General interconnect

The general interconnect of Virtex 2 is made of several kinds of similar, but not identical interconnect tiles. The tile types include:

  • INT.CLB, the interconnect tile associated with configurable logic blocks
  • INT.BRAM, the interconnect tile associated with block RAMs
  • INT.{IOI|IOI.CLK_B|IOI.CLK_T}, the interconnect tile associated with I/O tiles
  • INT.DCM.{V2|V2P}, the interconnect tiles associated with digital clock managers
  • INT.CNR, the interconnect tile associated with corner tiles
  • INT.PPC, the interconnect tile associated with PowerPC cores and multi-gigabit transceivers
  • INT.GT.CLKPAD, the interconnect tile associated with multi-gigabit transceivers in the I/O row

The various tile types have the same backbone, but differ in the types of input multiplexers they have, and the primitive outputs they accept.

Backbone

The core of the interconnect is made of the following wires, each of which is instantiated once per interconnect tile and is driven by a buffered multiplexer:

  • OMUX0 through OMUX15, per-tile output multiplexers. The inputs to those include various primitive outputs in the tile. They serve as single-hop interconnect wires — each OMUX wire is also visible in one or two of the (immediately or diagonally) adjacent interconnect tiles:

    WireDirectionWire in tile #1Wire in tile #2
    OMUX0SOMUX0.S
    OMUX1W, then SOMUX1.WOMUX1.WS
    OMUX2E and SOMUX2.EOMUX2.S
    OMUX3S, then EOMUX3.SOMUX3.SE
    OMUX4SOMUX4.S
    OMUX5S, then WOMUX5.SOMUX5.SW
    OMUX6WOMUX6.W
    OMUX7E, then SOMUX7.EOMUX7.ES
    OMUX8E, then NOMUX8.EOMUX8.EN
    OMUX9WOMUX9.W
    OMUX10N, then WOMUX10.NOMUX10.NW
    OMUX11NOMUX11.N
    OMUX12N, then EOMUX12.NOMUX12.NE
    OMUX13E and NOMUX13.EOMUX13.N
    OMUX14W, then NOMUX14.WOMUX12.WN
    OMUX15NOMUX15.N

  • Double lines going in the cardinal directions, 10 per direction, called DBL.[EWSN][0-9]. Each of them has three segments, called DBL.[EWSN][0-9].[0-2], where .0 is located in the source tile and is driven, .1 is in the next tile in the relevant direction, and .2 is in the next tile after that. Some of the lines additionally have a fourth segment:

    • DBL.W[89].3 is to the north of DBL.W[89].2
    • DBL.E[01].3 is to the south of DBL.E[01].2
    • DBL.S[01].3 is to the south of DBL.S[01].2
    • DBL.N[89].3 is to the north of DBL.N[89].2

    Only the .0 segment is driven. The inputs to the multiplexer include:

    • OMUX wires
    • .1, .2 and .3 segments of other DBL wires
    • .3, .6 and .7 segments of HEX wires
    • OUT.FAN wires

  • Hex lines going in the cardinal directions, 10 per direction, called HEX.[EWSN][0-9]. They are analogous to double lines, except they have 7 (or sometimes 8) segments, thus spanning a distance of 6 tiles. The lines with 8th segment include:

    • HEX.W[89].7 is to the north of HEX.W[89].6
    • HEX.E[01].7 is to the south of HEX.E[01].6
    • HEX.S[01].7 is to the south of HEX.S[01].6
    • HEX.N[89].7 is to the north of HEX.N[89].6

    Only the .0 segment is driven. The inputs to the multiplexer include:

    • OMUX wires
    • .3, .6 and .7 segments of other HEX wires
    • .0, .6, .12, .18 segments of LV wires (for vertical lines)
    • .0, .6, .12, .18 segments of LH wires (for horizontal lines)
    • OUT.FAN wires

    Some hex lines are associated with various kind of input multiplexers, such that a single hex line can drive all input multiplexers of given type through its .0 to .5 segments:

    • HEX.[SN]0 is associated with IMUX.SR*
    • HEX.[SN]1 is associated with IMUX.IOI.TS1*
    • HEX.[SN]3 is associated with IMUX.TI*, IMUX.TS*, and IMUX.IOI.ICLK*
    • HEX.[SN]4 is associated with IMUX.IOI.TS2*
    • HEX.[SN]5 is associated with IMUX.IOI.ICE*
    • HEX.[SN]6 is associated with IMUX.CLK* and IMUX.DCMCLK*
    • HEX.[SN]8 is associated with IMUX.IOI.TCE*
    • HEX.[SN]9 is associated with IMUX.CE*

For large-fanout nets or nets that need to span long distances, the interconnect also has long lines that span the whole width or height of the device. There are 24 vertical long lines, LV, per an interconnect column, and 24 horizontal long lines, LH, per an interconnect row. They are visible as LV.{0-23} and LH.{0-23} segments in each interconnect tile, in a rotating way: LH.0 in a given tile is visible as LH.1 or LH.23 in the horizontally adjacent tiles. Only .0, .6, .12, .18 segments of long wires are actually accessible to a tile — the rest are just passing through.

