Add README.md for flashing instructions

README.md

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+# Testing Firmware for the Sinara 1550 Kirdy
+
+- This repo is for testing only. Not intended for production use.
+
+## Building
+
+### Reproducible build with Nix
+
+kirdy firmware is packaged using the [Nix](https://nixos.org) Flakes system. Install Nix 2.4+ and enable flakes by adding ``experimental-features = nix-command flakes`` to ``nix.conf`` (e.g. ``~/.config/nix/nix.conf``). 
+
+Once you have Flakes enabled, you can use ``nix build`` to build the firmware.
+
+### Development environment
+
+Clone this repository and with Nix Flakes enabled, use the following commands:
+
+```shell
+nix develop
+cargo build
+```
+
+The resulting ELF file will be located under `target/thumbv7em-none-eabihf/release/kirdy`.
+
+Alternatively, you can install the Rust toolchain without Nix using rustup; see the Rust manifest file pulled in `flake.nix` to determine which Rust version to use.
+
+## Debugging
+
+Connect SWDIO/SWCLK/RST/GND to a programmer such as ST-Link v2.1. Run OpenOCD:
+```shell
+openocd -f interface/stlink.cfg -f target/stm32f4x.cfg
+```
+
+You may need to power up the programmer before powering the device.
+Leave OpenOCD running. Run the GNU debugger:
+```shell
+gdb target/thumbv7em-none-eabihf/release/kirdy
+
+(gdb) source openocd.gdb
+```
+
+## Flashing
+There are several options for flashing kirdy. DFU requires only a USB-C connector, whereas OpenOCD needs a JTAG/SWD adapter.
+
+### dfu-util on Linux
+* Install the DFU USB tool (dfu-util).
+* Convert firmware from ELF to BIN: `arm-none-eabi-objcopy -O binary kirdy kirdy.bin` (you can skip this step if using the BIN from Hydra)
+* Connect to the USB Type C connector to kirdy above the RJ45.
+* Add jumper to kirdy v2.0 across 2-pin jumper adjacent to JTAG connector.
+* Cycle board power to put it in DFU update mode
+* Push firmware to flash: `dfu-util -a 0 -s 0x08000000:leave -D kirdy.bin`
+* Remove jumper
+* Cycle power to leave DFU update mode
+
+### st.com DfuSe tool on Windows
+On a Windows machine install [st.com](https://st.com) DfuSe USB device firmware upgrade (DFU) software. [link](https://www.st.com/en/development-tools/stsw-stm32080.html).
+- add jumper to kirdy across 2-pin jumper adjacent to JTAG connector
+- cycle board power to put it in DFU update mode
+- connect USB Type C to PC
+- use st.com software to upload firmware
+- remove jumper
+- cycle power to leave DFU update mode
+
+### OpenOCD
+```shell
+openocd -f interface/stlink.cfg -f target/stm32f4x.cfg -c "program target/thumbv7em-none-eabihf/debug/kirdy verify reset; exit"
+```