Tri-state I/O

A triplet (O, OE, I) of one-way signals defining a tri-state I/O port is represented by the ``TSTriple`` object. Such objects are only containers for signals that are intended to be later connected to a tri-state I/O buffer, and cannot be used in fragments. Such objects, however, should be kept in the design as long as possible as they allow the individual one-way signals to be manipulated in a non-ambiguous way.

The object that can be used in a ``Fragment`` is ``Tristate``, and it behaves exactly like an instance of a tri-state I/O buffer that would be defined as follows: ::

Signals ``target``, ``o`` and ``i`` can have any width, while ``oe`` is 1-bit wide. The ``target`` signal should go to a port and not be used elsewhere in the design. Like modern FPGA architectures, Migen does not support internal tri-states.

A ``Tristate`` object can be created from a ``TSTriple`` object by calling the ``get_tristate`` method.

By default, Migen emits technology-independent behavioral code for a tri-state buffer. If a specific code is needed, the tristate generator function can be overriden using the ``tristate_handler`` parameter of the conversion function.

* A list of tri-states.