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cellref: Deprecate cell_library.rst

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Most of the word/coarse level cells have an assigned group and individual page.
The gate/fine level cells are all on one page.
Fix links to `cell_library.rst`.
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Coarse arithmetics
------------------
Multiply-accumulate
~~~~~~~~~~~~~~~~~~~
The `$macc` cell type represents a generalized multiply and accumulate
operation. The cell is purely combinational. It outputs the result of summing up
a sequence of products and other injected summands.
.. code-block::
Y = 0 +- a0factor1 * a0factor2 +- a1factor1 * a1factor2 +- ...
+ B[0] + B[1] + ...
The A port consists of concatenated pairs of multiplier inputs ("factors"). A
zero length factor2 acts as a constant 1, turning factor1 into a simple summand.
In this pseudocode, ``u(foo)`` means an unsigned int that's foo bits long.
.. code-block::
struct A {
u(CONFIG.mul_info[0].factor1_len) a0factor1;
u(CONFIG.mul_info[0].factor2_len) a0factor2;
u(CONFIG.mul_info[1].factor1_len) a1factor1;
u(CONFIG.mul_info[1].factor2_len) a1factor2;
...
};
The cell's ``CONFIG`` parameter determines the layout of cell port ``A``. The
CONFIG parameter carries the following information:
.. code-block::
struct CONFIG {
u4 num_bits;
struct mul_info {
bool is_signed;
bool is_subtract;
u(num_bits) factor1_len;
u(num_bits) factor2_len;
}[num_ports];
};
B is an array of concatenated 1-bit-wide unsigned integers to also be summed up.
Arbitrary logic functions
~~~~~~~~~~~~~~~~~~~~~~~~~
The `$lut` cell type implements a single-output LUT (lookup table). It
implements an arbitrary logic function with its ``\LUT`` parameter to map input
port ``\A`` to values of ``\Y`` output port values. In psuedocode: ``Y =
\LUT[A]``. ``\A`` has width set by parameter ``\WIDTH`` and ``\Y`` has a width
of 1. Every logic function with a single bit output has a unique `$lut`
representation.
The `$sop` cell type implements a sum-of-products expression, also known as
disjunctive normal form (DNF). It implements an arbitrary logic function. Its
structure mimics a programmable logic array (PLA). Output port ``\Y`` is the sum
of products of the bits of the input port ``\A`` as defined by parameter
``\TABLE``. ``\A`` is ``\WIDTH`` bits wide. The number of products in the sum is
set by parameter ``\DEPTH``, and each product has two bits for each input bit -
for the presence of the unnegated and negated version of said input bit in the
product. Therefore the ``\TABLE`` parameter holds ``2 * \WIDTH * \DEPTH`` bits.
For example:
Let ``\WIDTH`` be 3. We would like to represent ``\Y =~\A[0] + \A[1]~\A[2]``.
There are 2 products to be summed, so ``\DEPTH`` shall be 2.
.. code-block::
~A[2]-----+
A[2]----+|
~A[1]---+||
A[1]--+|||
~A[0]-+||||
A[0]+|||||
|||||| product formula
010000 ~\A[0]
001001 \A[1]~\A[2]
So the value of ``\TABLE`` will become ``010000001001``.
Any logic function with a single bit output can be represented with ``$sop`` but
may have variously minimized or ordered summands represented in the ``\TABLE``
values.
.. autocellgroup:: arith
:members:
:source:
:linenos:

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.. role:: verilog(code)
:language: Verilog
Debugging cells
---------------
The `$print` cell is used to log the values of signals, akin to (and
translatable to) the ``$display`` and ``$write`` family of tasks in Verilog. It
has the following parameters:
``FORMAT``
The internal format string. The syntax is described below.
``ARGS_WIDTH``
The width (in bits) of the signal on the ``ARGS`` port.
``TRG_ENABLE``
True if triggered on specific signals defined in ``TRG``; false if triggered
whenever ``ARGS`` or ``EN`` change and ``EN`` is 1.
If ``TRG_ENABLE`` is true, the following parameters also apply:
``TRG_WIDTH``
The number of bits in the ``TRG`` port.
``TRG_POLARITY``
For each bit in ``TRG``, 1 if that signal is positive-edge triggered, 0 if
negative-edge triggered.
``PRIORITY``
When multiple `$print` or `$check` cells fire on the same trigger, they
execute in descending priority order.
