i am using the flash 256K and ram 32k of NRF51822. the examples of SDK12.1.0 is includes dfu/bootloader_secure and ble_peripheral/ble_app_uart. when i was used in the software of nrfgo studio to download the two examples, I can not used the "DFU" to download the XXX.zip. please tell me this is why i can not used "DFU" in cellphone to download XXX.zip, I am not changed SDK.