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46 lines
1.7 KiB
ReStructuredText
46 lines
1.7 KiB
ReStructuredText
Arbitrary logic functions
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-------------------------
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The `$lut` cell type implements a single-output LUT (lookup table). It
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implements an arbitrary logic function with its ``\LUT`` parameter to map input
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port ``\A`` to values of ``\Y`` output port values. In psuedocode: ``Y =
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\LUT[A]``. ``\A`` has width set by parameter ``\WIDTH`` and ``\Y`` has a width
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of 1. Every logic function with a single bit output has a unique `$lut`
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representation.
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The `$sop` cell type implements a sum-of-products expression, also known as
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disjunctive normal form (DNF). It implements an arbitrary logic function. Its
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structure mimics a programmable logic array (PLA). Output port ``\Y`` is the sum
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of products of the bits of the input port ``\A`` as defined by parameter
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``\TABLE``. ``\A`` is ``\WIDTH`` bits wide. The number of products in the sum is
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set by parameter ``\DEPTH``, and each product has two bits for each input bit -
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for the presence of the unnegated and negated version of said input bit in the
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product. Therefore the ``\TABLE`` parameter holds ``2 * \WIDTH * \DEPTH`` bits.
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For example:
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Let ``\WIDTH`` be 3. We would like to represent ``\Y =~\A[0] + \A[1]~\A[2]``.
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There are 2 products to be summed, so ``\DEPTH`` shall be 2.
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.. code-block::
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~A[2]-----+
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A[2]----+|
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~A[1]---+||
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A[1]--+|||
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~A[0]-+||||
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A[0]+|||||
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|||||| product formula
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010000 ~\A[0]
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001001 \A[1]~\A[2]
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So the value of ``\TABLE`` will become ``010000001001``.
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Any logic function with a single bit output can be represented with ``$sop`` but
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may have variously minimized or ordered summands represented in the ``\TABLE``
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values.
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.. autocellgroup:: logic
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:members:
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:source:
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:linenos: |