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doc: tristates
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Sebastien Bourdeauducq committed Feb 19, 2013
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@@ -180,13 +180,33 @@ Options to ``get_port`` are:

Migen generates behavioural V*HDL code that should be compatible with all simulators and, if the number of ports is <= 2, most FPGA synthesizers. If a specific code is needed, the memory generator function can be overriden using the ``memory_handler`` parameter of the conversion function.
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: ::

Instance("Tristate",
Instance.Inout("target", target),
Instance.Input("o", o),
Instance.Input("oe", oe),
Instance.Output("i", i)
)

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.

Fragments
*********
A "fragment" is a unit of logic, which is composed of:

* A list of combinatorial statements.
* A list of synchronous statements, or a clock domain name -> synchronous statements dictionary.
* A list of instances.
* A list of tri-states.
* A list of memories.
* A list of simulation functions (see :ref:`simulating`).

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