The device resources schema is intended to be a complete description of an island-based FPGA design. It is made of many components, but the core description of the device is shown below.
┌─────────────────┐ │ Device │ │ ┌─────────────┐ │ │ │ Tile │ │ │ │ ┌─────────┐ │ │ │ │ │ Site │ │ │ │ │ │ ┌─────┐ │ │ │ │ │ │ │ BEL │ │ │ │ │ │ │ └─────┘ │ │ │ │ │ │ │ │ │ │ │ └─────────┘ │ │ │ │ │ │ │ └─────────────┘ │ │ │ └─────────────────┘
A device contains tiles
Tiles contains sites
Sites contains BELs
The schema contains the required information to answer questions such as:
Where are tiles located?
How are sites connected to the routing graph?
How are BELs connected to the boundary of the site?
How can cells be placed at BELs?
Device - A set of tiles and package pins.
Tiles - An instance of a tile type which contains wires and sites
Package pin - A boundry between the “interior” of the device and what is “outside” the package. Generally corresponds to a pin on a package, e.g. pin 1 on SOP-8 or A1 on CSG324.
Wire - Also known as a “tile wire” . A wire is a piece of conductive material totally contained within a tile. Wires can be part of nodes. Wires can connect to PIPs or site pins.
Node - A node is a set of 1 or more wires that are connected. Nodes can span multiple tiles. Nodes connect to PIPs or site pins via the wires that are part of the node.
PIP - PIP is an abbreviation for programable interconnect point. A PIP provides a connection between two wires. PIPs can be unidirectional or bidirectional. Unidirectional PIPs always connect wire0 to wire1. Bidirectional PIPs can connect wire0 to wire1 or wire1 to wire0.
Site - A collection of site pins, site wires and BELs.
Site pin - The connection between a site and a wire. Site pins may connect to 0 or more site port BELs.
Site wire - A piece of conductive material that connects to at most 1 output BEL pin and 0 or more input or inout BEL pins.
BEL - BEL is an abbreviation of basic logic element. A BEL can be one of 3 types, site port, logic, routing. A BEL contains 1 or more BEL pins.
BEL pin - A connection between a BEL and a site wire.
Logic BEL - A placable logic element. May be subject to 0 or more placement constraints.
Site port BEL - A site port BEL represents a connection to a site pin contained within the parent tile of the site. See Site port BEL.
Routing BEL - A routing BEL connects at most 1 input BEL pin to the output BEL pin. See Routing BEL.
Site PIP - A pair of input and output BEL pins belonging to a BEL that represents a logically connection.
Cell - A logical element of a design that contains some number of cell ports and some number of cell instances, and some number of nets.
Cell port - The boundary between the interior of a cell and the containing cell (if any).
Cell instance - A instance of a cell. The cell ports of may be connected to nets within the parent cell.
Net - A set of logically connected cell ports.
The place and route problem¶
The device resources schema is intended to provide a description for a tool solving the place and route problem. The definition of the problem used by this schema is described below:
There exists exactly 1 cell instance (the top instance) that contains 1 or more leaf cell instances that must be placed at BELs, such that no constraints are violated and the nets between the cell instances are routable. Routable means that site wires, site PIPs, nodes, PIPs can be assigned to at most 1 net such that each net driver BEL pin can reach each every net sink BEL pin on the net.
The device resource format describes how cell instances can be legally placed at BELs and how cell pins relate to BEL pins. When a cell instance is placed at a BEL, it may be subject to 0 or more constraints.
Nets are divided into 3 categories. A signal net represents a signal that is not either the constant logical 0 or constant logical 1 net. The constant logical 0 and constant logical 1 nets are special because they can have multiple drivers in the device description. Routing resources that are always part of the constant logical 0 or constant logical 1 net are explicitly defined in the device resources schema. The constant logical 0 net is listed in the schema as the “gnd” type. The constant logical 1 net is listed in the schema as the “vcc” type.
Rules for routing¶
Fully routed signal nets always begin at a output/inout BEL pin, and always end at an input/inout BEL pin. If a net enters a site, that net must end at an input/inout BEL pin. It is not legal for a net to enter and leave a site. If such a path is required, a pseudo PIP should be added to the schema.
The following is an example site for a SLICE for a non-existent device.
▲ CO│ │ ┌──────────────────────────┼─────────────────────────────────────┼──────┐ │ │ │ │ B0│ I0┌───────┐ │ BLUT ┌───────┐ ▼ CLK │ ─────┼────►│ │ ┌────────o─────────────────┬────►│ │ ┌────┐ │ B1│ I1│ │O │ │ │XOR │ │D D│ │Q │ FFOUT ─────┼────►│ BLUT3 ├───┤ │ ┌────────┬──o────►│ FFMUX ├──►│ FF ├───┼───► B2│ I2│ │ │ │ │ │ |ALUT │ │ │ │ │ ─────┼────►│ │ │ │ │ ┌──┬──o──o────►│ │ └────┘ │ │ └───────┘ │ ┌───┴───┐ │ │ │ │ │ └───────┘ │ │ │ SI│ │ │ │ │ │ │ │ │ └───►│ ├─┘ │ │ │ │ │ │ DX│ CARRY │ O │ │ │ │ │ │ ┌───►│ │ │ │ │ │ │ A0│ I0┌───────┐ │ └───┬───┘ │ │ │ │ ┌────────┐ │ ─────┼────►│ │ │ │ │ │ │ │ BLUT │ │ │ A1│ I1│ │O │ │ │ │ │ └─────►│ │OUT │ OUT ─────┼────►│ ALUT3 ├───┴────────o────────┘ │ │ XOR │ OUTMUX ├──────────┼────► B2│ I2│ │ │ │ └────────►│ │ │ ─────┼────►│ │ │ │ ALUT │ │ │ │ └───────┘ │ └───────────►└────────┘ │ │ │ │ │ │ │ └──────────────────────────┼────────────────────────────────────────────┘ CI│ │
In the above example, there are 17 BELs:
|BEL Name||Category||# Input||# Output|
Each site port BEL has a site pin, so the site pins are:
|Site Pin Name||Direction|
The BEL BLUT3 has 4 BEL pins:
|BEL pin name||Direction|
The BEL A0 has 1 BEL pin:
|BEL pin name||Direction|
The BEL OUTMUX has 4 BEL pins:
|BEL pin name||Direction|
There are 12 site PIPs:
|BEL name||In pin||Out pin|
The site wire BLUT3_O has 4 BEL pins:
The site wire B0 has 2 BEL pins:
Net routing summary¶
Each net start at the driver output/inout BEL pin. The BEL pin will be connected to exactly 1 site wire. If the net sink can be reached within the site, then the net can use site PIPs to reach a site wire connected to the input/inout BEL pin.
