Add signed/fixedpoint properties (#140)

* declared intwidth, fracwidth, and is_signed UDPs

* fix boolean type name in UDP definition

* generate hwif fields with fixedpoint indices

* make "counter" and "encode" properties mutualy exclusive with signed/fixedpoint

* add signed/unsigned to hwif

* improved fixedpoint error messages, added validation tests

* added fixedpoint tests

* fixedpoint/signed not allowed for signal components

* added signed/fixedpoint UDP docs

* handle single-bit fixedpoint numbers

* fix too many positional arguments lint

* changed spelling of fixedpoint to fixed-point

* use "logic" in place of "unsigned logic"

* split signed and fixedpoint docs, added examples

* allow enums with is_signed=false

* split signed and fixedpoint implementations

* assorted nits picked

* updated is_signed validation unit test

docs/udps/fixedpoint.rst

@@ -0,0 +1,103 @@
+.. _fixedpoint:
+
+Fixed-Point Fields
+==================
+
+`Fixed-point <https://en.wikipedia.org/wiki/Fixed-point_arithmetic>`_ numbers
+can be used to efficiently represent real numbers using integers. Fixed-point
+numbers consist of some combination of integer bits and fractional bits. The
+number of integer/fractional bits is usually implicitly tracked (not stored)
+for each number, unlike for floating-point numbers.
+
+For this SystemVerilog exporter, these properties only affect the signal type in
+the the ``hwif`` structs. There is no special handling in the internals of
+the regblock.
+
+Properties
+----------
+Fields can be declared as fixed-point numbers using the following two properties:
+
+.. literalinclude:: ../../hdl-src/regblock_udps.rdl
+    :lines: 46-54
+
+The :ref:`is_signed<signed>` property can be used in conjunction with these
+properties to declare signed fixed-point fields.
+
+These UDP definitions, along with others supported by PeakRDL-regblock, can be
+enabled by compiling the following file along with your design:
+:download:`regblock_udps.rdl <../../hdl-src/regblock_udps.rdl>`.
+
+.. describe:: intwidth
+
+    *   The ``intwidth`` property defines the number of integer bits in the
+        fixed-point representation (including the sign bit, if present).
+
+.. describe:: fracwidth
+
+    *   The ``fracwidth`` property defines the number of fractional bits in the
+        fixed-point representation.
+
+Representable Numbers
+^^^^^^^^^^^^^^^^^^^^^
+
+The range of representable real numbers is summarized in the table below.
+
+.. list-table:: Representable Numbers
+    :header-rows: 1
+
+    *   - Signedness
+        - Minimum Value
+        - Maximum Value
+        - Step Size
+
+    *   - Unsigned
+        - :math:`0`
+        - :math:`2^{\mathrm{intwidth}} - 2^{-\mathrm{fracwidth}}`
+        - :math:`2^{-\mathrm{fracwidth}}`
+
+    *   - Signed
+        - :math:`-2^{\mathrm{intwidth}-1}`
+        - :math:`2^{\mathrm{intwidth}-1} - 2^{-\mathrm{fracwidth}}`
+        - :math:`2^{-\mathrm{fracwidth}}`
+
+SystemVerilog Types
+^^^^^^^^^^^^^^^^^^^
+
+When either ``intwidth`` or ``fracwidth`` are defined for a field, that field's
+type in the generated SystemVerilog ``hwif`` struct is
+``logic (signed) [intwidth-1:-fracwidth]``. The bit at index :math:`i` contributes
+a weight of :math:`2^i` to the real number represented.
+
+Other Rules
+^^^^^^^^^^^
+*   Only one of ``intwidth`` or ``fracwidth`` need be defined. The other is
+    inferred from the field bit width.
+*   The bit width of the field shall be equal to ``intwidth`` + ``fracwidth``.
+*   If both ``intwidth`` and ``fracwidth`` are defined for a field,  it is an
+    error if their sum does not equal the bit width of the field.
+*   Either ``fracwidth`` or ``intwidth`` can be a negative integer. Because
+    SystemRDL does not have a signed integer type, the only way to achieve
+    this is to define one of the widths as larger than the bit width of the
+    component so that the other width is inferred as a negative number.
+*   The properties defined above are mutually exclusive with the ``counter``
+    property.
+*   The properties defined above are mutually exclusive with the ``encode``
+    property.
+
+Examples
+--------
+
+A 12-bit signed fixed-point field with 4 integer bits and 8 fractional bits
+can be declared with
+
+.. code-block:: systemrdl
+    :emphasize-lines: 3, 4
+
+    field {
+        sw=rw; hw=r;
+        intwidth = 4;
+        is_signed;
+    } fixedpoint_num[11:0] = 0;
+
+This field can represent values from -8.0 to 7.99609375
+in steps of 0.00390625.