❄️ Lighter ❄️

An automatic clock gating utility.

Table of contents

• Overview
• File structure
• Dependencies
• How to use
• How it works
• Power reduction analysis
• Authors
• Copyright and Licensing

📖 Overview

Electrical Power reduction in digital systems is significant for several reasons, including portability, reliability, and cost reduction. Because of this, power dissipation has become a critical parameter in low-power VLSI circuit designs. There are two sources for power dissipation in CMOS circuits: static power and dynamic power. Dynamic power is associated with the circuit switching activities due to the charging and discharging of internal node capacitances. On the other hand, static power is due to leakage current, or current that flows through the transistor when there is no activity. Dynamic power is the dominating component in mature fabrication processes such as sky130. It is still dominating in cutting-edge fabrication technologies. However, static power contribution to the total power is higher than mature technologies.

Several techniques can be utilized to reduce dynamic power through reducing the circuit switching activity. Clock gating is the most widely used technique here. It can be done manually or automatically. Automatic clock gating can be peformed for load-enabled registers.

Typically, RTL synthesizer maps load-enabled registers to flip-flops and multiplexors (or to load-enabled flip-flops if the standard cell library has them). In both cases, the dynamic power is very high as the flip-flops are connected to the clock, which is the fastest signal in the design. Instead of circulating the register output back to its input when the load condition is false (typically using multiplexors), the register clock can be enabled only when the load condition is true. This reduces the switching activities, which leads to lower dynamic power and less area (due to the elimination of the multiplexors). The following figure illustrates the automatic clock gating for a single flip flop.

Lighter is a Yosys plugin and technology mapping files that can perform automatic clock gating for registers to reduce the dynamic power. Currently, Lighter supports the following open-source standard cell libraries:

1. sky130_fd_sc_hd
2. sky130_fd_sc_hs
3. sky130_fd_sc_hvl
4. sky130_fd_sc_ms
5. gf180mcu_fd_sc_mcu7t5v0
6. gf180mcu_fd_sc_mcu9t5v0

Through extensive experimentation, Lighter has demonstrated significant power and area savings. To explore the detailed results, kindly refer to the following link: here

File structure

• designs / conatains verilog designs for benchmarking
• docs/ contains documentation
• platform/ contains standard cell libraries
• report_power/ contains power reporting python code (report_power.py)
  • stats/ contains full benchmarking results
• src/ contains clock gating Yosys plugin code
• validation/ contains automatic validation python code for clock-gated designs

🧱 Dependencies

You can find the installation steps in dependencies.md for:

• macos
• Linux
• Windows-10

🔍 How to use

Option one

First make sure to follow the dependancies section to install all requirements.

Generate the Yosys plugin using the following command:

yosys-config --build cg_plugin.so clock_gating_plugin.cc

Add the clock gating technology maping file to your project directory. For example, if you are using the sky130_fd_sc_hd standard cell library add the following file. You can find other supported libraries here.

• sky130_fd_sc_hd_ff_map.v

Add the flipflop clock gating command to your synthesis script:

reg_clock_gating sky130_fd_sc_hd_ff_map.v

For example:

read_verilog design
read_liberty -lib -ignore_miss_dir -setattr blackbox sky130_fd_sc_hd.lib
hierarchy -check
reg_clock_gating -map sky130_fd_sc_hd_ff_map.v
synth -top design
dfflibmap -liberty sky130_fd_sc_hd.lib
abc -D 1250 -liberty sky130_fd_sc_hd.lib
splitnets
opt_clean -purge
opt;; 
write_verilog -noattr -noexpr -nohex -nodec -defparam   design.gl.v

Run your Yosys synthesis script as follows:

yosys -m cg_plugin.so your_script.ys

Or TCL synthesis script as follows:

yosys -m cg_plugin.so your_script.tcl

Option two

Follow the same steps above for generating the Yosys plugin and adding the library files.

You can use the selection option to specify the flipflops you want to map in the clock gating step by doing the following:

• You need to add an attribute (pragma) to the module intended (inside the module declaration), for example:

    module test (...);
    (* clock_gate *)
    ...
    ...
    endmodule
  

• Then add the attribute to the clock gating command as a selection like:

    reg_clock_gating -map sky130_hd_ff_map.v a:clock_gate
  

For example:

read_verilog design
read_liberty -lib -ignore_miss_dir -setattr blackbox sky130_fd_sc_hd.lib
hierarchy -check
reg_clock_gating -map sky130_hd_ff_map.v a:clock_gate
synth -top design
dfflibmap -liberty sky130_fd_sc_hd.lib 
abc -D 1250 -liberty sky130_fd_sc_hd.lib 
splitnets
opt_clean -purge
opt;; 
write_verilog -noattr -noexpr -nohex -nodec -defparam  design.gl.v

🧐 How it works

A detailed guide can be found here

🔬 Power reduction analysis

Design	# Cells	# Added Clock Gates	Power reduction %	# Cells reduction %
AHB_SRAM	245	47	17.29%	25.71%
blabla	10589	1098	28.80%	3.72%
blake2s	14207	1872	33.70%	11.05%
blake2s_core	12971	1353	36.05%	12.53%
blake2s_m_select	4518	512	43.00%	22.29%
chacha	12857	1936	31.49%	4.28%
genericfir	143575	11624	32.62%	5.51%
i2c_master	758	106	13.29%	13.19%
jpeg_encoder	62472	4637	30.23%	11.78%
ldpcenc	20134	1273	18.04%	6.14%
NfiVe32_RF	3362	1024	30.46%	30.58%
picorv32a	14271	1244	25.42%	12.48%
PPU	10248	2845	19.74%	34.44%
prv32_cpu	2241	207	28.57%	23.47%
rf_64x64	13475	4096	32.01%	32.07%
sha512	20187	3669	30.53%	12.85%
spi_master	175	43	9.16%	17.14%
y_huff	11004	2345	19.55%	21.33%
y_quantizer	8281	2816	30.96%	30.85%
zigzag	3807	769	31.95%	58.21%

Further Stats:

Average Power Reduction %	Average cell Reduction %
27.14%	19.48%

Max Power Reduction %	Min Power Reduction %
43.00%	9.16%

Max cell Reduction %	Min cell Reduction %
58.21%	3.72%

To access the complete benchmarking data, methodology, and all the necessary details, we have prepared a dedicated file called benchmarks.md.

Authors

• Mohamed Shalan
• Youssef Kandil

⚖️ Copyright and Licensing

Copyright 2022 AUC Open Source Hardware Lab

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at:

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.