Each interconnect tile can optionally drive the long line segments accessible to it via buffered multiplexers. The inputs to LH driver multiplexers include:

  • OMUX wires
  • .1 segments of DBL wires

The inputs to LV driver multiplexers include:

  • OMUX wires
  • .1 segments of DBL wires
  • various segments of HEX.E1 and HEX.W7 wires

The every-6-tiles nature of long wires combined with existence of hex wires allows for easy distribution of signals everywhere on the FPGA.

Input multiplexers

Every interconnect tile also contains input multiplexers, which drive the associated primitive inputs. The exact set of available input multiplexers depends on the type of interconnect tile.

The baseline set of input muxes is present in the INT.CLB tile:

  • IMUX.CLK[0-3]: four clock inputs. In CLBs, they correspond to the four SLICE\ s. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • GCLK0 through GCLK7, the clock interconnect global lines
    • various segments of DBL lines
    • any segment of HEX.S6 and HEX.N6 lines

    The IMUX.CLK multiplexers have a programmable inverter.

  • IMUX.SR[0-3]: four set/reset inputs. In CLBs, they correspond to the four SLICE\ s. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • various segments of DBL lines
    • any segment of HEX.S0 and HEX.N0 lines

    The IMUX.SR multiplexers have a programmable inverter.

  • IMUX.CE[0-3]: four clock enable inputs. In CLBs, they correspond to the four SLICE\ s. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • various segments of DBL lines
    • any segment of HEX.S9 and HEX.N9 lines

    The IMUX.CE multiplexers have a programmable inverter.

  • IMUX.TI[0-1]: two tristate buffer data inputs. In CLBs, they correspond to the two TBUF\ s. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • OMUX wires
    • various segments of DBL lines
    • any segment of HEX.S3 and HEX.N3 lines

    The IMUX.TI multiplexers have a programmable inverter.

  • IMUX.TS[0-1]: two tristate buffer enable inputs. In CLBs, they correspond to the two TBUF\ s. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • various segments of DBL lines
    • any segment of HEX.S3 and HEX.N3 lines

    The IMUX.TS multiplexers have a programmable inverter.

  • IMUX.S[0-3].B[XY]: 8 bypass inputs, specific to CLBs. They are special in that they can be used both as primitive inputs and as extra routing resources to reach other primitive inputs. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • OMUX wires
    • various segments of DBL lines
    • other IMUX.S[0-3].B[XY] wires

    These inputs have a programmable inverter. However, the inverter only affects the SLICE input — it doesn't affect the value seen when this wire is used as input to another IMUX.

    TODO: verify the inverter behavior, just in case

  • IMUX.S[0-3].[FG][0-3]: 32 LUT inputs, specific to CLBs. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • OMUX wires
    • various segments of DBL lines
    • IMUX.S[0-3].B[XY] wires
    • OUT.FAN wires

The INT.CNR and INT.PPC tiles have a similar set of input multiplexers, with some differences:

  • the IMUX.S[0-3].* wires are not present

  • IMUX.G[0-3].FAN[0-1] wires replace IMUX.S[0-3].B[XY]. They are similar, but do not include the programmable inverter. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • OMUX wires
    • various segments of DBL lines
    • other IMUX.G[0-3].FAN[0-1] wires

  • IMUX.G[0-3].DATA[0-7] wires replace IMUX.S[0-3].[FG][0-3]. They are multplexed from the same sources as IMUX.G[0-3].FAN[0-1] wires (thus removing the OUT.FAN sources present in CLBs).

