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Using HCS08 TPM Timer Module to generate 1 second interrupt

Question asked by Sergey Pilipchuk on May 13, 2010
Latest reply on Jul 6, 2012 by bigmac



I am trying to use TPM1 Module on MC9S08JM60 to generate an interrupt every second.

I am operating in PEE mode using 12MHz external crystal and bus frequency of 24MHz.

But since 24MHz seems too high to get 1 second interrupt usint TPM I'm using "Fixed system clock" source for TPM which runs at 1.5MHz since RDIV value is 8.

So, I figured, if I set TPM prescaler to divide by 32: 1.5MHz/32=46,875 or 0xB71B, all I need is to set Modulo value to 0xB71B and that should give me one overflow (TOF) per second?


No matter what value I set TPM1MOD to it seems to have no effect on terminal count. I mean, looking at TPM1CNT in emulator it counts up from 0 to about 32767 then counts from -32767 to 0 and repeats???


  TPM1SC_TOF = 0;               //timer has not overflowed
  TPM1SC_CLKSA = 0;          //select fixed system clock (1.5MHz)
  TPM1SC_PS = 0b100;         //clock source divided 16
  TPM1MOD = 0x0FFF;            //set counter modulus (terminal count)
  TPM1SC_TOIE = 1;               //enable TPM1 timer overflow interrupts




  //*** Configure Multi-purpose Clock Generator (MCG) for 24MHz bus frequency
  //Switching from FEI Mode to PEE Mode
  //FEI --> FBE
  MCGC2 = 0x36;           
  while(!MCGSC_OSCINIT);              //loop until crystal has been initialized, OSCINIT = 1
  DisableInterrupts;                             //disable interrupts so no extra steps occur in 1.5MHz range
  MCGC1 = 0x98;                                 //CLKS=10 (Ext. ref); RDIV=011 (12MHz/8=1.5MHz)
  while(MCGSC_IREFST == 1);        //loop until IREFST = 0; Ext. reference is source
  while(MCGSC_CLKST != 0b10);   //loop until external ref. clock is selected to feed MCGOUT
  //FBE --> BLPE
  MCGC2 = 0x3E;                                 //LP = 1; BLPE MODE ON
  MCGC3 = 0x48;                                 //1.5MHz*32=48MHz; PLL is selected (prepared for PBE)
  while(!MCGSC_PLLST);                  //loop until current source for PLLS clock is PLL; PLLST=1
  //BLPE --> PBE
  MCGC2 = 0x36;                                 //LP = 0; BLPE MODE OFF, PBE MODE ENTERED
  while(!MCGSC_LOCK);                   //loop until PLL acquired lock; LOCK=1
  //PBE --> PEE
  MCGC1 = 0x18;                                 //CLKS = 00; output of PLL or FLL is selected
  while(MCGSC_CLKST != 0b11);   //loop until PLL output is selected to feed MCGOUT