clarify Xilinx path, flake

src/hw/mirny_almazny.md

@@ -15,6 +15,49 @@
 }
 ```
 
+## Building firmware
+
+Once you get your hands on the firmware, you will need to work around few shortcomings of Nix, mainly not being able to run dynamically linked executables.
+
+You will need:
+- Xilinx ISE 14.7 installed on your system (this guide is assuming ``/opt/Xilinx`` path),
+- an environment with Migen.
+
+One way to do it is to create an FHS environment, like ARTIQ does for Vivado, within ARTIQ's ``flake.nix`` (to leverage Migen already being there), by adding these lines:
+
+```
+iseEnv = pkgs.buildFHSEnv {
+  name = "ise-env";
+  targetPkgs = vivadoDeps;
+};
+
+ise = pkgs.buildFHSEnv {
+  name = "ise";
+  targetPkgs = vivadoDeps;
+  profile = "set -e; source /opt/Xilinx/14.7/ISE_DS/settings64.sh";
+  runScript = "ise";
+};
+```
+
+Add them below ``vivadoEnv``. Then add ``iseEnv`` and ``ise`` to the dev shell's build inputs. Call ``nix develop`` on that.
+
+Then you can build Mirny:
+
+```shell
+nix develop
+ise-env
+cd ../mirny  # or wherever your source is at
+source /opt/Xilinx/14.7/ISE_DS/settings64.sh
+python mirny_impl.py
+```
+
+### Flashing
+
+```shell
+nix-shell -p xc3sprog
+xc3sprog -c jtaghs2 -m /opt/Xilinx/14.7/ISE_DS/ISE/xbr/data -v build/mirny.jed
+```
+
 ## Testing
 
 ### Without Almazny
@@ -49,7 +92,7 @@ After running `artiq_sinara_test`:
 
 ![](../img/mirny_gqrx.png)
 
-### With Almazny
+### With Almazny (ARTIQ 7)
 
 At first, `artiq_sinara_test` will prompt you for testing Mirnies as the would be without Almazny.
 After that, it will prompt you with testing the Almazny:
@@ -98,3 +141,5 @@ You should also see differences in various modes, but that may require disabling
 ### Tips
 
 ~~Mirnies often fail `ValueError: MUXOUT not high`, in that case restart the tests or reboot the board(s).~~ - fixed in [9569cfb](https://github.com/m-labs/artiq/commit/9569cfb26329c0acdc1705d3256d2506b7bccce5)
+
+For Almazny v1.2+ support, CPLD firmware above 0.3.0 (with fixes) must be flashed onto Mirny.