Clock Tree Synthesis#

The clock tree synthesis module in OpenROAD (cts) is based on TritonCTS 2.0. It is available from the clock_tree_synthesis command. TritonCTS 2.0 performs on-the-fly characterization. Thus, there is no need to generate characterization data. The on-the-fly characterization feature can be optionally controlled by parameters specified by the configure_cts_characterization command. Use set_wire_rc command to set the clock routing layer.

Commands#

Note

Parameters in square brackets [-param param] are optional.
Parameters without square brackets -param2 param2 are required.

Configure CTS Characterization#

Configure key CTS characterization parameters, for example maximum slew and capacitance, as well as the number of steps they will be divided for characterization.

configure_cts_characterization 
    [-max_slew max_slew]
    [-max_cap max_cap]
    [-slew_steps slew_steps]
    [-cap_steps cap_steps]

Options#

Switch Name	Description
`-max_slew`	Max slew value (in the current time unit) 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`	Max capacitance value (in the current capacitance unit) 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_steps`	Number of steps that `max_slew` will be divided into for characterization. The default value is `12`, and the allowed values are integers `[0, MAX_INT]`.
`-cap_steps`	Number of steps that `max_cap` will be divided into for characterization. The default value is `34`, and the allowed values are integers `[0, MAX_INT]`.

Clock Tree Synthesis#

Perform clock tree synthesis.

clock_tree_synthesis 
    [-wire_unit wire_unit]
    [-buf_list <list_of_buffers>]
    [-root_buf root_buf]
    [-clk_nets <list_of_clk_nets>]
    [-tree_buf <buf>]
    [-distance_between_buffers]
    [-branching_point_buffers_distance]
    [-clustering_exponent]
    [-clustering_unbalance_ratio]
    [-sink_clustering_size cluster_size]
    [-sink_clustering_max_diameter max_diameter]
    [-macro_clustering_size cluster_size]
    [-macro_clustering_max_diameter max_diameter]
    [-sink_clustering_enable]
    [-balance_levels]
    [-sink_clustering_levels levels]
    [-num_static_layers]
    [-sink_clustering_buffer]
    [-obstruction_aware]
    [-apply_ndr strategy]
    [-insertion_delay]
    [-dont_use_dummy_load]
    [-sink_buffer_max_cap_derate derate_value]
    [-delay_buffer_derate derate_value]
    [-library liberty_library_name]
    [-repair_clock_nets]
    [-no_insertion_delay]

Options#

Switch Name	Description
`-buf_list`	Tcl list of master cells (buffers) that will be considered when making the wire segments (e.g. `{BUFXX, BUFYY}`).
`-root_buffer`	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`	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`.
`-distance_between_buffers`	Distance (in microns) between buffers that `cts` 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`	Distance (in microns) 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`	Value that determines the power used on the difference between sink and means on the CKMeans clustering algorithm. The default value is `4`, and the allowed values are integers `[0, MAX_INT]`.
`-clustering_unbalance_ratio`	Value determines each cluster’s maximum capacity during CKMeans. A value of `0.5` (i.e., 50%) means that each cluster will have exactly half of all sinks for a specific region (half for each branch). The default value is `0.6`, and the allowed values are floats `[0, 1.0]`.
`-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 for the register tree. The allowed values are integers `[0, MAX_INT]`. If this is not specified the size will be automatically chosen between `10, 20 or 30` based on the tree buffer max cap.
`-sink_clustering_max_diameter`	Specifies maximum diameter (in microns) of sink cluster for the register tree. The allowed values are integers `[0, MAX_INT]`. If this is not specified the diameter will be automatically chosen between `50, 100 or 200`, based on the tree buffer max cap.
`-macro_clustering_size`	Specifies the maximum number of sinks per cluster for the macro tree. The default value is `4`, and the allowed values are integers `[0, MAX_INT]`.
`-macro_clustering_max_diameter`	Specifies maximum diameter (in microns) of sink cluster for the macro tree. The default value is `50`, and the allowed values are integers `[0, MAX_INT]`.
`-balance_levels`	Attempt to keep a similar number of levels in the clock tree across non-register cells (e.g., clock-gate or inverter). The default value is `False`, and the allowed values are bool.
`-clk_nets`	String containing the names of the clock roots. If this parameter is omitted, `cts` looks for the clock roots automatically.
`-num_static_layers`	Set the number of static layers. The default value is `0`, and the allowed values are integers `[0, MAX_INT]`.
`-sink_clustering_buffer`	Set the sink clustering buffer(s) to be used.
`-obstruction_aware`	Enables obstruction-aware buffering such that clock buffers are not placed on top of blockages or hard macros. This option may reduce legalizer displacement, leading to better latency, skew or timing QoR. The default value is `False`, and the allowed values are bool.
`-apply_ndr`	Applies 2X spacing non-default rule to clock nets except leaf-level nets following some strategy. There are four strategy options: `none, root_only, half, full`. If this is not specified, the default value is `none`.
`-dont_use_dummy_load`	Don’t apply dummy buffer or inverter cells at clock tree leaves to balance loads. The default values is `False`.
`-sink_buffer_max_cap_derate`	Use this option to control automatic buffer selection. To favor strong(weak) drive strength buffers use a small(large) value. The default value is `0.01`, meaning that buffers are selected by derating max cap limit by 0.01. The value of 1.0 means no derating of max cap limit.
`-delay_buffer_derate`	This option balances latencies between macro cells and registers by inserting delay buffers. The default value is `1.0`, meaning all needed delay buffers are inserted. A value of 0.5 means only half of necessary delay buffers are inserted. A value of 0.0 means no insertion of delay buffers.
`-library`	This option specifies the name of Liberty library from which clock buffers will be selected, such as the LVT or uLVT library. It is assumed that the library has already been loaded using the read_liberty command. If this option is not specified, clock buffers will be chosen from the currently loaded libraries, which may not include LVT or uLVT cells.
`-repair_clock_nets`	This option includes fixing long wires inside CTS prior to latency adjustment with delay buffers. This can lead to a more balanced clock tree. The default is not to perform clock net repair.
`-no_insertion_delay`	Ignore sink insertion delay in clock tree construction and balancing.

