OpenROAD

TritonCTS 2.0¶

TritonCTS 2.0 is available under the OpenROAD app as clock_tree_synthesis command. The following tcl snippet shows how to call TritonCTS. TritonCTS 2.0 performs on-the-fly characterization. Thus there is no need to generate characterization data. On-the-fly characterization feature could still be optionally controlled by parameters specified to configure_cts_characterization command. Use set_wire_rc command to set clock routing layer.

Commands¶

Configure CTS Characterization¶

configure_cts_characterization [-max_slew <max_slew>] \
                               [-max_cap <max_cap>] \
                               [-slew_inter <slew_inter>] \
                               [-cap_inter <cap_inter>]

Argument description:

-max_slew is the max slew value (in seconds) that the characterization will test. If this parameter is omitted, the code would use max slew value for specified buffer in buf_list from liberty file.
-max_cap is the max capacitance value (in farad) that the characterization will test. If this parameter is omitted, the code would use max cap value for specified buffer in buf_list from liberty file.
-slew_inter is the time value (in seconds) that the characterization will consider for results. If this parameter is omitted, the code gets the default value (5.0e-12). Be careful that this value can be quite low for bigger technologies (>65nm).
-cap_inter is the capacitance value (in farad) that the characterization will consider for results. If this parameter is omitted, the code gets the default value (5.0e-15). Be careful that this value can be quite low for bigger technologies (>65nm).

Clock Tree Synthesis¶

clock_tree_synthesis -buf_list <list_of_buffers> \
                     [-root_buf <root_buf>] \
                     [-wire_unit <wire_unit>] \
                     [-clk_nets <list_of_clk_nets>] \
                     [-out_path <lut_path>] \
                     [-post_cts_disable] \
                     [-distance_between_buffers] \
                     [-branching_point_buffers_distance] \
                     [-clustering_exponent] \
                     [-clustering_unbalance_ratio] \
                     [-sink_clustering_enable] \
                     [-sink_clustering_size <cluster_size>] \
                     [-sink_clustering_max_diameter <max_diameter>]

Argument description:

-buf_list are the master cells (buffers) that will be considered when making the wire segments.
-root_buffer is the master cell of the buffer that serves as root for the clock tree. If this parameter is omitted, the first master cell from -buf_list is taken.
-wire_unit is the minimum unit distance between buffers for a specific wire. If this parameter is omitted, the code gets the value from ten times the height of -root_buffer.
-clk_nets is a string containing the names of the clock roots. If this parameter is omitted, TritonCTS looks for the clock roots automatically.
-out_path is the output path (full) that the lut.txt and sol_list.txt files will be saved. This is used to load an existing characterization, without creating one from scratch.
-post_cts_disable is a flag that, when specified, disables the post-processing operation for outlier sinks (buffer insertion on 10% of the way between source and sink).
-distance_between_buffers is the distance (in micron) between buffers that TritonCTS should use when creating the tree. When using this parameter, the clock tree algorithm is simplified, and only uses a fraction of the segments from the LUT.
-branching_point_buffers_distance is the distance (in micron) that a branch has to have in order for a buffer to be inserted on a branch end-point. This requires the -distance_between_buffers value to be set.
-clustering_exponent is a value that determines the power used on the difference between sink and means on the CKMeans clustering algorithm. If this parameter is omitted, the code gets the default value (4).
-clustering_unbalance_ratio is a value that determines the maximum capacity of each cluster during CKMeans. A value of 50% means that each cluster will have exactly half of all sinks for a specific region (half for each branch). If this parameter is omitted, the code gets the default value (0.6).
-sink_clustering_enable enables pre-clustering of sinks to create one level of sub-tree before building H-tree. Each cluster is driven by buffer which becomes end point of H-tree structure.
-sink_clustering_size specifies the maximum number of sinks per cluster. Default value is 20.
sink_clustering_max_diameter specifies maximum diameter (in micron) of sink cluster. Default value is 50.
-clk_nets is a string containing the names of the clock roots. If this parameter is omitted, TritonCTS looks for the clock roots automatically.

Report CTS¶

Another command available from TritonCTS is report_cts. It is used to extract metrics after a successful clock_tree_synthesis run. These are: Number of Clock Roots, Number of Buffers Inserted, Number of Clock Subnets, and Number of Sinks. The following tcl snippet shows how to call report_cts.

report_cts [-out_file <file>]

Argument description:

-out_file (optional) is the file containing the TritonCTS reports. If this parameter is omitted, the metrics are shown on the standard output.

Example scripts¶

clock_tree_synthesis -root_buf "BUF_X4" \
                     -buf_list "BUF_X4" \
                     -wire_unit 20

report_cts "file.txt"

Regression tests¶

Limitations¶

FAQs¶

Check out GitHub discussion about this tool.

External references¶

LEMON - Library for Efficient Modeling and Optimization in Networks
Capacitate k-means package from Dr. Jiajia Li (UCSD). Published here.

Authors¶

TritonCTS 2.0 is written by Mateus Fogaça, PhD student in the Graduate Program on Microelectronics from the Federal University of Rio Grande do Sul (UFRGS), Brazil. Mr. Fogaça advisor is Prof. Ricardo Reis

Many guidance provided by (alphabetic order):

Andrew B. Kahng
Jiajia Li
Kwangsoo Han
Tom Spyrou

License¶

BSD 3-Clause License. See LICENSE file.

Table Of Contents

TritonCTS 2.0

OpenDP

CTS LUT characterization