docs: add SCPI application documentation

Documentation/applications/industry/scpi/index.rst

@@ -1,3 +1,92 @@
 ====================================
 ``scpi`` SCPI instrument side parser
 ====================================
+
+Overview
+========
+
+The ``scpi`` application integrates the
+`scpi-parser <https://github.com/j123b567/scpi-parser>`_ library with the
+NuttX application build system. SCPI, the Standard Commands for Programmable
+Instruments, is commonly used by test and measurement equipment to parse
+instrument commands and responses.
+
+The application provides the SCPI parser library for NuttX applications and can
+optionally build the upstream interactive demo program.
+
+Source Location
+===============
+
+The NuttX application wrapper is located in ``apps/industry/scpi``. It
+downloads and builds ``scpi-parser`` version 2.2 unless an unpacked
+``scpi-parser`` source tree is already present in that directory.
+
+Basic Configuration
+===================
+
+Enable the parser with:
+
+- ``CONFIG_SCPI_PARSER`` - Build the SCPI instrument-side parser library.
+
+Optional demo support is controlled by:
+
+- ``CONFIG_SCPI_PARSER_DEMO`` - Build the SCPI parser interactive demo program.
+- ``CONFIG_SCPI_PARSER_DEMO_PRIORITY`` - Demo task priority.
+- ``CONFIG_SCPI_PARSER_DEMO_STACKSIZE`` - Demo task stack size.
+
+Unit Parser Configuration
+=========================
+
+``scpi-parser`` can include or exclude optional unit groups at compile time.
+NuttX exposes these as Kconfig options and maps each option to the matching
+upstream ``USE_UNITS_*`` compile-time define.
+
+The following unit groups are configurable:
+
+- ``CONFIG_SCPI_PARSER_UNITS_IMPERIAL`` - Imperial units.
+- ``CONFIG_SCPI_PARSER_UNITS_ANGLE`` - Angle units.
+- ``CONFIG_SCPI_PARSER_UNITS_PARTICLES`` - Particle units.
+- ``CONFIG_SCPI_PARSER_UNITS_DISTANCE`` - Distance units.
+- ``CONFIG_SCPI_PARSER_UNITS_MAGNETIC`` - Magnetic units.
+- ``CONFIG_SCPI_PARSER_UNITS_LIGHT`` - Light units.
+- ``CONFIG_SCPI_PARSER_UNITS_ENERGY_FORCE_MASS`` - Energy, force, and mass
+  units.
+- ``CONFIG_SCPI_PARSER_UNITS_TIME`` - Time units.
+- ``CONFIG_SCPI_PARSER_UNITS_TEMPERATURE`` - Temperature units.
+- ``CONFIG_SCPI_PARSER_UNITS_RATIO`` - Ratio units.
+- ``CONFIG_SCPI_PARSER_UNITS_POWER`` - Power units.
+- ``CONFIG_SCPI_PARSER_UNITS_FREQUENCY`` - Frequency units.
+- ``CONFIG_SCPI_PARSER_UNITS_ELECTRIC`` - Electric units.
+- ``CONFIG_SCPI_PARSER_UNITS_ELECTRIC_CHARGE_CONDUCTANCE`` - Electric charge
+  and conductance units.
+
+By default, ``CONFIG_SCPI_PARSER_UNITS_POWER``,
+``CONFIG_SCPI_PARSER_UNITS_FREQUENCY``, and
+``CONFIG_SCPI_PARSER_UNITS_ELECTRIC`` are enabled. The other optional unit
+groups are disabled by default and may be enabled individually.
+
+For example, enabling ``CONFIG_SCPI_PARSER_UNITS_TIME`` passes
+``USE_UNITS_TIME=1`` to ``scpi-parser`` during compilation.
+
+Example Configuration
+=====================
+
+A minimal parser configuration can enable only the library:
+
+.. code-block:: text
+
+   CONFIG_SCPI_PARSER=y
+
+To build the parser with time-unit support:
+
+.. code-block:: text
+
+   CONFIG_SCPI_PARSER=y
+   CONFIG_SCPI_PARSER_UNITS_TIME=y
+
+To include the interactive demo program:
+
+.. code-block:: text
+
+   CONFIG_SCPI_PARSER=y
+   CONFIG_SCPI_PARSER_DEMO=y