The INT.BRAM tile is a variant of INT.CNR with the following differences:

  • IMUX.G[0-3].DATA[0-1] wires are not present

  • IMUX.BRAM_ADDR[AB][0-3] replace the above. They are used for blockram address inputs. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • OMUX wires
    • various segments of DBL lines
    • IMUX.G[0-3].FAN[0-1] wires
    • IMUX.BRAM_ADDR[AB][0-3] lines of the interconnect tile 4 tiles to the south or to the north of this tile (with matching [0-3] index, but any [AB] letter)

    The extra cascading input allows for distribution of identical addresses across a whole column of BRAMs without using general routing resources.

    The PowerPC cores are special: when a blockram is adjacent to a PowerPC core, the multiplexer inputs that would normally source from the adjacent blockram's IMUX.BRAM_ADDR* are instead connected to the PowerPC core's primitive outputs that drive OCM addresses. This once again allows routing resource savings.

The INT.DCM.* and INT.GT.CLKPAD tiles are a variant of INT.CNR with the following differences:

  • IMUX.CE[0-1], IMUX.CLK[0-3] and IMUX.TS[0-1] are not present

  • IMUX.DCMCLK[0-3] wires replace IOMUX.CLK[0-3]. They are multiplexed from:

    • PULLUP, a dummy always-1 wire
    • GCLK0 through GCLK7
    • DCM.CLKPAD[0-7], direct inputs from clock I/O pads (see clock interconnect)
    • various segments of DBL lines
    • any segment of HEX.S6 and HEX.N6 lines

    The IMUX.DCMCLK multiplexers have a programmable inverter.

The INT.IOI* tiles are a variant of INT.CNR with the following differences:

  • IMUX.TI[0-1] and IMUX.TS[0-1] are not present

  • IMUX.G[0-3].DATA[0-4] are not present

  • new multiplexers are added:

    • IMUX.IOI.ICLK[0-3]: four more clock inputs (used for I/O input clocks, while IMUX.CLK are used for output clocks). Multiplexed from:

      • PULLUP, a dummy always-1 wire
      • GCLK0 through GCLK7, the clock interconnect global lines
      • various segments of DBL lines
      • any segment of HEX.S3 and HEX.N3 lines

    • IMUX.IOI.TS[12][0-3]: tristate inputs. Multiplexed from:

      • PULLUP, a dummy always-1 wire
      • various segments of DBL lines
      • TS1: any segment of HEX.S1 and HEX.N1 lines
      • TS2: any segment of HEX.S4 and HEX.N4 lines
      • IMUX.G[0-3].FAN[0-1] wires

    • IMUX.IOI.[IT]CE[0-3]: clock enable inputs. Multiplexed from:

      • PULLUP, a dummy always-1 wire
      • various segments of DBL lines
      • ICE: any segment of HEX.S5 and HEX.N5 lines
      • TCE: any segment of HEX.S8 and HEX.N8 lines
      • IMUX.G[0-3].FAN[0-1] wires

Primitive outputs

Primitive outputs are wires that go from the various primitives into the general interconnect. The set of available primitive outputs depends on the type of the interconnect tile. The OUT.FAN* outputs can be used as inputs to many interconnect multiplexers, while other outputs can only be routed via OMUX multiplexers.

The INT.CLB tile has the following primitive outputs:

  • OUT.FAN[0-7]: the main combinational LUT outputs (X and Y); they have access to many more routing resources than other outputs
  • OUT.SEC[8-23]: the remaining SLICE outputs (XQ, YQ, XB, YB)
  • OUT.TBUS: a tap of one of the tristate lines passing through the CLB

Every OMUX multiplexer can mux from all OUT.FAN wires, and all but one of the remaining wires (in a rotating manner).

Note that the bandwidth limitation of 16 OMUX wires per tile means that it is not possible to use all primitive outputs in a CLB simultanously (there is one output too many).

The INT.IOI* tiles have the following primitive outputs:

  • OUT.FAN[0-7]: as above
  • OUT.SEC[8-23]: routed to all OMUX wires

There is no OMUX bottleneck in these tiles.

The INT.CNR tile has the following primitive outputs:

  • OUT.FAN[0-7]: as above
  • OUT.HALF[8-17].[01]: various outputs; .0 outputs are routed only to OMUX[0-7] while .1 outputs are routed only to OMUX[8-15], creating a possible bottleneck

The OMUX bottleneck is worse in this kind of tile. However, it only matters when accessing test primitives (specifically, the DCI primitives).

The INT.BRAM tile has the following primitive outputs:

  • OUT.FAN[0-7]
  • OUT.SEC[12-23]: routed to all OMUX wires
  • OUT.HALF[8-11].[01]: as above

The OMUX bottleneck in this tile type means it's not possible to instantiate the multiplier together with the blockram with maximum port width. However, this is in practice also prevented by IMUX resource sharing.

