OpenFPGA/docs/source/manual/arch_lang/config_protocol.rst

.. _config_protocol:

Configuration Protocol
----------------------

Configuration protocol is the circuitry designed to program an FPGA.
As an interface, configuration protocol could be really different in FPGAs, depending on the application context.
OpenFPGA supports versatile configuration protocol, providing different trade-offs between speed and area. 

Template
~~~~~~~~

.. code-block:: xml

  <configuration_protocol>
    <organization type="<string>" circuit_model_name="<string>"/>
  </configuration_protocol>

.. option:: type="scan_chain|memory_bank|standalone"

  Specify the type of configuration circuits.

  OpenFPGA supports different types of configuration protocols to program FPGA fabrics:
    - ``scan_chain``: configurable memories are connected in a chain. Bitstream is loaded serially to program a FPGA
    - ``frame_based``: configurable memories are organized by frames. Each module of a FPGA fabric, e.g., Configurable Logic Block (CLB), Switch Block (SB) and Connection Block (CB), is considered as a frame of configurable memories. Inside each frame, all the memory banks are accessed through an address decoder. Users can write each memory cell with a specific address. Note that the frame-based memory organization is applid hierarchically. Each frame may consists of a number of sub frames, each of which follows the similar organization.
    - ``memory_bank``: configurable memories are organized in an array, where each element can be accessed by an unique address to the BL/WL decoders
    - ``standalone``: configurable memories are directly accessed through ports of FPGA fabrics. In other words, there are no protocol to control the memories. This allows full customization on the configuration protocol for hardware engineers.

  .. note:: Avoid to use ``standalone`` when designing an FPGA chip. It will causes a huge number of I/Os required, far beyond any package size. It is well applicable to eFPGAs, where designers do need customized protocols between FPGA and processors. 

.. warning:: Currently FPGA-SPICE only supports standalone memory organization.

.. warning:: Currently RRAM-based FPGA only supports memory-bank organization for Verilog Generator.

.. option:: circuit_model_name="<string>"

  Specify the name of circuit model to be used as configurable memory.

  - ``scan_chain`` requires a circuit model type of ``ccff``
  - ``frame_based`` requires a circuit model type of ``sram``
  - ``memory_bank`` requires a circuit model type of ``sram``
  - ``standalone`` requires a circuit model type of ``sram``

.. option:: num_regions="<int>"

  Specify the number of configuration regions to be used across the fabrics. By default, it will be only 1 configuration region. Each configuration region contains independent configuration protocols, but the whole fabric should employ the same type of configuration protocols. For example, an FPGA fabric consists of 4 configuration regions, each of which includes a configuration chain. The more configuration chain to be used, the fast configuration runtime will be, but at the cost of more I/Os in the FPGA fabrics. The organization of each configurable region can be customized through the fabric key (see details in :ref:`fabric_key`).

  .. warning:: Currently, multiple configuration regions is not applicable to ``standalone`` configuration protocol.


Configuration Chain Example
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The following XML code describes a scan-chain circuitry to configure the core logic of FPGA, as illustrated in :numref:`fig_ccff_fpga`.
It will use the circuit model defined in :numref:`fig_ccff_config_chain`.

.. code-block:: xml

  <configuration_protocol>
    <organization type="scan_chain" circuit_model_name="ccff" num_regions="<int>"/>
  </configuration_protocol>

.. _fig_ccff_fpga:

.. figure:: figures/ccff_fpga.png
   :scale: 60%
   :alt: map to buried treasure
 
   Example of a configuration chain to program core logic of a FPGA 


.. figure:: figures/multi_region_config_chains.png
   :scale: 100%
   :alt: map to buried treasure
 
   Examples of single- and multiple- region configuration chains


Frame-based Example
~~~~~~~~~~~~~~~~~~~
The following XML code describes frame-based memory banks to configure the core logic of FPGA.
It will use the circuit model defined in :numref:`fig_config_latch`.

