I followed this blog entry ( devzone.nordicsemi.com/.../) setup IAR to debug in monitor mode (by adding Jlink_monitor.c, jlink_monitor.h, jlink_monitor_isa_iar.s, add EnableMonMode.mac etc..) . It has improved (I can hit breakpoint after ble_stack_init and step over) without restarting.... But when I hit breakpoint on power_manage(); and step over, then system restarting again.
I used C:\allsrc\nRF5_SDK_12.0.0_12f24da\examples\ble_peripheral\ble_app_hrs example code. and selected IAR workspace: ble_app_hrs_pca10040_s132 - nrf52832_xxaa