Ports:
``TRG``
The signals that control when this `$print` cell is triggered.
If the width of this port is zero and ``TRG_ENABLE`` is true, the cell is
triggered during initial evaluation (time zero) only.
``EN``
Enable signal for the whole cell.
``ARGS``
The values to be displayed, in format string order.
.. autocellgroup:: debug
:members:
:source:
:linenos:
Format string syntax
~~~~~~~~~~~~~~~~~~~~
The format string syntax resembles Python f-strings. Regular text is passed
through unchanged until a format specifier is reached, starting with a ``{``.
Format specifiers have the following syntax. Unless noted, all items are
required:
``{``
Denotes the start of the format specifier.
size
Signal size in bits; this many bits are consumed from the ``ARGS`` port by
this specifier.
``:``
Separates the size from the remaining items.
justify
``>`` for right-justified, ``<`` for left-justified.
padding
``0`` for zero-padding, or a space for space-padding.
width\ *?*
(optional) The number of characters wide to pad to.
base
* ``b`` for base-2 integers (binary)
* ``o`` for base-8 integers (octal)
* ``d`` for base-10 integers (decimal)
* ``h`` for base-16 integers (hexadecimal)
* ``c`` for ASCII characters/strings
* ``t`` and ``r`` for simulation time (corresponding to :verilog:`$time` and
:verilog:`$realtime`)
For integers, this item may follow:
``+``\ *?*
(optional, decimals only) Include a leading plus for non-negative numbers.
This can assist with symmetry with negatives in tabulated output.
signedness
``u`` for unsigned, ``s`` for signed. This distinction is only respected
when rendering decimals.
ASCII characters/strings have no special options, but the signal size must be
divisible by 8.
For simulation time, the signal size must be zero.
Finally:
``}``
Denotes the end of the format specifier.
Some example format specifiers:
+ ``{8:>02hu}`` - 8-bit unsigned integer rendered as hexadecimal,
right-justified, zero-padded to 2 characters wide.
+ ``{32:< 15d+s}`` - 32-bit signed integer rendered as decimal, left-justified,
space-padded to 15 characters wide, positive values prefixed with ``+``.
+ ``{16:< 10hu}`` - 16-bit unsigned integer rendered as hexadecimal,
left-justified, space-padded to 10 characters wide.
+ ``{0:>010t}`` - simulation time, right-justified, zero-padded to 10 characters
wide.
To include literal ``{`` and ``}`` characters in your format string, use ``{{``
and ``}}`` respectively.
It is an error for a format string to consume more or less bits from ``ARGS``
than the port width.
Values are never truncated, regardless of the specified width.
Note that further restrictions on allowable combinations of options may apply
depending on the backend used.
For example, Verilog does not have a format specifier that allows zero-padding a
string (i.e. more than 1 ASCII character), though zero-padding a single
character is permitted.
Thus, while the RTLIL format specifier ``{8:>02c}`` translates to ``%02c``,
``{16:>02c}`` cannot be represented in Verilog and will fail to emit. In this
case, ``{16:> 02c}`` must be used, which translates to ``%2s``.

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Formal verification cells
-------------------------
.. TODO:: Describe formal cells
`$check`, `$assert`, `$assume`, `$live`, `$fair`, `$cover`, `$equiv`,
`$initstate`, `$anyconst`, `$anyseq`, `$anyinit`, `$allconst`, and `$allseq`.
Also `$ff` and `$_FF_` cells.
.. autocellgroup:: formal
:members:
:source:
:linenos:

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Finite state machines
---------------------
.. TODO:: Describe `$fsm` cell
.. autocellgroup:: fsm
:members:
:source:
:linenos:

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.. role:: verilog(code)
:language: Verilog
.. _sec:memcells:
Memories
~~~~~~~~
Memories are either represented using ``RTLIL::Memory`` objects, `$memrd_v2`,
`$memwr_v2`, and `$meminit_v2` cells, or by `$mem_v2` cells alone.
In the first alternative the ``RTLIL::Memory`` objects hold the general metadata
for the memory (bit width, size in number of words, etc.) and for each port a
`$memrd_v2` (read port) or `$memwr_v2` (write port) cell is created. Having
individual cells for read and write ports has the advantage that they can be
consolidated using resource sharing passes. In some cases this drastically
reduces the number of required ports on the memory cell. In this alternative,
memory initialization data is represented by `$meminit_v2` cells, which allow
delaying constant folding for initialization addresses and data until after the
frontend finishes.