Instance Name Prefixes#

clock_tree_synthesis uses the following prefixes for the instances that it inserts:

Instance Prefix	Purpose
clkbuf_regs	Splitting registers from macros
clkload	Dummy loads to help balance the clock tree
delaybuf	Delay buffers to help balance the tree

Report CTS#

This command is used to extract the following metrics after a successful clock_tree_synthesis run.

Number of Clock Roots
Number of Buffers Inserted
Number of Clock Subnets
Number of Sinks.

report_cts 
    [-out_file file]

Options#

Switch Name	Description
`-out_file`	The file to save `cts` reports. If this parameter is omitted, the report is streamed to `stdout` and not saved.

Set CTS configuration#

This command is used to set the configuration of CTS.

set_cts_config
    [-apply_ndr strategy]
    [-buf_list <list_of_buffers>]
    [-branching_point_buffers_distance distance]
    [-clock_buffer_footprint string]
    [-clock_buffer_string string]
    [-clustering_exponent power]
    [-clustering_unbalance_ratio capacity]
    [-delay_buffer_derate derate_value]
    [-distance_between_buffers distance]
    [-library liberty_library_name]
    [-macro_clustering_max_diameter max_diameter]
    [-macro_clustering_size cluster_size]
    [-num_static_layers num_layers]
    [-root_buf root_buf]
    [-sink_buffer_max_cap_derate derate_value]
    [-sink_clustering_levels levels]
    [-sink_clustering_max_diameter max_diameter]
    [-sink_clustering_size cluster_size]
    [-skip_nets <list_of_clk_nets_to_skip>]
    [-tree_buf <buf>]
    [-wire_unit wire_unit]

