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TMS320F8335 SPUFB0D PLL Ambiguity

Angelo Dattilo58579
Prodigy 30 points

Hi

I'm using TMS320F8335 microcontoller. On my board I 'm using a 25 Mhz external oscillator clock. Now I must set PLL register to set the cpu CLKIN.

On  page 43 I find this :

"PLL enabled mode
When the PLL control register is non-zero (PLLCR = n, where n ≠ 0x0000), the PLL is enabled. In this
mode, OSCCLK*n, OSCCLK*n/2, or OSCCLK*n/4 is connected to CLKIN of the CPU. If OSCCLK is
detected as missing, the missing clock detect status bit will be set and the device will generate a
missing clock reset. The device will now run at one-half of the PLL limp mode frequency."

So if PLLCR[DIV]= 6 and PLLSTS[DIVSEL]=3   CLKIN=(25 *6)/1=150 Mhz

But on page 41 I have found table 20 where is written :

PLL Enabled         Achieved by writing a non-zero value n into the PLLCR register. Upon                             0, 1 OSCCLK*n/4
                                writing to the PLLCR, the device will switch to PLL Bypass mode until                              2 OSCCLK*n/2
                                the PLL locks.

So it seems I can't set the THE PLLSTS[DIV]=3 and PLLCR[DIV]= 6 to obtain a CLKIN =150Mhz

Can somebody explain me which is the wrong configuration and which the correct?

Can I set PLLSTS[DIVSEL]=3  and PLLCR[DIV]= 6?

Thanks

Angelo

  • 0
    TI__Expert 3760 points

    Angelo -

    The correct answer is that, on TMS320F2833x/F2823x devices, PLLSTS[DIVSEL] = 3 (/1 mode) should NOT be used.  The reason for this is in the data-sheet 4th note under Table 3-16. PLL Settings:

    (4) A divider at the output of the PLL is necessary to ensure correct duty cycle of the clock fed to the core. For this reason, a DIVSEL value
    of 3 is not allowed when the PLL is active.

     Basically, since the 2833x/2823x devices run at such a high frequency, if there is no PLL divider (via the DIVSEL bits) of at least /2, the system clock duty cycle will not be maintained properly - which will cause timing issues in the core and for certain timing-sensitive peripherals.

    Note: The data sheet is always the "golden" resource when compared to the user guides.  I agree that the statement in SPRUFB0 is ambiguous and will mark that as something to be updated.