The `$memrd_v2` cells have a clock input ``CLK``, an enable input ``EN``, an
address input ``ADDR``, a data output ``DATA``, an asynchronous reset input
``ARST``, and a synchronous reset input ``SRST``. They also have the following
parameters:
``MEMID``
The name of the ``RTLIL::Memory`` object that is associated with this read
port.
``ABITS``
The number of address bits (width of the ``ADDR`` input port).
``WIDTH``
The number of data bits (width of the ``DATA`` output port). Note that this
may be a power-of-two multiple of the underlying memory's width -- such ports
are called wide ports and access an aligned group of cells at once. In this
case, the corresponding low bits of ``ADDR`` must be tied to 0.
``CLK_ENABLE``
When this parameter is non-zero, the clock is used. Otherwise this read port
is asynchronous and the ``CLK`` input is not used.
``CLK_POLARITY``
Clock is active on the positive edge if this parameter has the value ``1'b1``
and on the negative edge if this parameter is ``1'b0``.
``TRANSPARENCY_MASK``
This parameter is a bitmask of write ports that this read port is transparent
with. The bits of this parameter are indexed by the write port's ``PORTID``
parameter. Transparency can only be enabled between synchronous ports sharing
a clock domain. When transparency is enabled for a given port pair, a read
and write to the same address in the same cycle will return the new value.
Otherwise the old value is returned.
``COLLISION_X_MASK``
This parameter is a bitmask of write ports that have undefined collision
behavior with this port. The bits of this parameter are indexed by the write
port's ``PORTID`` parameter. This behavior can only be enabled between
synchronous ports sharing a clock domain. When undefined collision is enabled
for a given port pair, a read and write to the same address in the same cycle
will return the undefined (all-X) value.This option is exclusive (for a given
port pair) with the transparency option.
``ARST_VALUE``
Whenever the ``ARST`` input is asserted, the data output will be reset to
this value. Only used for synchronous ports.
``SRST_VALUE``
Whenever the ``SRST`` input is synchronously asserted, the data output will
be reset to this value. Only used for synchronous ports.
``INIT_VALUE``
The initial value of the data output, for synchronous ports.
``CE_OVER_SRST``
If this parameter is non-zero, the ``SRST`` input is only recognized when
``EN`` is true. Otherwise, ``SRST`` is recognized regardless of ``EN``.
The `$memwr_v2` cells have a clock input ``CLK``, an enable input ``EN`` (one
enable bit for each data bit), an address input ``ADDR`` and a data input
``DATA``. They also have the following parameters:
``MEMID``
The name of the ``RTLIL::Memory`` object that is associated with this write
port.
``ABITS``
The number of address bits (width of the ``ADDR`` input port).
``WIDTH``
The number of data bits (width of the ``DATA`` output port). Like with
`$memrd_v2` cells, the width is allowed to be any power-of-two multiple of
memory width, with the corresponding restriction on address.
``CLK_ENABLE``
When this parameter is non-zero, the clock is used. Otherwise this write port
is asynchronous and the ``CLK`` input is not used.
``CLK_POLARITY``
Clock is active on positive edge if this parameter has the value ``1'b1`` and
on the negative edge if this parameter is ``1'b0``.
``PORTID``
An identifier for this write port, used to index write port bit mask
parameters.
``PRIORITY_MASK``
This parameter is a bitmask of write ports that this write port has priority
over in case of writing to the same address. The bits of this parameter are
indexed by the other write port's ``PORTID`` parameter. Write ports can only
have priority over write ports with lower port ID. When two ports write to
the same address and neither has priority over the other, the result is
undefined. Priority can only be set between two synchronous ports sharing
the same clock domain.
The `$meminit_v2` cells have an address input ``ADDR``, a data input ``DATA``,
with the width of the ``DATA`` port equal to ``WIDTH`` parameter times ``WORDS``
parameter, and a bit enable mask input ``EN`` with width equal to ``WIDTH``
parameter. All three of the inputs must resolve to a constant for synthesis to
succeed.
``MEMID``
The name of the ``RTLIL::Memory`` object that is associated with this
initialization cell.
