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.. _Understanding:
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Understanding the flow
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======================
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This section provides valuable information on how each of the commands used to compile and build
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designs in F4PGA work. It is especially helpful for debugging or for using methods
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other than a makefile to build your designs, such as a bash or python script.
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The following describes the commands for running each of the steps for a full design flow
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(synthesis, place and route, and generate bitstream) as well as giving a description of the most
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common flags for those commands. If you would like a more detailed break down of how the design
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flow for F4PGA works take a look at the
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`FPGA Design Flow page <https://f4pga.readthedocs.io/en/latest/toolchain-desc/design-flow.html>`_.
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.. note::
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Files created by synthesis, implementation, and bitstream generation will be dumped into
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the directory from which the command is run by default. To keep all of the files generated by
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the toolchain separate from your design files, you might consider running the toolchain
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commands in a separate directory from your design files.
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Synthesis
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----------
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To synthesize your designs run the ``symbiflow_synth`` command. The command has the following
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flags:
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.. table:: symbiflow_synth
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+------+---------------------------------------------------------------+
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| Flag | Argument |
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+======+===============================================================+
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| -t | Defines the name for the top level module |
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+------+---------------------------------------------------------------+
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| -v | A list of paths to verilog files for the design |
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+------+---------------------------------------------------------------+
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| -d | FPGA family (i.e. artix7 or zynq7) |
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+------+---------------------------------------------------------------+
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| -p | The part number for the FPGA (i.e xc7a35tcsg324-1) |
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+------+---------------------------------------------------------------+
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| -x | Optional command: path to xdc files for design |
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+------+---------------------------------------------------------------+
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An example of how to run synthesis on a design containing two separate
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verilog HDL files is below. The design is built for a basys3 board which comes from the artix7
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family and uses the xc7a35tcpg236-1 chip.
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.. code-block:: bash
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symbiflow_synth -t top -v example.v top_example.v -d artix7 -p xc7a35tcpg236-1 -x design_constraint.xdc
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Synthesis is carried out using the Yosys open source tool. ``symbiflow_synth`` generates
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an .eblif file, a few verilog netlists that describe the gate level design for your project, and a log
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file. For more information on Yosys and its relation to F4PGA go to the
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`F4PGA-Yosys page <https://f4pga.readthedocs.io/en/latest/toolchain-desc/yosys.html>`_.
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.. note::
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The build files generated by the toolchain (for example .eblif from synthesis, .net from
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packing, .bit from generate bitstream) are named using the top module specified in
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symbiflow_synth. For example if you specified ``switch_top`` as the top level module name
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during synthesis using the ``-t`` flag, the build files generated by the toolchain would be
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named switch_top.eblif, switch_top.net, etc.
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Place and Route
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----------------
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The three steps for implementing a design are internally handled by the open source VPR
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(Versatile Place and Route) tool. For more information go to
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`the F4PGA VPR page <https://f4pga.readthedocs.io/en/latest/vtr-verilog-to-routing/doc/src/vpr/index.html>`_.
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Pack
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+++++
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Packing is run by the ``symbiflow_pack`` command and generates several files containing
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a pin usage report, a timing report, a log file, and a netlist. The various flags for the
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pack command are as follows:
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.. table:: symbiflow_pack
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+------+--------------------------------------------------------------------+
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| Flag | Argument |
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+======+====================================================================+
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| -e | Path to .eblif file generated by synthesis |
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+------+--------------------------------------------------------------------+
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| -d | Fabric definition for the board (i.e. xc7a100t_test) |
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+------+--------------------------------------------------------------------+
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| -s | Optional: SDC file path |
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+------+--------------------------------------------------------------------+
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Note that the -d option for this step (defining the fabric definition) is different
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from the -d from synthesis (defining the FPGA family).
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The following example runs packing on the basys3 board:
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.. code-block:: bash
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symbiflow_pack -e top.eblif -d xc7a35t_test
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Place
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++++++
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Placement generates several files describing the location of design elements
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as well as a log file. Placement is run using ``symbiflow_place`` which utilizes
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the following flags:
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.. table:: symbiflow_place
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+------+----------------------------------------------------+
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| Flag | Argument |
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+======+====================================================+
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| -e | Path to .eblif file generated by synthesis |
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+------+----------------------------------------------------+
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| -d | Fabric definition (xc7a50t_test) |
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+------+----------------------------------------------------+
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| -p | Optional: PCF file path |
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+------+----------------------------------------------------+
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| -n | Path to the .net file generated by pack step |
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+------+----------------------------------------------------+
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| -P | The part number for the FPGA (i.e xc7a35tcsg324-1) |
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+------+----------------------------------------------------+
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| -s | Optional: SDC file path |
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+------+----------------------------------------------------+
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For the basys3:
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.. code-block:: bash
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symbiflow_pack -e top.eblif -d xc7a35t_test -p design.pcf -n top.net -P xc7a35tcpg236-1 -s design.sdc
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Route
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++++++
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Routing produces several timing reports as well as a post routing netlist and log file.
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``symbiflow_route`` uses the -e, -d, and the optional -s flags. The arguments for these flags
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are the same as in the placement step (.eblif, fabric definition, and SDC file path respectively).
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The following is an example:
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.. code-block:: bash
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symbiflow_route -e top.eblif -d xc7a35t_test -s design.sdc
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Generating Bitstream
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----------------------
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Generating the bitstream consists of two steps. First, run ``symbiflow_write_fasm`` to generate
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the .fasm file used to create the bitstream. ``symbiflow_write_fasm`` uses the -e and -d flags
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with the same arguments as the placing and routing steps (.eblif path, and fabric definition).
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Second, run ``symbiflow_write_bitstream`` which has the following flags:
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.. table:: symbiflow_write_bitstream
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+------+-------------------------------------------------------+
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| Flag | Argument |
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+======+=======================================================+
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| -d | FPGA family (i.e. artix7 or zynq7) |
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+------+-------------------------------------------------------+
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| -f | The path to the .fasm file generated in by write_fasm |
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+------+-------------------------------------------------------+
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| -p | The FPGA part number (i.e xc7a35tcsg324-1) |
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+------+-------------------------------------------------------+
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| -b | Name of the file to write the bitstream to |
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+------+-------------------------------------------------------+
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Notice that the specification for the part number is a lowercase ``-p`` instead of a capital
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``-P`` as in the placement step. Also note that the ``-d`` in write_bitstream defines the FPGA
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family instead of the fabric as in the write_fasm step.
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.. warning::
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If you change the name of the output for your bitstream to something other than top.bit then the
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openFPGALoader command used in the examples would need to change too. For example if I used
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``-b my_module_top`` in symbiflow_write_bitstream then my openFPGALoader command would change to:
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.. code-block:: bash
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openFPGALoader -b $OFL_BOARD my_module_top.bit
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Note that the only part of the command that changes is "<top module name>.bit;"
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The following example generates a bitstream file named example.bit for the basys3 board:
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.. code-block:: bash
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symbiflow_write_fasm -e top.eblif -d xc7a50t_test
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symbiflow_write_bitstream -d artix7 -f top.fasm -p xc7a35tcpg236-1 -b example.bit
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Karol Gugala
GitHub