.. code-block:: xml

  <configuration_protocol>
    <organization type="frame_based" circuit_model_name="config_latch"/>
  </configuration_protocol>

Through frame-based configuration protocol, each memory cell can be accessed with an unique address given to decoders.
:numref:`fig_frame_config_protocol_example` illustrates an example about how the configurable memories are organizaed inside a Logic Element (LE) shown in :numref:`fig_k4n4_arch`.
The decoder inside the LE will enable the decoders of the Look-Up Table (LUT) and the routing multiplexer, based on the given address at ``address[2:2]``.
When the decoder of sub block, e.g., the LUT, is enabled, each memory cells can be accessed throught the ``address[1:0]`` and the data to write is provided at ``data_in``.

.. _fig_frame_config_protocol_example:

.. figure:: figures/frame_config_protocol_example.png
   :scale: 25%
   :alt: map to buried treasure
 
   Example of a frame-based memory organization inside a Logic Element

:numref:`fig_frame_config_protocol` shows a hierarchical view on how the frame-based decoders across a FPGA fabric. 

.. _fig_frame_config_protocol:

.. figure:: figures/frame_config_protocol.png
   :scale: 60%
   :alt: map to buried treasure
 
   Frame-based memory organization in a hierarchical view

.. note:: Frame-based decoders does require a memory cell to have 

  -  two outputs (one regular and another inverted)
  -  a Bit-Line input to load the data
  -  a Word-Line input to enable data write 

.. warning:: Please do NOT add inverted Bit-Line and Word-Line inputs. It is not supported yet!

When multiple configuration region is applied, the configuration frames will be grouped into different configuration regions. Each region has a separated data input bus and dedicated address decoders. As such, the configuration frame groups can be programmed in parallel.

Memory bank Example
~~~~~~~~~~~~~~~~~~~
The following XML code describes a memory-bank circuitry to configure the core logic of FPGA, as illustrated in :numref:`fig_memory_bank`.
It will use the circuit model defined in :numref:`fig_sram_blwl`.
Users can customized the number of memory banks to be used across the fabrics. By default, it will be only 1 memory bank. :numref:`fig_memory_bank` shows an example where 4 memory banks are defined. The more memory bank to be used, the fast configuration runtime will be, but at the cost of more I/Os in the FPGA fabrics. The organization of each configurable region can be customized through the fabric key (see details in :ref:`fabric_key`).

.. code-block:: xml

  <configuration_protocol>
    <organization type="memory_bank" circuit_model_name="sram_blwl"/>
  </configuration_protocol>

.. _fig_memory_bank:

.. figure:: figures/memory_bank.png
   :scale: 30%
   :alt: map to buried treasure
 
   Example of (a) a memory organization using memory decoders; (b) single memory bank across the fabric; and (c) multiple memory banks across the fabric.

.. note:: Memory-bank decoders does require a memory cell to have 

  -  two outputs (one regular and another inverted)
  -  a Bit-Line input to load the data
  -  a Word-Line input to enable data write 

.. warning:: Please do NOT add inverted Bit-Line and Word-Line inputs. It is not supported yet!

Standalone SRAM Example
~~~~~~~~~~~~~~~~~~~~~~~
In the standalone configuration protocol, every memory cell of the core logic of a FPGA fabric can be directly accessed at the top-level module, as illustrated in :numref:`fig_vanilla_config_protocol`.

.. _fig_vanilla_config_protocol:

.. figure:: figures/vanilla_config_protocol.png
   :scale: 30%
   :alt: map to buried treasure
 
   Vanilla (standalone) memory organization in a hierarchical view

The following XML code shows an example where we use the circuit model defined in :numref:`fig_sram_blwl`.

.. code-block:: xml

  <configuration_protocol>
    <organization type="standalone" circuit_model_name="sram_blwl"/>
  </configuration_protocol>

.. note:: The standalone protocol does require a memory cell to have 

  -  two outputs (one regular and another inverted)
  -  a Bit-Line input to load the data
  -  a Word-Line input to enable data write 

.. warning:: Please do NOT add inverted Bit-Line and Word-Line inputs. It is not supported yet!

.. warning:: This is a vanilla configuration method, which allow users to build their own configuration protocol on top of it.