``ABITS``
The number of address bits (width of the ``ADDR`` input port).
``WIDTH``
The number of data bits per memory location.
``WORDS``
The number of consecutive memory locations initialized by this cell.
``PRIORITY``
The cell with the higher integer value in this parameter wins an
initialization conflict.
The HDL frontend models a memory using ``RTLIL::Memory`` objects and
asynchronous `$memrd_v2` and `$memwr_v2` cells. The `memory` pass (i.e. its
various sub-passes) migrates `$dff` cells into the `$memrd_v2` and `$memwr_v2`
cells making them synchronous, then converts them to a single `$mem_v2` cell and
(optionally) maps this cell type to `$dff` cells for the individual words and
multiplexer-based address decoders for the read and write interfaces. When the
last step is disabled or not possible, a `$mem_v2` cell is left in the design.
The `$mem_v2` cell provides the following parameters:
``MEMID``
The name of the original ``RTLIL::Memory`` object that became this `$mem_v2`
cell.
``SIZE``
The number of words in the memory.
``ABITS``
The number of address bits.
``WIDTH``
The number of data bits per word.
``INIT``
The initial memory contents.
``RD_PORTS``
The number of read ports on this memory cell.
``RD_WIDE_CONTINUATION``
This parameter is ``RD_PORTS`` bits wide, containing a bitmask of "wide
continuation" read ports. Such ports are used to represent the extra data
bits of wide ports in the combined cell, and must have all control signals
identical with the preceding port, except for address, which must have the
proper sub-cell address encoded in the low bits.
``RD_CLK_ENABLE``
This parameter is ``RD_PORTS`` bits wide, containing a clock enable bit for
each read port.
``RD_CLK_POLARITY``
This parameter is ``RD_PORTS`` bits wide, containing a clock polarity bit for
each read port.
``RD_TRANSPARENCY_MASK``
This parameter is ``RD_PORTS*WR_PORTS`` bits wide, containing a concatenation
of all ``TRANSPARENCY_MASK`` values of the original `$memrd_v2` cells.
``RD_COLLISION_X_MASK``
This parameter is ``RD_PORTS*WR_PORTS`` bits wide, containing a concatenation
of all ``COLLISION_X_MASK`` values of the original `$memrd_v2` cells.
``RD_CE_OVER_SRST``
This parameter is ``RD_PORTS`` bits wide, determining relative synchronous
reset and enable priority for each read port.
``RD_INIT_VALUE``
This parameter is ``RD_PORTS*WIDTH`` bits wide, containing the initial value
for each synchronous read port.
``RD_ARST_VALUE``
This parameter is ``RD_PORTS*WIDTH`` bits wide, containing the asynchronous
reset value for each synchronous read port.
``RD_SRST_VALUE``
This parameter is ``RD_PORTS*WIDTH`` bits wide, containing the synchronous
reset value for each synchronous read port.
``WR_PORTS``
The number of write ports on this memory cell.
``WR_WIDE_CONTINUATION``
This parameter is ``WR_PORTS`` bits wide, containing a bitmask of "wide
continuation" write ports.
``WR_CLK_ENABLE``
This parameter is ``WR_PORTS`` bits wide, containing a clock enable bit for
each write port.
``WR_CLK_POLARITY``
This parameter is ``WR_PORTS`` bits wide, containing a clock polarity bit for
each write port.
``WR_PRIORITY_MASK``
This parameter is ``WR_PORTS*WR_PORTS`` bits wide, containing a concatenation
of all ``PRIORITY_MASK`` values of the original `$memwr_v2` cells.
The `$mem_v2` cell has the following ports:
``RD_CLK``
This input is ``RD_PORTS`` bits wide, containing all clock signals for the
read ports.
``RD_EN``
This input is ``RD_PORTS`` bits wide, containing all enable signals for the
read ports.
``RD_ADDR``
This input is ``RD_PORTS*ABITS`` bits wide, containing all address signals
for the read ports.
``RD_DATA``
This output is ``RD_PORTS*WIDTH`` bits wide, containing all data signals for
the read ports.
``RD_ARST``
This input is ``RD_PORTS`` bits wide, containing all asynchronous reset
signals for the read ports.
``RD_SRST``
This input is ``RD_PORTS`` bits wide, containing all synchronous reset
signals for the read ports.
``WR_CLK``
This input is ``WR_PORTS`` bits wide, containing all clock signals for the
write ports.