Options#

Switch Name	Description
`-apply_ndr`	Applies 2X spacing non-default rule to clock nets except leaf-level nets following some strategy. There are four strategy options: `none, root_only, half, full`. If this is not specified, the default value is `none`.
`-buf_list`	Tcl list of master cells (buffers) that will be considered when making the wire segments (e.g. `{BUFXX, BUFYY}`).
`-branching_point_buffers_distance`	Distance (in microns) 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.
`-clock_buffer_footprint`	sub-string that identifies clock buffers by liberty cell_footprint attribute. This option is mutually exclusive with -clock_buffer_string.
`-clock_buffer_string`	sub-string that identifies clock buffers by name. The default is CLKBUF. This option is mutally exclusive with -clock_buffer_footprint.
`-clustering_exponent`	Value that determines the power used on the difference between sink and means on the CKMeans clustering algorithm. The default value is `4`, and the allowed values are integers `[0, MAX_INT]`.
`-clustering_unbalance_ratio`	Value determines each cluster’s maximum capacity during CKMeans. A value of `0.5` (i.e., 50%) means that each cluster will have exactly half of all sinks for a specific region (half for each branch). The default value is `0.6`, and the allowed values are floats `[0, 1.0]`.
`-delay_buffer_derate`	This option balances latencies between macro cells and registers by inserting delay buffers. The default value is `1.0`, meaning all needed delay buffers are inserted. A value of 0.5 means only half of necessary delay buffers are inserted. A value of 0.0 means no insertion of delay buffers.
`-distance_between_buffers`	Distance (in microns) between buffers that `cts` 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.
`-library`	This option specifies the name of Liberty library from which clock buffers will be selected, such as the LVT or uLVT library. It is assumed that the library has already been loaded using the read_liberty command. If this option is not specified, clock buffers will be chosen from the currently loaded libraries, which may not include LVT or uLVT cells.
`-macro_clustering_max_diameter`	Specifies maximum diameter (in microns) of sink cluster for the macro tree. The default value is `50`, and the allowed values are integers `[0, MAX_INT]`.
`-macro_clustering_size`	Specifies the maximum number of sinks per cluster for the macro tree. The default value is `4`, and the allowed values are integers `[0, MAX_INT]`.
`-num_static_layers`	Set the number of static layers. The default value is `0`, and the allowed values are integers `[0, MAX_INT]`.
`-root_buffer`	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.
`-sink_buffer_max_cap_derate`	Use this option to control automatic buffer selection. To favor strong(weak) drive strength buffers use a small(large) value. The default value is `0.01`, meaning that buffers are selected by derating max cap limit by 0.01. The value of 1.0 means no derating of max cap limit.
`-sink_clustering_levels`	…
`-sink_clustering_max_diameter`	Specifies maximum diameter (in microns) of sink cluster for the register tree. The allowed values are integers `[0, MAX_INT]`. If this is not specified the diameter will be automatically chosen between `50, 100 or 200`, based on the tree buffer max cap.
`-sink_clustering_size`	Specifies the maximum number of sinks per cluster for the register tree. The allowed values are integers `[0, MAX_INT]`. If this is not specified the size will be automatically chosen between `10, 20 or 30` based on the tree buffer max cap.
`-sink_clustering_levels`	…
`-skip_nets`	Tcl list containing the clock nets to be skipped (e.g. `{net1, net2}`). If the net is a root clock net, the whole clock is skipped, otherwise only the subnet specified is skipped.
`-wire_unit`	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`.

Report CTS configuration#

This command is used to extract the configuration of CTS.

report_cts_config

Reset CTS configuration#

This command is used to reset the configuration of CTS. The flags determine which configurations will be reset to their default values. Using it without a flag means resetting all configurations.

reset_cts_config
    [-apply_ndr]
    [-buf_list]
    [-branching_point_buffers_distance]
    [-clock_buffer_footprint]
    [-clock_buffer_string]
    [-clustering_exponent]
    [-clustering_unbalance_ratio]
    [-delay_buffer_derate]
    [-distance_between_buffers]
    [-library]
    [-macro_clustering_max_diameter]
    [-macro_clustering_size]
    [-num_static_layers]
    [-root_buf]
    [-sink_buffer_max_cap_derate]
    [-sink_clustering_levels]
    [-sink_clustering_max_diameter]
    [-sink_clustering_size]
    [-skip_nets]
    [-tree_buf]
    [-wire_unit]

Useful Developer Commands#

If you are a developer, you might find these useful. More details can be found in the source file or the swig file.

Command Name	Description
`clock_tree_synthesis_debug`	Option to plot the CTS to GUI.

Example scripts#

clock_tree_synthesis -root_buf "BUF_X4" \
                     -buf_list "BUF_X4" \
                     -wire_unit 20
report_cts "file.txt"

Regression tests#

There are a set of regression tests in ./test. For more information, refer to this section.

Simply run the following script:

./test/regression

Limitations#

FAQs#

Check out GitHub discussion about this tool.

References#

LEMON - Library for Efficient Modeling and Optimization in Networks
Kahng, A. B., Li, J., & Wang, L. (2016, November). Improved flop tray-based design implementation for power reduction. In 2016 IEEE/ACM International Conference on Computer-Aided Design (ICCAD) (pp. 1-8). IEEE. (.pdf)

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’s 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.

Clock Tree Synthesis

Contents

Clock Tree Synthesis#

Commands#

Configure CTS Characterization#

Options#

Clock Tree Synthesis#

Options#

Instance Name Prefixes#

Report CTS#

Options#

Set CTS configuration#

Options#

Report CTS configuration#

Reset CTS configuration#

Useful Developer Commands#

Example scripts#

Regression tests#

Limitations#

FAQs#

References#

Authors#

License#