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MC9S08DZ128 PLL ocasionally fails to lock/select on reset (4MHz xtal, 20MHz bus).

Question asked by Juan Luis Alvarez on Jul 30, 2015
Latest reply on Aug 25, 2015 by Juan Luis Alvarez

I am currently having problems with a MC9S08DZ128 based board where the MCU fails to boot one out of ten times. The code generated by processor expert gets stuck in the following `while` loop, trying to wait for PLL:


  clrReg8Bits(MCGC3, 0x10U);      

  /* MCGC3: LOLIE=0,PLLS=1,CME=0,DIV32=0,VDIV=5 */

  setReg8(MCGC3, 0x45U);           /* Set MCGC3 register */
  while(MCGSC_PLLST == 0U) {       /* Wait until PLL is selected */



Often times the startup code fails to lock, getting stuck in the follwoing `while` loop:


  clrReg8Bits(MCGC2, 0x08U);      
  while(MCGSC_LOCK == 0U) {        /* Wait until PLL is locked */



Whenever the MCU gets stuck on this place, a power-off/power-on or triggering the RESET pin tends to "fix" the issue by giving the MCU a chance to try again.


If I change to using FLL instead (16MHz bus frequency) the MCU starts just fine every time.