``WR_EN``
This input is ``WR_PORTS*WIDTH`` bits wide, containing all enable signals for
the write ports.
``WR_ADDR``
This input is ``WR_PORTS*ABITS`` bits wide, containing all address signals
for the write ports.
``WR_DATA``
This input is ``WR_PORTS*WIDTH`` bits wide, containing all data signals for
the write ports.
The `memory_collect` pass can be used to convert discrete `$memrd_v2`,
`$memwr_v2`, and `$meminit_v2` cells belonging to the same memory to a single
`$mem_v2` cell, whereas the `memory_unpack` pass performs the inverse operation.
The `memory_dff` pass can combine asynchronous memory ports that are fed by or
feeding registers into synchronous memory ports. The `memory_bram` pass can be
used to recognize `$mem_v2` cells that can be implemented with a block RAM
resource on an FPGA. The `memory_map` pass can be used to implement `$mem_v2`
cells as basic logic: word-wide DFFs and address decoders.
.. autocellgroup:: mem
:members:
:source:
:linenos:

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.. role:: verilog(code)
:language: Verilog
Multiplexers
------------
Multiplexers are generated by the Verilog HDL frontend for ``?:``-expressions.
Multiplexers are also generated by the proc pass to map the decision trees from
RTLIL::Process objects to logic.
The simplest multiplexer cell type is `$mux`. Cells of this type have a
``WITDH`` parameter and data inputs ``A`` and ``B`` and a data output ``Y``, all
of the specified width. This cell also has a single bit control input ``S``. If
``S`` is 0 the value from the input ``A`` is sent to the output, if it is 1 the
value from the ``B`` input is sent to the output. So the `$mux` cell implements
the function :verilog:`Y = S ? B : A`.
The `$pmux` cell is used to multiplex between many inputs using a one-hot select
signal. Cells of this type have a ``WIDTH`` and a ``S_WIDTH`` parameter and
inputs ``A``, ``B``, and ``S`` and an output ``Y``. The ``S`` input is
``S_WIDTH`` bits wide. The ``A`` input and the output are both ``WIDTH`` bits
wide and the ``B`` input is ``WIDTH*S_WIDTH`` bits wide. When all bits of ``S``
are zero, the value from ``A`` input is sent to the output. If the :math:`n`\
'th bit from ``S`` is set, the value :math:`n`\ 'th ``WIDTH`` bits wide slice of
the ``B`` input is sent to the output. When more than one bit from ``S`` is set
the output is undefined. Cells of this type are used to model "parallel cases"
(defined by using the ``parallel_case`` attribute or detected by an
optimization).
The `$tribuf` cell is used to implement tristate logic. Cells of this type have
a ``WIDTH`` parameter and inputs ``A`` and ``EN`` and an output ``Y``. The ``A``
input and ``Y`` output are ``WIDTH`` bits wide, and the ``EN`` input is one bit
wide. When ``EN`` is 0, the output is not driven. When ``EN`` is 1, the value
from ``A`` input is sent to the ``Y`` output. Therefore, the `$tribuf` cell
implements the function :verilog:`Y = EN ? A : 'bz`.
Behavioural code with cascaded if-then-else- and case-statements usually results
in trees of multiplexer cells. Many passes (from various optimizations to FSM
extraction) heavily depend on these multiplexer trees to understand dependencies
between signals. Therefore optimizations should not break these multiplexer
trees (e.g. by replacing a multiplexer between a calculated signal and a
constant zero with an `$and` gate).
.. autocellgroup:: mux
:members:
:source:
:linenos:

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.. autocellgroup:: word_other
:caption: Other word-level cells
:members:
:source:
:linenos:

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.. role:: verilog(code)
:language: Verilog
Registers
---------
SR-type latches are represented by `$sr` cells. These cells have input ports
``SET`` and ``CLR`` and an output port ``Q``. They have the following
parameters:
``WIDTH``
The width of inputs ``SET`` and ``CLR`` and output ``Q``.
``SET_POLARITY``
The set input bits are active-high if this parameter has the value ``1'b1``
and active-low if this parameter is ``1'b0``.
``CLR_POLARITY``
The reset input bits are active-high if this parameter has the value ``1'b1``
and active-low if this parameter is ``1'b0``.
