[doc] add pcf file format to documentation

docs/source/manual/file_formats/index.rst

@@ -29,3 +29,5 @@ OpenFPGA widely uses XML format for interchangable files
    bitstream_distribution_file
 
    bus_group_file
+
+   pcf_file

docs/source/manual/file_formats/pcf_file.rst

@@ -0,0 +1,24 @@
+.. _file_format_pcf_file:
+
+Pin Constraints File (.pcf)
+---------------------------
+
+.. note:: This file is in a different usage than the Pin Constraints File in XML format (see details in :ref:`file_format_pin_constraints_file`)
+
+The PCF file is the file which users should craft to assign their I/O constraints
+
+An example of the file is shown as follows.
+
+.. code-block:: xml
+
+  set_io a pad_fpga_io[0]
+  set_io b[0] pad_fpga_io[4]
+  set_io c[1] pad_fpga_io[6]
+
+.. option:: set_io <net> <pin>
+
+  Assign a net (defined as an input or output in users' HDL design) to a specific pin of an FPGA device (typically a packaged chip).
+
+  .. note:: The net should be single-bit and match the port declaration of the top-module in users' HDL design
+
+  .. note:: FPGA devices have different pin names, depending their naming rules. Please contact your vendor about details.

docs/source/manual/file_formats/pin_constraints_file.rst

@@ -3,7 +3,8 @@
 Pin Constraints File (.xml)
 ---------------------------
 
-The *Pin Constraints File* (PCF) aims to create pin binding between an implementation and an FPGA fabric
+The *Pin Constraints File* (PCF) aims to create pin binding between an implementation and an FPGA fabric.
+It is a common file format used by FPGA vendors, for example, `QuickLogic<https://docs.verilogtorouting.org/en/latest/vpr/file_formats/#placement-file-format-place>`_.
 
 An example of design constraints is shown as follows.
 

docs/source/manual/openfpga_shell/openfpga_commands/setup_commands.rst

@@ -242,7 +242,7 @@ write_fabric_io_info
 pcf2place
 ~~~~~~~~~
 
-  Convert a Pin Constraint File (.pcf, see details in :ref:`file_format_pcf`) to a `placement file<https://docs.verilogtorouting.org/en/latest/vpr/file_formats/#placement-file-format-place>`_)
+  Convert a Pin Constraint File (.pcf, see details in :ref:`file_format_pcf_file`) to a `placement file<https://docs.verilogtorouting.org/en/latest/vpr/file_formats/#placement-file-format-place>`_)
   
   .. option:: --pcf <string>