CC2640R2F: How Read Register of reset ?

Part Number: CC2640R2F

Hi 

I want to read  Register of "AON_SYSCTL:RESETCTL.RESET_SRC " to see reason of reset my device . i try this but not get reasonable result .

 reset=HWREG(AON_SYSCTL_RESETCTL_RESET_SRC_PIN_RESET) ;

 or

 reset=HWREG((AON_SYSCTL_O_RESETCTL);