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PDK_STRUCTURE.md

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Porting a PDK

This readme describes how to port a PDK to openlane.

Folder structure

This is the expected folder structure for a PDK:

  • <pdk_name>
    • libs.tech
      • openlane
        • config.tcl
        • common_pdn.tcl
        • <standard cell library>
          • config.tcl
          • tracks.info
          • no_synth.cells
          • drc_exclude.cells
    • libs.ref
      • lef
      • techLEF
      • lib

Configuration files

  • config.tcl PDK configuration file contains common information for all standard cell libraries under this PDK
  • common_pdn.tcl Pdn configuration file. Refer to PDN configuration section.
  • <standard cell library>/config.tcl standard cell library configuration file which contains information specific to that SCL. It can override PDK configuration file.
  • <standard cell library>/tracks.info Contains information about the metal layers offsets and pitches. Refer to tracks configuration file section
  • <standard cell library>/no_synth.cells that should contain the list of newline-separated cell names to trim during synthesis (to not use them in synthesis). More in this section.

PDK Variables

This section defines the neccessary variables for PDK configuration file

Variable Description
DEF_UNITS_PER_MICRON Defines the unit distance microns. Used during floorplanning for proper def file generation.
VDD_PIN Defines the power pin of the cells.
GND_PIN Defines the ground pin of the cells.
TRACKS_INFO_FILE Points to the path of the tracks file. Used by the floorplanner to generate tracks
TECH_LEF Points to the path of the tech lef.
CELLS_LEF A list of paths to the cells lef views. Recommended to use wild card to catch all the files as follows: [glob "$::env(PDK_ROOT)/sky130A/libs.ref/$::env(STD_CELL_LIBRARY)/lef/*.lef"]
GDS_FILES A list of paths to the cells GDS-II views. Recommended to use wild card to catch all the files as follows: [glob "$::env(PDK_ROOT)/sky130A/libs.ref/$::env(STD_CELL_LIBRARY)/gds/*.gds"]
MAGIC_TECH_FILE Points to the magic tech file which mainly has drc rules.
KLAYOUT_TECH Points to the klayout tech file (.lyt).
KLAYOUT_PROPERTIES Points to the klayout properties file (.lyp).
MAGIC_MAGICRC Points to the magicrc file that is sourced while running magic in the flow.
GPIO_PADS_LEF A list of the pads lef views. For example:[glob "$::env(PDK_ROOT)/sky130A/libs.ref/sky130_fd_io/lef/sky130_fd_io.lef"]
NETGEN_SETUP_FILE Points to the setup file for netgen(lvs), that can exclude certain cells etc..
FP_TAPCELL_DIST The distance between tapcell columns. Used in floorplanning in tapcell insertion.
GLB_RT_L1_ADJUSTMENT Reduction in the routing capacity of the edges between the cells in the global routing graph but specific to li1 layer in sky130A. Values range from 0 to 1
(Default: 0)
CTS_SQR_CAP Defines the capacitance per square micron, used in CTS.
CTS_SQR_RES Defines the resistance per square micron, used in CTS.
DEFAULT_MAX_TRAN Defines the default maximum transition value, used in CTS & synthesis.
FP_PDN_RAIL_OFFSET Defines the rail offset for met1 used in PDN.
Default: 0.
FP_PDN_VWIDTH Defines the strap width for the vertical layer used in PDN.
Default: 1.6.
FP_PDN_HWIDTH Defines the strap width for the horizontal layer used in PDN.
Default: 1.6.
FP_PDN_CORE_RING_VWIDTH Defines the vertical width for the vertical layer used to create the core ring in the PDN.
Default: 20.
FP_PDN_CORE_RING_HWIDTH Defines the horizontal width for the horizontal layer used to create the core ring in the PDN.
Default: 20.
FP_PDN_CORE_RING_VSPACING Defines the spacing for the vertical layer used to create the core ring in the PDN.
Default: 5.
FP_PDN_CORE_RING_HSPACING Defines the spacing for the horizontal layer used to create the core ring in the PDN.
Default: 5.
FP_PDN_CORE_RING_VOFFSET Defines the offset for the vertical layer used to create the core ring in the PDN.
Default: 20.
FP_PDN_CORE_RING_HOFFSET Defines the offset for the horizontal layer used to create the core ring in the PDN.
Default: 20.
WIRE_RC_LAYER The metal layer used in estimate parastics set_wire_rc.
Default: met1.

Standard cell library-specific variables

This section defines the necessary variables to configure a standard cell library for use with OpenLane:

