Structure
=========

Directories
-----------

* `XXX/device/` - Full architecture definitions of a given device for
  [Verilog To Routing](https://verilogtorouting.org/)

  * `XXX/device/YYYY-virt` - Verilog to Routing architecture definitions
    generally are not able to able to generate the **exact** model of many
    FPGA routing interconnects, but this is a pretty close.
* `XXX/primitives/` - The primitives that make up the architecture. These
  are generally used inside the tiles.
* `XXX/tiles/` - The tiles found in the architecture.
* `XXX/tests/` - Tests for making sure the architecture specific features
  works with VPR.
* [`vpr`](vpr) - Common defines used by multiple architectures.

Files
-----

* `pb_type.xml` - The Verilog to Routing
  [Complex Block](https://docs.verilogtorouting.org/en/latest/arch/reference/#complex-blocks)
  defintinition.

  * Inside `primitives` directory they should be intermediate or primitive
    `<pb_type>` and thus allow setting the `num_pb` attribute.
  * Inside `tiles` directory they should be top level `<pb_type>` and thus have,

    - `capacity` (if a pin type),
    - `width` & `height` (and maybe `area`)
* `model.xml` - The Verilog to Routing
  [Recognized BLIF Models](https://docs.verilogtorouting.org/en/latest/arch/reference/#recognized-blif-models-models)
  defintinition.
* `sim.v` - A Verilog definition of the object. It should;

  - [ ] Match the definition in `model.xml` (should be one `module` in
    `sim.v` for every `model` in `model.xml`)
  - [ ] Include a `ifndef BLACKBOX` section which actually defines how the
    Verilog works.
* `macro.v` - A Verilog definition of the object which a user might
  instantiate in their own code when specifying a primitive. This should match
  the definition provided by a manufacturer. Examples would be the definitions
  in;
  
  - [Lattice iCE Technology Library](http://www.latticesemi.com/~/media/LatticeSemi/Documents/TechnicalBriefs/SBTICETechnologyLibrary201504.pdf)
  - [UG953: Vivado Design Suite 7 Series FPGA and Zynq-7000 All Programmable SoC Libraries Guide](https://www.xilinx.com/support/documentation/sw_manuals/xilinx2017_3/ug953-vivado-7series-libraries.pdf)

Names
-----

* ``BLK_MB-block_1_name-block_2_name`` - ``BLOCK`` which is a "**m**\ ega **b**\ lock". A "mega block" is a top level block which is made up of other blocks.
* ``BLK_XX-name``       - ``BLOCK`` which is the hierarchy. Maps to ``BLK_SI`` -> ``SITE`` and ``BLK_TI`` -> ``TILE`` in Xilinx terminology.
* ``BLK_IG-name``       - ``BLOCK`` which is ignored. They don't appear in the output hierarchy and are normally used when something is needed in the description which doesn't match actual architecture.
* ``BEL_RX-mux_name``   - ``BEL`` which is a **r**\ outing mu\ **x**. Routing muxes are statically configured at PnR time.
* ``BEL_MX-mux_name``   - ``BEL`` which is a **m**\ u\ **x** .
* ``BEL_LT-lut_name``   - ``BEL`` which is a **l**\ ook up **t**\ able.
* ``BEL_MM-mem_name``   - ``BEL`` which is a **m**\ e**m**\ ory.
* ``BEL_FF-ff_name``    - ``BEL`` which is a **f**\ lip **f**\ lop (``FF``).
* ``BEL_LL-latch_name`` - ``BEL`` which is a **l**\ atch (``LL``).
* ``BEL_BB-name``       - ``BEL`` which is a **b**\ lack **b**\ ox (``BB``).
* ``PAD_IN-name``       - A signal input location.
* ``PAD_OT-name``       - A signal output location.

Notes
-----

* Unless there is a good reason otherwise, all muxes should be generated via
  [`mux_gen.py`](utils/mux_gen.py)

* DRY (Don't repeat yourself) - Uses
  [XML XIncludes](https://en.wikipedia.org/wiki/XInclude) to reuse stuff!