DFU from application

Question

Hello Expert, 
 I am using SDK 6.1 and Softdevice 7.1.0 (s110) for our 256K/32K nrf51822 platform. As of now I wanna port DFU feature into our project. Of course, when burnt with bootloader, nRF toolbox can find device and upgrade application correctly. But you know, I dont wanna add DFU service in my application since the limitted flash memeory size and extra BLE power consuming. if so, please guide me how to enter DFU mode in my application. AFAIK, I must handle uncompleted flash operation and ble disconnection, right? And then, any points to be shared with me? Directly excute NVIC_SystemReset? But how does bootloader know it's from DFU not WDT or power-up? 
 Another question I found is that when my Android phone installed both nRF toolbox and nRF UART application, DFU APP cannot connect nrf51822 DFU service, it will disconnect unexpectedly in a very short time. My Android version is Lollipop. Any clues? 
 Thanks,
Alex

Hung Bui · Accepted Answer

@Alex Zhang: Yes, you should set IS_SRVC_CHANGED_CHARACT_PRESENT to 1 for both application and bootloader. However this one would have no effect if the device is not bonded.
But to do that you would need to share bond information between the application and bootloader.
You can refer to our button-less example in our SDK. Our approach doesn't do a NVIC soft reset but jump directly to the bootloader.
Another solution you can use is to change the address of the device to another address, so that the phone will treat it as a new device. This is what we do when the device is started directly in bootloader (with a button press for examle).
This way you can get away with the ATT cache issue. But the link when doing DFU is not encrypted. 
 If you want to use an app to tell the device to restart in bootloader mode. You just have to define a "enter bootloader" command, and use a write command to write to one of your characteristic to trigger the "restart to bootloader".