Add initial object and docstrings for timing model.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

prjxray/timing.py

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+""" Route timing delay definitions.
+
+Routing delay is formed from two parts in this model:
+
+- Intristic delay of the element
+- Capactive loading delay of the net
+
+Intristic delay is a time value (e.g. nanoseconds), does not vary based on
+routing fanout.  It does vary based on the PVT (process, voltage, temperature)
+corner.  PvtCorner and IntristicDelay objects are used to model intristic
+delay of elements.
+
+Capactive loading is the elmore delay from the RC tree formed by interconnect.
+The RC tree is made up of 5 types of RC nodes:
+
+1. Out-pins
+2. Buffer switches
+3. Pass-through switches
+4. Wires
+5. In-pins
+
+Outpins have intristic delays and an driver resistence.
+
+Buffer switches have instrinsic delays, internal capacitance, and output driver
+resistence.
+
+Pass-through switches have instrinsic delays and a resistence and capacitance.
+
+Wires have a resistence and capacitance.
+
+Inpins have instrinsic delays and a sink capacitance.
+
+The elmore delay is the RC tree formed by these 5 components.  Out pins and
+buffer switches are the roots of the elmore tree. Buffer switches
+and inpins are leafs of the elmore tree. Wires and pass-through switches are
+nodes in the tree, and share their capacitance upstream and downstream using a
+pi-model.
+
+"""
+import enum
+from collections import namedtuple
+
+class PvtCorner(enum.Enum):
+    """ Process/voltage/temperature corner definitions. """
+
+    # Corner where device operates with fastest intristic delays.
+    FAST = "FAST"
+
+    # Corner where device operates with slowest intristic delays.
+    SLOW = "SLOW"
+
+class IntristicDelay(namedtuple('IntristicDelay', 'min max')):
+    """ An intristic delay instance.
+
+    Represents is the intristic delay through an element (e.g. a site pin or
+    interconnect pip).
+
+    The intristic delay of an element is generally modelled at a particular
+    "corner" of a design.  The "corner" generally speaking is modelled over
+    process, voltage and temperature PVT.  The IntristicDelay object
+    reperesents the minimum or maximum delay through all instances of the
+    element at 1 corner.
+
+    Attributes
+    ----------
+
+    min : float
+        Minimum instrinsic delay (nsec)
+
+    max : str
+        Maximum instrinsic delay (nsec)
+    """
+
+class RcElement(namedtuple('RcElement', 'resistence capacitance')):
+    """ One part of an RcNode, embedded within an RcTree.
+
+    """
+    pass
+
+
+def fast_slow_tuple_to_corners(arr):
+    """ Convert delay 4-tuple into two IntristicDelay objects.
+
+    Returns
+    -------
+
+    corners : dict of PvtCorner to IntristicDelay
+        Dictionary keys of FAST and SLOW, mapping to the instrinsic delay
+        for each corner.
+
+    """
+
+    fast_min, fast_max, slow_min, slow_max = map(float, arr)
+
+    return {
+            PvtCorner.FAST: IntristicDelay(
+                min=fast_min,
+                max=fast_max,
+                ),
+            PvtCorner.SLOW: IntristicDelay(
+                min=slow_min,
+                max=slow_max,
+                ),
+            }