Buttonless DFU Error 8


I have been testing unbounded buttonless DFU with BL652 using SDK 15.0 and SDK15.2 and I kept getting Error 8(0x8): GATT CONN TIMEOUT after bootloader is entered.I traced back the problem: The device restarts itself after NVIC_SystemReset() in shutdown_process() is called. I am using Android phone and latest version of NRF Connect. 

The buttonless Secure_Bootloader_BLE is tested to be working on its own. I followed the procedures to generate the setting and merge with bootloader. The things I modified are:

    1. The software device clock source changing them to RC source with CTIV 16 and TEMP_CTIV 2. 

    2. The LEDs and Buttons mapping in pca10040.

I dont know how to proceed furthur to solve this problem. If anyone can shine some lights on the issue I will greatly appreciate it. 

Thank you in advance