PLL Frequency Maximum? --> Datasheet vs. system_XMC4700.c

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User13334
Level 2
Level 2
Hello forum members and all of you first a nice St. Nicholas day.

The below command is out from the above mentioned c-file



/*******************************************************************************
* Default clock initialization
* fPLL = 288MHz => fSYS = 144MHz => fCPU = 144MHz
* => fPB = 144MHz
* => fCCU = 144MHz
* => fETH = 72MHz
* => fUSB = 48MHz
* => fEBU = 72MHz
*
* fUSBPLL Disabled, only enabled if SCU_CLK_USBCLKCR_USBSEL_USBPLL is selected
*
* fOFI = 24MHz => fWDT = 24MHz
*******************************************************************************/



In the demo from Infineon the PLL Setup was realized with the Parameter




#elif OSCHP_FREQUENCY == 12000000U
#define PLL_PDIV (1U)
#define PLL_NDIV (47U)
#define PLL_K2DIV (0U)




If I made the calculation I also get the 288 MHz for the PLL frequency

But in the Manual XMC4000 Family V1.0 2016-01 in the Cappter 3.3.4 Phase Locked Loop (PLL) Characteristics

The MAX. PLL base frequency is 140 MHz is this OK, in my opinion no.

But also in the DAVE app and in the examples you can set this frequency or I miss something

Thanks for your Help

With best regards

EbbeSand
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2 Replies
User13334
Level 2
Level 2
Possibly written too complicated

In the Manual the Maximum base PLL fequency is 140 MHz. (see Picture)

3345.attach

Why then, in most examples, are PLL operated at 288 or 144 MHz?

Table 11-5 (see Picture below) in the reference manual indicate the possible
dividers for the different clocks. There is always mentionted of 144 MHz.

3346.attach

Therefore, once again, my question we reach 144 MHz
when the maximum pll frequency is 140 MHz.

Where is my thinking error.

Best regards

EbbeSand
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User13334
Level 2
Level 2
So I think I found my mistake and now understood at what points in the data sheet are importent for answer my question


  • pll base frequency
    Without a clock Input fOSC, the PLL gradually slows down to its VCO base frequency and remains there
  • the VCO frequency range
    min. 260 MHz max 520 MHz --> This correspondence with the pll frequency 288 MHz from the examples
    The assumption with the 144 MHz is therefore wrong one must operate the PLL always larger or equal 260 MHz.
    And that's 288 MHz a good choice.


One should first understand before asking too fast questions that one can answer to oneself

Best regards,

EbbeSand
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