CCS/F28M35H52C: LOW SPEED ACCURACY USING QEP?

Part Number:F28M35H52C

Tool/software: Code Composer Studio

The precondition is : encoder is 2500 lines and MCU is F28M35H52C1.

We use qep to test a pmsm motor,we find the speed accuracy when low  is very bad : +/-20rpm for 50rpm.

So we want to know what is the low speed accuracy for this IC using qep? do you have example code or inner block for qep?

Thanks for your help

Longfei