz) .

Why ?

[martin_kovar]

Hello,

could you please clarify, which microcontroller you use?

Regards,

Martin

[ypchen]

hello:

the microcontroller is MPC5554MZP132.