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LibreCell - Lib

Characterization kit for CMOS cells. This Python package comes with a some stand-alone command-line tools:

  • Most notably lctime for recognition and characterization of combinational and sequential cells.
  • sp2bool: Recognition ('reverse engineering') of transistor networks. This is intended for analyzis and debugging.
  • libertyviz: Visualization of NDLM tables.

Getting started

See install instructions in top-project.

Characterize a cell

An ready-to-run example can be found in the examples folder. The script should characterize the INVX1 inverter.

The following example determines the input capacitances and timing delays of a combinational cell.

It is assumed that FreePDK45 is installed in the users home directory.

Required inputs are:

  • --liberty: A template liberty file which defines how the cells should be characterized.
  • --include: SPICE files or models to be included.
  • --spice: A SPICE file which contains the transistor level circuit of the cell (best including extracted parasitic capacitances).
  • --cell: Name of the cell to be characterized.
  • --output: Output liberty file which will contain the characterization data.

Characterize a single cell:

lctime --liberty ~/FreePDK45/osu_soc/lib/files/gscl45nm.lib \
	--include ~/FreePDK45/osu_soc/lib/files/gpdk45nm.m \
    --output-loads "0.05, 0.1, 0.2, 0.4, 0.8, 1.6" \
    --slew-times "0.1, 0.2, 0.4, 0.8, 1.6, 3.2" \
	--spice ~/FreePDK45/osu_soc/lib/source/netlists/AND2X1.pex.netlist \
	--cell AND2X1 \
	--output /tmp/and2x1.lib

Characterize multiple cells in the same run:

lctime --liberty ~/FreePDK45/osu_soc/lib/files/gscl45nm.lib \
	--include ~/FreePDK45/osu_soc/lib/files/gpdk45nm.m \
    --output-loads "0.05, 0.1, 0.2, 0.4, 0.8, 1.6" \
    --slew-times "0.1, 0.2, 0.4, 0.8, 1.6, 3.2" \
	--spice ~/FreePDK45/osu_soc/lib/source/netlists/*.pex.netlist \
	--cell INVX1 AND2X1 XOR2X1 \
	--output /tmp/invx1_and2x1_xor2x1.lib

Cell recognition

Cell types can be recognized automatically such that only a minimal liberty file needs to be supplied.

cd examples
lctime --liberty template.lib \
    --analize-cell-function \
    --include gpdk45nm.m \
    --spice INVX1.pex.netlist \
    --cell INVX1 \
    --output-loads "0.05, 0.1, 0.2, 0.4, 0.8, 1.6" \
    --slew-times "0.1, 0.2, 0.4, 0.8, 1.6, 3.2" \
    --output invx1.lib

Sequential cells

Characterization of sequential cells involves finding hold, setup, removal and recovery constraints.

For an example see examples/


Vizualize the result:

libertyviz -l /tmp/and2x1.lib --cell AND2X1 --pin Y --related-pin A --table cell_rise

Characterize a cell with differential inputs

Differential inputs can be specified in the liberty template with the complementary_pin attribute. Only the non-inverted pin should appear in the liberty file.

Differential pairs can also be recognized based on their naming. For example if pairs are named with suffixes _p for the non-inverted pin and _n for the inverted pin:

lctime --diff %_p,%_n ...

Merging liberty files

lctime will output a liberty file containing only one cell. The libertymerge command allows to merge this kind of output file back into the liberty template.

The following example will take base_liberty.lib as a template and update its cell entries with the data found in the liberty files in the characterization directory.

libertymerge -b base_liberty.lib \
    -o output_liberty.lib \
    -u characterization/*.lib

This approach allows to run characterization runs of multiple cells independently and in parallel (e.g using make).

Recognize a cell

lctime can recognize the boolean function of cells based on the transistor network. Besides combinational functions also memory-loops can be found and abstracted into latches or flip-flops. The sp2bool command can be used to analyze cells and dump information about their behaviour. This can be useful for debugging and verification.


# Analyze a combinational cell. 
sp2bool sp2bool --spice ~/FreePDK45/osu_soc/lib/files/cells.sp --cell NAND2X1

# Analyze a flip-flop with asynchronous set and reset signals.
sp2bool sp2bool --spice ~/FreePDK45/osu_soc/lib/files/cells.sp --cell DFFSR

For cells with differential inputs the --diff argument must be used to specify differential pairs.

License of FreePDK45

Files of the FreePDK45 in ./test_data/freepdk45 and ./examples are licensed under the APACHE-2.0 licence.

The according copyright notice is:

FreePDK 45nm verion 1.4 (2011-04-07)
(Subversion Repository revision 173)

Copyright 2007 - W. Rhett Davis, Paul Franzon, Michael Bucher,
                 and Sunil Basavarajaiah, North Carolina State University
Copyright 2008 - W. Rhett Davis, Michael Bucher, and Sunil Basavarajaiah,
                 North Carolina State University (ncsu_basekit subtree)
                 James Stine, and Ivan Castellanos,
                 and Oklahoma State University (osu_soc subtree)
Copyright 2011 - W. Rhett Davis, and Harun Demircioglu,
                 North Carolina State University