The timer interrupt period inaccurate??

hi dear all,

 I have a question about the interrupt period accuracy.

I am using 28035, and use one gpio to output a pwm signal. (please not  persuad me to use the epwm module)

here is part of my code:

=====================================================================

interrupt void servo_ISR(void)//2015.5.10 last modified for control servo, output pulse

{

     switch(GpioDataRegs.GPADAT.bit.GPIO10)

     {

     case 0:/*when the output is low*/

         CpuTimer2Regs.PRD.all=(long)60*1500;/*GPIO10 on for 1500usec*/

         CpuTimer2Regs.TCR.bit.TRB = 1    ;/*reload interrput period*/

         CpuTimer2Regs.TCR.bit.TSS = 0;    /* 1 = Stop timer, 0 = Start/Restart Timer*/

         GpioDataRegs.GPADAT.bit.GPIO10=1;

         break;

     case 1:/* when the output is high*/

         CpuTimer2Regs.PRD.all=(long)60*15000; // GPIO10 off for 15msec;

         CpuTimer2Regs.TCR.bit.TRB = 1;                //reload interrput period

         CpuTimer2Regs.TCR.bit.TSS = 0;             // 1 = Stop timer, 0 = Start/Restart Timer

GpioDataRegs.GPADAT.bit.GPIO10=0;

         break;

     }/*end of case*/

}/*end of servo_ISR *

=========================================================================

the initialize of CpuTimer.etc.. are done in main(), so let us pass it.

and  ConfigCpuTimer(&CpuTimer2, 60, 20000); is set in main().

and the output wil be:

however according to my design, the pos width should be 1500sec.

and I try different value of pulse width and make plot:

the red dot are the pulse width value i got, and black are predict value. with different interrupt perion desire

and the red line is the linera fitting, which slope is 0.9829.

So the value 0.9829/60(usec) is my question, it means how much time the timer2tim decrement by one, which ideally should be 1/60(usec)

so either the the timer is faster, or the system clock greater than 60MHz?

Or my code is wrong??