Add troubleshooting note about fixing locked up core

README.md

@@ -68,3 +68,60 @@ softdevice has to flashed to the chip.
   [nrf-recover](https://docs.rs/crate/nrf-recover/latest) utility
 - [Pinning to use nRF52DK as programmer](https://devzone.nordicsemi.com/f/nordic-q-a/14058/external-programming-using-nrf52-dk)
 - For examples of how to use embassy to program nRF chips, see https://github.com/embassy-rs/embassy/tree/main/examples (and look for nrfXXX)
+
+## Troubleshooting
+
+### Core locking up
+
+One possible cause for the core to lock up is to have a flash memory
+configuration that doesn't match the selected softdevice.
+
+#### Background
+
+When the device powers up, it looks for certain things at certain positions
+in memory, namely the interrupt vector table and the program entrypoint.
+The interrupt vector table and entrypoint it finds are from the softdevice.
+After initialization, the softdevice then attempts to hand over control to the
+user program. The user program again has an interrupt vector table and an
+entrypoint that are expected to be at a certain position in memory.
+Specifically, they are expected to be right after the softdevice. If they are
+not there, the device enters a hard fault state and locks up the core.
+
+This means that the flash memory configuration in `memory.x` has to be correct
+for the program to start up properly:
+
+```
+MEMORY
+{
+  FLASH : ORIGIN = 0x00000000 + <softdevice size>, LENGTH = <flash memory size> - <softdevice size>
+  ...
+}
+```
+
+The softdevice size can be found in the release notes of the softdevice. This
+is a pdf that is downloaded along with the hex file from the nordic website.
+The flash memory size depends on the model of microcontroller used.
+
+#### Solution
+
+1. Make sure `memory.x` is configured correctly for the selected softdevice. E.g., for `S112` running on an `nRF52832` chip:
+    ```
+    MEMORY
+    {
+      FLASH : ORIGIN = 0x00000000 + 100K, LENGTH = 512K - 100K
+      RAM : ORIGIN = 0x20000000 + 30K, LENGTH = 64K - 30K
+    }
+    ```
+2. Recover the device from it's locked state. This erases the flash memory:
+    ```
+    $ probe-rs erase --chip nRF52832_xxAA --allow-erase-all
+    ```
+4. Compile and flash the program:
+    ```
+    $ cargo run --bin blinky --release
+    ```
+    The program will not run successfully at this time, as it requires the softdevice to be flashed as well.
+2. Flash the desired softdevice, e.g., `S112`:
+    ```
+    $ probe-rs download --verify --binary-format hex --chip nRF52832_xxAA s112_nrf52_7.3.0_softdevice.hex
+    ```