how to configuratethe eQEP interrupt in 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.

thanks.