Variable Description
LIB_SYNTH Points to the lib file used during synthesis.
LIB_SLOWEST Points to the lib file, corresponding to the slowest corner, for max delay calculation during STA.
LIB_FASTEST Points to the lib file, corresponding to the fastest corner, for min delay calculation during STA.
LIB_TYPICAL Points to the lib file for typical delay calculation during STA.
NO_SYNTH_LIST Specifies the file that contains the don't-use-cell-list to be excluded from the liberty file during synthesis and timing optimizations. If it's not defined, this path is searched $::env(PDK_ROOT)/$::env(PDK)/libs.tech/openlane/$::env(STD_CELL_LIBRARY)/no_synth.cells and if it's not found, then the original liberty will be used as is.
PLACE_SITE Defines the main site used by the cells. Used during floorplanning to generate the rows.
PLACE_SITE_WIDTH Defines the main site width.Used during floorplanning to generate the rows.
PLACE_SITE_HEIGHT Defines the main site height.Used during floorplanning to generate the rows.
FP_WELLTAP_CELL Defines the tapcell to be used in tapcell insertion.
If this is not defiend then tapcell insertion will be skipped but the flow will resume normally
FP_ENDCAP_CELL Defines the decapcell. Inserted during floorplanning at the sides of the design.
SYNTH_DRIVING_CELL Defines the cell to drive the input ports. Used in synthesis
SYNTH_DRIVING_CELL_PIN Defines the driving cell output pin. Used in synthesis
SYNTH_CAP_LOAD Defines the capacitive load on the output ports in femtofarads. Used in synthesis
SYNTH_MIN_BUF_PORT Defines the buffer, followed by its input port and output port to be used by ins_buf statements by yosys. It insert buffer cells into the design for directly connected wires. Example: sky130_fd_sc_hd__buf_2 A X
SYNTH_TIEHI_PORT Defines the tie high cell followed by the port that implements the tie high functionality. Used in synthesis. Example: sky130_fd_sc_hd__conb_1 HI
SYNTH_TIELO_PORT Defines the tie low cell followed by the port that implements the tie high functionality. Used in synthesis. Example: sky130_fd_sc_hd__conb_1 LO
CELL_CLK_PORT Defines the name of clk port of the flip flops and other cells. Used in CTS.
PL_LIB Points to the lib view used in time driven placement.
FILL_CELL Defines the fill cell. Used in fill insertion. Can use a wild card to define a class of cells. Example sky130_fd_sc_hd__fill_*
DECAP_CELL Defines the decap cell used for fill insertion. Can use a wild card to define a class of cells. Example sky130_fd_sc_hd__fill_*
CELL_PAD Defines the number of sites to pad the cells lef views with.
CELL_PAD_EXCLUDE Defines the cells to exclude from padding.
CTS_ROOT_BUFFER Defines the cell inserted at the root of the clock tree. Used in CTS.
CLK_BUFFER Defines the clock buffer cell. Used in CTS.
CLK_BUFFER_INPUT Defines the clock buffer cell input port. Used in CTS.
CLK_BUFFER_OUTPUT Defines the clock buffer cell output port. Used in CTS.
CTS_CLK_BUFFER_LIST Defines the list of clock buffers to be used in CTS.
CTS_MAX_CAP Defines the maximum capacitance, used in CTS.
FP_PDN_RAIL_WIDTH Defines the rail width for met1 used in PDN.
SYNTH_LATCH_MAP A pointer for the file contianing the latch mapping for yosys.
TRISTATE_BUFFER_MAP A pointer for the file contianing the tri-state buffer mapping for yosys.
NO_SYNTH_CELL_LIST Specifies the file that contains the don't-use-cell-list to be excluded from the liberty file during synthesis. If it's not defined, this path is searched $::env(PDK_ROOT)/$::env(PDK)/libs.tech/openlane/$::env(STD_CELL_LIBRARY)/no_synth.cells and if it's not found, then the original liberty will be used as is.
DRC_EXCLUDE_CELL_LIST Specifies the file that contains the don't-use-cell-list to be excluded from the liberty file during synthesis and timing optimizations. If it's not defined, this path is searched $::env(PDK_ROOT)/$::env(PDK)/libs.tech/openlane/$::env(STD_CELL_LIBRARY)/drc_exclude.cells and if it's not found, then the original liberty will be used as is. In other words, DRC_EXCLUDE_CELL_LIST contain the only excluded cell list in timing optimizations.

Tracks Info File

The tracks files defines the metal layers pitches and offsets. This information should be extracted from the PDK's tech lef. The following is the format of the files

<layer name> X|Y <offset> <pitch>

PDN Configuration File

A sample PDN configuration file exists here.

More on the structure of the file here

No Synthesis Cells File

There are some cell types that you don't want to use in synthesis like, for example, delay cells and clock buffers (since CTS is a separate step that would insert the clock buffers).

Also, other cells don't have a default mapping in yosys, so they were excluded to reduce the liberty file size until we add a tech mapping file for each.

The smallest sizes of the cells were also removed to prevent the synthesizer from using them, which allows for a larger floorplan. These cells however are accessible by the optimizer which will in turn replace the bigger cells with these smaller sizes when needed. Eventually, this process will produce clean routing without forcing the users of OpenLane to re-configure their designs with smaller FP_CORE_UTIL and PL_TARGET_DENSITY values. Nonetheless, this change will come some time in the future.

However, this list is likely over-constraining, and if you have done experiments allowing smaller sizes incrementally and still got clean routed layouts, please let us know your findings, or better yet, submit a pull request at open_pdks with a suggested no_synth list.

This list is only used during synthesis.

You can also point your custom no_synth.cells by setting the value for NO_SYNTH_CELL_LIST to point to it.

DRC Exclude Cells File

Some of the cells, back when this list was created, had hard-to-access pin shapes, so the detailed router didn't manage to do routing cleanly.

Others had DRC violations within the cell definitions.

We excluded all cells of size _0 for the same reasons explained in the previous section.

The lpflow cells were also excluded because the flow is still unable to deal with them and connect a KAPWR supply.

This list is used for both synthesis and timing optimizations.

You can also point your custom no_synth.cells by setting the value for DRC_EXCLUDE_CELL_LIST to point to it.