If the net sink is in another site, then the net must first reach a site port input BEL pin using site PIPs to reach the site wire connected to the site port. From there the net leaves the site via the site port and is now on the first node via the site pin matching the site port.
From there the net must use PIPs to expand to new nodes until arriving at a node attached to valid site pin for the sink. This would be a site pin that is part of the same site that the sink BEL is part of, and that the site port wire can reach the sink BEL pin (via 0 or more site PIPs). The site can be entered via the site port corresponding to the site pin. The first site wire in the site will be the site wire attached to the output BEL pin of the site port. From there site routing continues per above.
Use of site PIPs¶
It is important to note that site PIPs can only be used to access placed cells inside that site. Site PIPs cannot be used as general route-thrus, to route from site input to output. General route-thrus across entire sites should use tile pseudo PIPs as described below - even if a site pin is being validly used for one sink pin of a net that is located inside the site; site PIPs cannot also be used to leave the site again to reach other sinks.
LUT route-thrus, for example, might require both site PIPs and tile pseudo PIPs. The site PIP would be used to route through the LUT in order to access an associated flipflop input inside the site. The tile PIP would be used to route across the entire site as part of the general, inter-tile, routing problem.
A diagram illustrating the legal and illegal uses is shown below.
Tile Types and site types¶
To reduce data duplication in the device schema, both tiles and sites have a type. Most of the definition of the tile and site is in the type rather than repeated at each instance. This does cause some more complicated indirection, so the following section provides some additional explanation here.
Sites, site types and alternative site types¶
The most complicated relationship in the schema is likely the relationship between sites, site types and alternative site types.
Most of the site type description is independent of the tile / tile type that the site type is within. See the “SiteType” struct definition for the independent portion of the schema. The important exception is the relationship between wires and site pins.
Each site within a tile has a “primary site type”, which is found in the “SiteTypeInTileType” struct definition, contained in the “TileType” struct. The site within the tile will specify which “SiteTypeInTileType” to use for that particular site.
The primary site type contains a list of “alternative” site types that may be used (”altSiteTypes” in “SiteType”). The primary site type must always contains the complete list of site pins used in any of the alternative site types.
The site pins to wire relationship is always done via the primary site type. When an alternative site type is used, the site pins of that alternative site type must be first be mapped to a site pin of the primary site type.
The “SiteTypeInTileType” defines the relationship between the primary site type and the wires. It also defines the relationship from the alternative site type to the primary site type.
It first defines the primary site type (”primaryType”). It defines a map between the site pins in the primary site type and wires in the tile type that contains the site (”primaryPinsToTileWires”). Last it defines the map between the alternative site pin and the primary site pin (”altPinsToPrimaryPins”).
Important: When solving the place and route problem, only the primary or one of the alternative site types can be used at a time for a particular site.
A routing BEL represents statically configurable site routing by connecting a site wire connected to one of the input BEL pins to the output BEL pin of the BEL. Routing BELs must have 1 output BEL pin.
Inverting routing BELs¶
Some routing BELs can invert signals that pass through them. Defined the “inverting” field in the “BEL” struct with the BEL pins that invert and do not invert.
A site port BEL represents a connection to a site pin contained within the parent tile of the site. Site port BELs have exactly 1 BEL pin. The BEL name and pin name should be the same. The name of the BEL should match the name of the site pin that the site port connects too. The direction of the BEL pin should be the opposite of the site pin direction.
An input site pin “I0” would have a site port BEL named “I0” with 1 BEL output pin named “I0”.
The device resources schema also covers some important data required for handling common cases found in island based FPGAs.
It is fairly common for fabrics to contain site local signal inverters. Depending on the architecture, the place and route tool may be expected to leverage inverters or may even require it. The device resources schemas provides a description for how cells express inversion and how to use site local inverters to implement the requested inversion.
LUTs are common to every island-based FPGA, and many place and route tasks depend on having knowledge of how the LUTs are arranged. The LUT definition section of the device resources defines where LUTs exist as BELs and what cells can be placed at those BELs. This is important is at least two place and route tasks. The first is that LUTs can be trivially turned into site pips from the input of the LUT to the output of the LUT, subject to constraints and LUT equation sharing. The second is that LUTs can be trivially turned into constant sources from the output pin.
Some parameters attached to cell instances may be relevant for the place and route problem. A common example is LUT equation sharing, which can happen on fracturable LUTs. See the schema for details.
It may be important within a device to represent PIPs that “route-thru” one or more BELs. This can be modelled as placing a cell in a particular configuration at a BEL, subject to the normal cell placement rules. The “PseudoCell” struct defines what resources are used by using PIPs.
All pseudo PIPs must define at least 1 pseudo cell. Pseudo cells should include the site port BEL that the pseudo PIP used to enter the site.