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docs/Customization/Firrtl-Transforms.rst

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+.. _firrtl-transforms:
+
+Adding a Firrtl Transform
+=========================
+
+After generating a ``.fir`` file from the Chisel generator, this ``.fir`` file is passed to
+FIRRTL. FIRRTL then goes ahead and modifies the ``.fir`` IR with a series of tranforms that can modify
+the circuit and then converts it to Verilog.
+
+Where to add transforms
+-----------------------
+
+The main location to add tranforms is within the ``tools/barstools`` project.
+Inside the ``tranforms`` package, the FIRRTL compiler is called twice,
+first on the top-level circuit, then on the test harness. This is because we want to separate out
+the harness and all the modules associated with it from the top-level design under test.
+
+If you want to add transforms to just modify the DUT, you can add them to ``topTransforms``.
+
+If you want to add transforms to just modify the test harness, you can add them to ``harnessTransforms``.
+
+Examples of transforms
+----------------------
+
+There are multiple examples of transforms that you can apply and are spread across the FIRRTL ecosystem.
+Within FIRRTL there is a default set of supported transforms located in https://github.com/freechipsproject/firrtl/tree/master/src/main/scala/firrtl/transforms.
+This includes transforms that can flatten modules (``Flatten``), group modules together (``GroupAndDedup``), and more.
+
+Transforms can be standalone or can take annotations as input. Annotations are FIRRTL specific ``json`` files that
+are used to pass information into FIRRTL transforms (e.g. what modules to flatten, group, etc). Annotations can be added to the code by
+adding them to your Chisel source (which will generate the ``json`` code for you) or by creating the ``.json`` file and adding it to the FIRRTL compiler.
+
+Here is an example of grouping a series of modules by specifying the ``.json`` file:
+
+.. code-block:: json
+
+   [
+       {
+           "class": "firrtl.transforms.GroupAnnotation",
+           "components": [
+               "BeagleChipTop.BeagleChipTop.instance1",
+               "BeagleChipTop.BeagleChipTop.instance2",
+           ],
+           "newModule": "NewModule",
+           "newInstance": "newModInst"
+       }
+    ]
+
+
+Once created then you can add this to the FIRRTL compiler by modifying ``common.mk`` and adding the particu
+
+.. code-block:: none
+
+	-faf yourAnnoFile.json
+
+
+
+If you are interested in writing FIRRTL transforms please refer to the FIRRTL documentation located here:
+https://github.com/freechipsproject/firrtl/wiki.
+
+The main Chipyard tranforms that are applied to the top and are located in the ``tools/barstools``.
+
+- Talk about .json annotations
+