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## Cell, BEL and Site Design
One of the key concepts within the FPGA interchange device resources is the
relationship between the cell library and the device BEL and site definitions.
A well designed cell library and a flexible but concise BEL and site
definition is important for exposing the hardware in an efficient way that
enables a place and route tool to succeed.
Good design is hard to capture, but this document will talk about some of the
considerations.
## Granularity of the cell library
It is important to divide the place and route problem and the synthesis
problem, at least as defined for the purpose of the FPGA interchange. The
synthesis tool operates on the **cell library**, which should be designed to
expose logic elements at a useful level of granularity.
As a concrete example, a LUT4 element is technically just two LUT3 elements,
connected by a mux (e.g. MUXF4), a LUT3 element is just two LUT2 elements,
connected by a mux (e.g. MUXF3), etc. If the outputs of those interior muxes
are not accessible to the place and route tool, then exposing those interior
function muxes as cells in the cell library is not as useful.
Cell definitions should be granular enough that the synthesis can map to
them, but not so granular that the place and route tool will be making few if
any choices. If there is only one legal placement of the cell, it's value is
relatively low.
## Drawing site boundaries
When designing an FPGA interchange device resource for a new fabric, one
important consideration is where to draw the site boundary. The primary goal
of lumping BELs within a site is to capture some local congestion due to
fanout limitations. Interior static routing muxes and output muxes may
accommodate significantly fewer signals than the possible number of BELs that
drive them. In this case, it is important to draw the site boundary large
enough to capture these cases so as to enable the local congestion to be
resolved during either packing for clustered approaches, or during placement
during unclustered approaches. In either case, local congestion that is
strongly placement dependant must be resolved prior to general routing,
unless a fused placement and routing algorithm is used.
### FF control sets routing
A common case worth exploring is FF control sets, e.g. SR type signals and CE
type signals. In most fabric SLICE types, the SR and CE control signals are
shared among multiple rows of the SLICE. This is a common example of local
site congestion, and the site boundary should typically encompass all BELs
that share this kind of local routing for all the reasons discussed above.
Another consideration with control signals is the presence of control signal
constraints that cannot be expressed as local routing congestion. For
example, if a set of BELs share whether the SR control line is a set or reset
(or async set or async reset), it is common to expand the site boundary to
cover the BELs that share these implicit configurations. The constraint
system in the device resources is designed to handle this kind of non-routing
driven configuration.
## Drawing BEL boundaries
BEL definitions require creating a boundary around primitive elements of
the fabric. The choice of where to place that boundary has a strong influence
on the design of the cell library in the FPGA interchange.
In general, the smaller the BEL boundary, the more complexity is exposed to
the place and route tool. In some cases exposing this complexity is
important, because it enables some goal. For example, leaving static routing
muxes outside of BELs enables a place and route tool to have greater
flexibility when resolving site congestion. But as a counter point, if only
a handful of static mux configurations are useful and those choices can be
made at synthesis time, then lumping those muxes into synthesis reduces the
complexity required in the place and route tool.
The most common case where the static routing muxes are typically lumped into
the BEL is BRAM's and FIFO's address and routing configuration. At synthesis
time, a choice is made about the address and data widths, which are encoded as
parameters on the cell. The place and route tool does not typically make
meaningful choices on the configuration of those static routing muxes, but
they do exist in the hardware.
The most common case where the static routing muxes are almost never lumped
into the BEL is SLICE-type situations. The remainder of this document will
show examples of why the BEL boundary should typically exclude the static
routing muxes, and leave the choice to the place and route tooling.
## Static routing muxes and bitstream formats
Something to keep in mind when drawing BEL boundaries to include or exclude
static routing muxes is the degree of configurability present in the
underlying bitstream. Some static routing muxes share configuration bits in
the bitstream, and so expressing them as two seperate static routing muxes
potentially gives the place and route tool flexibility than the underlying
fabric cannot express. This will result in physical netlists that cannot be
converted to bitstream.
In some cases this can be handled through tight coupling of the cell and
BEL library. The idea is to limit cell port to BEL pin mappings that avoid
illegal static routing mux configurations. This approach has its limits.
In general, considering how the bitstream expresses static routing muxes must
be accounted for when drawing BEL boundaries.
### Stratix II and Stratix 10 ALM
Consider both Stratix II and Stratix 10 logic sites. The first thing to note
is that the architectures at this level are actually mostly the same. Though
it isn't immediately apparent, both designs are structured around 4 4-LUT
elements.
Take note that of the following structure:
This is actually just two LUT4 elements, where the top select line is
independent.
See the following two figures:
In Stratix 10, the LUT4 element is still present, but the top select line
fracturing was removed.
So now consider the output paths from the the 4 LUT4 elements in the Stratix
II site. Some of the LUT4 outputs route directly to the carry element, so it
will be important for the place and route tool be able to place a LUT4 or
smaller to access that direct connection. But if the output is not used in
the carry element, then it can only be accessed in Stratix II via the MUXF5
(blue below) and MUXF6 (red below) elements.
So given the Stratix II site layout, the following BELs will be required:
- 4 LUT4 BELs that connect to the carry
- 2 LUT6 BELs that connect to the output FF or output MUX.
The two LUT6 BELs are shown below:
Drawing a smaller BEL boundary has little value, because a LUT5 element would
still always require routing through the MUXF6 element.
Now consider the Stratix 10 output arrangement. The LUT4 elements direct to
the carry element is the same, so those BELs would be identical. The Stratix
10 site now has an output tap directly on the top LUT5, similiar to the Xilinx
Versal LUT6 / LUT5 fracture setup. See diagram below. LUT5 element is shown
in blue, and LUT6 element is shown in red.
So given the Stratix 10 site layout, the following BELs will be required:
- 4 LUT4 BELs that connect to the carry
- 2 LUT5 BELs that connect to the output FF or output MUX
- 1 LUT6 BELs that connect to the output FF or output MUX
### Versal ACAP
The Versal ACAP LUT structure is fairly similiar to the Stratix 10 combitorial
elements.
Unlike the Stratix 10 ALM, it appears only 1 of the LUT4's connects to the
carry element (the prop signal). The O6 output also has a dedicate
connection to the carry. See image below:
The Versal LUT structure likely should be decomposed into 4 BELs, shown in
the next figures:
So given the Versal site layout, the following BELs will be required (per SLICE row):
- 1 LUT4 BELs that connect to the carry
- 2 LUT5 BELs that connect to the output FF or output MUX
- 1 LUT6 BELs that connect to the output FF or output MUX
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