the eQEP interrupt problem of DSP 28335

I want  to use the eQEP interrupt::IEL  Index event latch interrupt enable;

the configuration as follows:

EQep1Regs.QEINT.bit.IEL=1;      

EQep1Regs.QEPCTL.bit.PCRM=00;   /*  Position counter reset on an index event  */

the IEL interrupt can be triggered and interrupt is responsed;

but when I change the PCRM:

EQep1Regs.QEPCTL.bit.PCRM=01;   /*  Position counter reset on the maximum position  */

the eQEP interrupt can not be triggered. The same problem occur when  PCRM be set 10 or 11;

in tests other registers except EQep1Regs.QEPCTL.bit.PCRM remain unchanged.

 the following test :

first .  set the value of EQep1Regs.QEPCTL.bit.PCRM  to 0, the eQEP's Index event latch interrupt  occurs.

second. set the value of EQep1Regs.QEPCTL.bit.PCRM  to 1 or 2,3 ,the eQEP's Index event latch interrupt  could not occur,

third. reset the value of EQep1Regs.QEPCTL.bit.PCRM  to 0, the eQEP's Index event latch interrupt  occurs again.  

Who can help me solve this problem. what's wrong with my configurations.