nextpnr/docs/netlist.md

nextpnr netlist structures documentation

The current in-memory design in nextpnr uses several basic structures. See the FAQ for more info on terminology.

See also the Arch API reference for information on developing new architectures and accessing the architecture database.

• CellInfo: instantiation of a physical block in the netlist (currently, all cells in nextpnr are blackboxes.)
• NetInfo: a connection between cell ports. Has at most one driver; and zero or more users
• BaseCtx: contains all cells and nets, subclassed by Arch and then becomes Context

Other structures used by these basic structures include:

• Property: stores a numeric or string value - isomorphic to RTLIL::Const in Yosys
• PortInfo: stores the name, direction and connected net (if applicable) of cell ports
• PortRef: used to reference the source/sink ports of a net; refers back to a cell and a port name

CellInfo

CellInfo instances have the following fields:

• name and type are IdStrings containing the instance name, and type
• ports is a map from port name IdString to PortInfo structures for each cell port
• bel and belStrength contain the ID of the Bel the cell is placed onto; and placement strength of the cell; if placed. Placement/ripup should always be done by Arch::bindBel and Arch::unbindBel rather than by manipulating these fields.
• params and attrs store parameters and attributes - from the input JSON or assigned in flows to add metadata - by mapping from parameter name IdString to Property.
• The constr_ fields are for relative constraints:
  • constr_parent is a reference to the cell this cell is constrained with respect to; or nullptr if not relatively constrained. If not nullptr, this cell should be in the parent's constr_children.
  • constr_children is a list of cells relatively constrained to this one. All children should have constr_parent == this.
  • constr_x and constr_y are absolute (constr_parent == nullptr) or relative (constr_parent != nullptr) tile coordinate constraints. If set to UNCONSTR then the cell is not constrained in this axis (defaults to UNCONSTR)
  • constr_z is an absolute (constr_abs_z) or relative (!constr_abs_z) 'Z-axis' (index inside tile, e.g. logic cell) constraint
• region is a reference to a Region if the cell is constrained to a placement region (e.g. for partial reconfiguration or out-of-context flows) or nullptr otherwise.

NetInfo

NetInfo instances have the following fields:

• name is the IdString name of the net - for nets with multiple names, one name is chosen according to a set of rules by the JSON frontend
• driver refers to the source of the net using PortRef; driver.cell == nullptr means that the net is undriven. Nets must have zero or one driver only. The corresponding cell port must be an output and its PortInfo::net must refer back to this net.
• users contains a list of PortRef references to sink ports on the net. Nets can have zero or more sinks. Each corresponding cell port must be an input or inout; and its PortInfo::net must refer back to this net.
• wires is a map that stores the routing tree of a net, if the net is routed.
  • Each entry in wires maps from sink wire in the routing tree to its driving pip, and the binding strength of that pip (e.g. how freely the router may rip up the pip)
  • Manipulation of this structure is done automatically by Arch::bindWire, Arch::unbindWire, Arch::bindPip and Arch::unbindPip; which should almost always be used in lieu of manual manipulation
• attrs stores metadata about the wire (which may come from the JSON or be added by passes)