How do we use the external Flash loader in STM32 link utility?... We already have/created a separate hex file for ext flash..
But, to flash that via ST-Utility, How do we specify the start address ? and how do we do for custom ext flash chips?
In the ST-Link utility installation directory you'll find a ExternalLoader subdirectory with examples and sources. There are hints how to do in the documentation (UM0892 Developing custom loaders for external memory).
Developing custom loaders for external memory
The header inside the External Loader modules indicates the address the memory resides in, and the .HEX should provide the addresses as constructed by the Linker.
For custom boards and chips you'll have to code a loader, this in my opinion is a non trivial task requiring significant skill and experience. You should build your own flashing code within an application, and test that by processing .HEX file streams or X-MODEM. Once you've managed the external memory there port the functionality into the "external loader" model/framework. You're options testing/debugging the latter are very limited, outputting telemetry via a serial port being the primary route to understand what happens internally.
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