The INT.DCM* tiles have the following primitive outputs:

  • OUT.SEC[2-13]: routed to all OMUX wires
  • OUT.HALF[14-17].[01]: as above

OUT.SEC[2-11] correspond to the DCM's clock outputs, which are also routable via dedicated clock routing, and would usually not use the general interconnect.

The OMUX bottleneck in this tile type means that it's not possible to access all the outputs via general interconnect at once. However, this usually doesn't matter, as the clock outputs are generally used via dedicated clock routing, this doesn't matter in practice.

This tile type contains U-turns: some IMUX lines can be routed directly to OMUX for test purposes. They are:

  • IMUX.SR[0-3]
  • IMUX.TI[0-1]
  • IMUX.DCMCLK[0-3]
  • IMUX.CE[2-3]
  • IMUX.G[0-3].DATA[0-7]

The INT.PPC and INT.GT.CLKPAD tiles have the following primitive outputs:

  • OUT.FAN[0-7]
  • OUT.SEC[8-15], routed to all OMUX wires
  • OUT.TEST[0-15], used for test outputs, and routed to 2 OMUX wires each

The OMUX bottleneck means that it's not possible to access all outputs at once when the OUT.TEST outputs are used.

This tile type contains U-turns: some IMUX lines can be routed directly to OMUX for test purposes. They are:

  • IMUX.SR[0-3]
  • IMUX.TI[0-1]
  • IMUX.TS[0-1]
  • IMUX.CLK[0-3] (INT.PPC)
  • IMUX.DCMCLK[0-3] (INT.GT.CLKPAD)
  • IMUX.CE[0-3]
  • IMUX.G[0-3].DATA[0-7]

Additionally, this tile type has associated INTF.* interface tiles that contain testing U-turn logic that can rewire OUT.FAN* and OUT.SEC* to be mirrors of some IMUX pins instead of primitive outputs. There are four types of INTF.* tiles which have identical functionality, but differ in bitstream layout.

Terminators

The edges of the device contain special TERM.[EWSN] tiles that handle interconnect lines going out-of-bounds:

  • DBL lines get reflected — eg. northbound lines "bounce off" the top edge and become southbound lines

  • HEX lines outgoing from the TERM tile have special multiplexers, with the following choices per each line:

    • reflection: another, incoming HEX line is reflected onto this line
    • long line (two different taps): one of the LH.* or LV.* segments is driven onto this line
    • (some lines only) OUT.PCI[01]: one of the PCILOGIC outputs is driven onto this line

The PCI logic primitives effectively live outside of normal interconnect tiles, and use TERM.[EW] as their interconnect tiles. They reuse DBL and OMUX interconnect lines for their inputs, and have special OUT.PCI[0-1] primitive outputs that can be connected to outgoing HEX lines in the terminator tiles.

PowerPC holes

The PowerPC cores create holes in the interconnect structure — the area they occupy has no interconnect tiles. Not all interconnect lines can cross the gap across the PPC core. There are four special tile types around the core:

  • PPC.N: present on the bottom edge of the core
  • PPC.S: present on the top edge of the core
  • PPC.E: present on the left edge of the core
  • PPC.W: present on the right edge of the core

The long lines pass through the core undisturbed. The DBL lines are reflected as in the TERM tiles. The OMUX lines are likewise reflected. The HEX lines contain a multiplexer with the following choices per each line:

  • passthrough: the line passes through the core and connects to the corresponding line on the other side
  • reflection: another, incoming HEX line is reflected onto this line
  • long line (two different taps): one of the LH.* or LV.* segments is driven onto this line

Clock spine top and bottom

The clock spine top and bottom (containing BUFGMUX primitives) live horizontally in between two normal interconnect tiles. They have their own input multiplexers:

  • CLK.IMUX.SEL[0-7]: used for the BUFGMUX S input, multiplexed from:

    • PULLUP
    • various DBL horizontal segments

  • CLK.IMUX.CLK[0-7]: used for the BUFGMUX I[0-1] inputs when not using dedicated clock interconnect, multiplexed from:

    • PULLUP
    • various DBL horizontal segments

They also have their own primitive outputs:

  • CLK.OUT.[0-7]: special BUFGMUX primitive outputs

They have eight OMUX lines, multiplexed from the 8 primitive outputs above. The OMUX wires are connected to the neighbouring general interconnect tiles as-if they came from the otherwise out-of-bounds tiles above or below them.