Both set and reset inputs have separate bits for every output bit. When both the
set and reset inputs of an `$sr` cell are active for a given bit index, the
reset input takes precedence.
D-type flip-flops are represented by `$dff` cells. These cells have a clock port
``CLK``, an input port ``D`` and an output port ``Q``. The following parameters
are available for `$dff` cells:
``WIDTH``
The width of input ``D`` and output ``Q``.
``CLK_POLARITY``
Clock is active on the positive edge if this parameter has the value ``1'b1``
and on the negative edge if this parameter is ``1'b0``.
D-type flip-flops with asynchronous reset are represented by `$adff` cells. As
the `$dff` cells they have ``CLK``, ``D`` and ``Q`` ports. In addition they also
have a single-bit ``ARST`` input port for the reset pin and the following
additional two parameters:
``ARST_POLARITY``
The asynchronous reset is active-high if this parameter has the value
``1'b1`` and active-low if this parameter is ``1'b0``.
``ARST_VALUE``
The state of ``Q`` will be set to this value when the reset is active.
Usually these cells are generated by the `proc` pass using the information in
the designs RTLIL::Process objects.
D-type flip-flops with synchronous reset are represented by `$sdff` cells. As
the `$dff` cells they have ``CLK``, ``D`` and ``Q`` ports. In addition they also
have a single-bit ``SRST`` input port for the reset pin and the following
additional two parameters:
``SRST_POLARITY``
The synchronous reset is active-high if this parameter has the value ``1'b1``
and active-low if this parameter is ``1'b0``.
``SRST_VALUE``
The state of ``Q`` will be set to this value when the reset is active.
Note that the `$adff` and `$sdff` cells can only be used when the reset value is
constant.
D-type flip-flops with asynchronous load are represented by `$aldff` cells. As
the `$dff` cells they have ``CLK``, ``D`` and ``Q`` ports. In addition they also
have a single-bit ``ALOAD`` input port for the async load enable pin, a ``AD``
input port with the same width as data for the async load data, and the
following additional parameter:
``ALOAD_POLARITY``
The asynchronous load is active-high if this parameter has the value ``1'b1``
and active-low if this parameter is ``1'b0``.
D-type flip-flops with asynchronous set and reset are represented by `$dffsr`
cells. As the `$dff` cells they have ``CLK``, ``D`` and ``Q`` ports. In addition
they also have multi-bit ``SET`` and ``CLR`` input ports and the corresponding
polarity parameters, like `$sr` cells.
D-type flip-flops with enable are represented by `$dffe`, `$adffe`, `$aldffe`,
`$dffsre`, `$sdffe`, and `$sdffce` cells, which are enhanced variants of `$dff`,
`$adff`, `$aldff`, `$dffsr`, `$sdff` (with reset over enable) and `$sdff` (with
enable over reset) cells, respectively. They have the same ports and parameters
as their base cell. In addition they also have a single-bit ``EN`` input port
for the enable pin and the following parameter:
``EN_POLARITY``
The enable input is active-high if this parameter has the value ``1'b1`` and
active-low if this parameter is ``1'b0``.
D-type latches are represented by `$dlatch` cells. These cells have an enable
port ``EN``, an input port ``D``, and an output port ``Q``. The following
parameters are available for `$dlatch` cells:
``WIDTH``
The width of input ``D`` and output ``Q``.
``EN_POLARITY``
The enable input is active-high if this parameter has the value ``1'b1`` and
active-low if this parameter is ``1'b0``.
The latch is transparent when the ``EN`` input is active.
D-type latches with reset are represented by `$adlatch` cells. In addition to
`$dlatch` ports and parameters, they also have a single-bit ``ARST`` input port
for the reset pin and the following additional parameters:
``ARST_POLARITY``
The asynchronous reset is active-high if this parameter has the value
``1'b1`` and active-low if this parameter is ``1'b0``.
``ARST_VALUE``
The state of ``Q`` will be set to this value when the reset is active.
D-type latches with set and reset are represented by `$dlatchsr` cells. In
addition to `$dlatch` ports and parameters, they also have multi-bit ``SET`` and
``CLR`` input ports and the corresponding polarity parameters, like `$sr` cells.
.. autocellgroup:: reg
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9
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Specify rules
-------------
.. TODO:: `$specify2`, `$specify3`, and `$specrule` cells.
.. autocellgroup:: spec
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