it can't boot to dfu mode.

[xaqfan]

I have a Nordic 52840 hardware, which comes with the bootloader. Learned that the register GPREGRET address is 0x4000051C. The app is written in rust language, and the address is assigned 0xB1 directly in the app. But  it can't  enter the dfu mode.
code show as below:

let gpregret = 0x4000051Cusize;
         let pa = gpregret as * mut i32;
         unsafe {
             * pa = 0xB1;
         };

Am I writing it wrong?

thank you!

[xaqfan]

My purpose is that HW enter DFU mode with button less by program some code in app.

Learned that set  GPREGRET register's value 0xB1 ,and bootload enter DFU mode in office sdk.

But it doesn't work.