RTL Design, Synthesis and Optimization using Yosys and SKYWATER130 PDKs

In VLSI industry Front-end digital design inlcudes RTL design, synthesis, optimization and verification. In the current repository i have included the deisgn, synthesis and optimization work on various digital modules, from very simple digital logic (mux, decoder, encoder etc.) to complex designs(barrel shifter, booth multiplier, dual port ram etc). All the work has been done using open source tools:-

• iverilog for RTL simulation.
• gtkwave for reading .vcd file and waveform generation.
• yosys for synthesis and netlist generation.
• skywater 130nm open source pdk.

Introduction Of OPEN-SOURCE HARDWARE And TOOLS

With the introduction of open-source tools in the ASIC flow, now VLSI engineers have opportunity to design and craft their own ideas starting from RTL 2 GDSII. Moreover further inclusion of VLSI enthusiast and learners have propelled the need of open-source community for hardware also. Organizations such as:-

• open source hardware association
• SiFive
• FOSSI
• OpenPOWER Foundation
• VSD
• RISCV
• Redwood EDA
• Efabless etc

are committed to bring revolution in the Silicon industry.

RTL DESIGN AND SYNTHESIS USING SKYWATER130 PDK

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I have categorized my work in day wise learning, according to the workshop conducted by VSD.

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Table of Contents

• Day 01

  • Introduction to verilog RTL design and synthesis
    • RTL Design
    • Synthesis
    • Netlist
    • Lib file
  • Synthesis using yosys
    • Invoke yosys shell
    • Commands for synthesis using yosys
    • Results

• Day 02

  • Timing library hierarchical vs flat synthesis and efficient flop coding styles
    • Introduction to timing library
    • Skywater Foundary provided standard cell libraries
    • Hierarchical vs Flat Synthesis
    • Various Flip Flop coding styles and optimization
    • Optimization in constant multiplier circuit

• Day 03

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DAY 01

Introduction to verilog RTL design and synthesis

RTL Design

It's a way of writng hdl code for digital design in a way to exploit the behavioral features offered by the language and also utilizing the structural description for design accuracy. So it's a trade off between behavioral constructs and structural description while writing hardware code.

Synthesis

Translation and optimization of the RTL code into the gate level netlist. The synthesis tool(yosys) takes hdl code, liberty file(.lib) and design constraints to generate gate level netlist.

Netlist

Representation of the design in terms of actual standard cells and their connections.

Lib file

Liberty Timing file(.lib) consist of timing and power parameters associated with standard cells specific to a particular technology node. Generally it includes definition of an inverter, nand2, nand3, nand4, nor2, nor3, nor4, aoi12, aoi22, oai12, oai22, mux, dff etc.

NOTE

Why Standard Cell library includes various(in terms of PPA) types of gate cells?

In the digital circuit some combinational paths requires cells that are very fast in performance(but requires more area and power) and likewise some combinational paths requires cells that are slower in performance(but better in terms of area and power). So adhering to the timing conditions, which standard cells are to be used the decision is made. These decisions are written in constraint file and are passed to synthesis tool(yosys) for guided synthesis.

Synthesis using yosys

Invoke yosys shell

   $ yosys

yosys>

Commands for synthesis using yosys

    yosys> read_liberty -lib ../../path_to_the_standard_library_file.lib         // reading the liberty file.
    yosys> read_verilog ../../path_to_verilog_file_that_is_to_be_synthesized.v   // reading the verilog file.
    yosys> synth -top module_name                                                // the module name that is to be synthesized.
    yosys> abc -liberty  ../../path_to_to_liberty_file.lib                       // netlist generation.
    yosys> show                                                                  // a gui based representation of logic synthesized.
    yosys> write_verilog file_name.v                                             // write the generated netlist to new a module/verilog
                                                                                 // file for verification.

Results

result of the command "yosys> synth -top module_name"

• result of the command "yosys> abc -liberty ../../path_to_liberty_file.lib" NOTE

• the difference in synthesis performed by various version of yosys.

• synthesis performed by yosys 0.7.

• logical representation of the deisgn by yosys 0.7.

• synthesis performed by yosys 0.9.

• logical representation of the design by yosys 0.9.

NOTE

the synthesis performed by yosys 0.7 is choosing different standard cells as compared to synthesis by yosys 0.9.

• netlist generated by yosys 0.7.

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Day 02

Timing library hierarchical vs flat synthesis and efficient flop coding styles

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Introduction to timing library

sky130_fd_sc_hd_tt_025C_1v80.lib

the liberty timing file(.lib) is an ASCII file consists of detailed information of timing and power parameters about any standard cell of a particular technology node. The parameters are affected by PVT variations (process, voltage, temperature). These variations are factorized while designing IC. So the Standard Cell library is characterized to model these variations. The feature size in this technology node is 130nm. The terms in the .lib file are:-

• fd - the skywater foundary
• sc - digital standard cell
• hd - high density
• tt - typical process
• 025C - temperature
• 1v - voltage

library in the sky130 pdk are named using the following scheme:

"" Process name _ Library Source Abbreviation _ Library Type Abbreviation _ Library Name ""

Library Source	Abbreviation
The SkyWater Foundary	fd
Efabless	ef
Oklahoma State University	osu

Library Type	Abbreviation
Primitive Cells	pr
Digital Standard Cells	sc
Build Space(Flash, SRAM, etc)	sp
IO and Periphery	io
High Densiry	hd

Skywater Foundary provided standard cell libraries

• sky130_fd_sc_hd - High Density Standard Cell Library.
• sky130_fd_sc_hdll - High Density, Low Leakage Standard Cell Library.
• sky130_fd_sc_hs - Low Voltage (<2.0V), High Speed, Standard Cell Library.
• sky130_fd_sc_ms - Low Voltage (<2.0V), Medium Speed, Standard Cell Library.
• sky130_fd_sc_ls - Low Voltage (<2.0V), Low Speed, Standard Cell Library.
• sky130_fd_sc_lp - Low Voltage (<2.0V), Low Power, Standard Cell Library.
• sky130_fd_sc_hvl - High Voltage (5V), Standard Cell Library.

more details can be found at skywater-pdf.

standard cell definition of and3_or1_invert:-

area comparison of different types of and cells.

Hierarchical vs Flat Synthesis

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hierarchical synthesis

the synthesis results obtained after hierarchical synthesis shows that the sub-modules and the hierarchy of it's instantiation is preserved. The design is partitioned into a much higher level of abstraction.

logical description of hierarchical synthesis

netlist of hierarchical synthesis

Note: in the Hierarchical Synthesis the design is partitioned into sub-modules hence it becomes easy to trace back each component of the design. Also the design complexity is less and easy to comprehend.

flat synthesis

$ yosys> flatten                                            // to invoke flat synthesis after netlist generation
$ yosys> write_verilog -noattr multiple_modules_flat.v      // to write verilog netlist without attributes(clean)

in the flat synthesis, instantiation of standard cells take place and the design is partitioned into a much lower level of abstraction.

logical description of flat synthesis

netlist comparison of flat synthesis vs hierarchical synthesis

Sub-module level synthesis

$ yosys> synth -top sub_module1           // to synthesize only the submodule

• sub-module synthesis is preferred if:
  • there are multiple instances of same module. So it's better to synthesize the module once and then use it multiple times.
  • if desing and conquer approach is required i.e. if the design is very massive and complex, so it's better to synthesize module by module and then finally stitch together all the modules into the top module.

Various Flip Flop coding styles and optimization

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asynchronous reset dff

always@(posedge clk or posedge async_reset) begin 
    if(async_reset)  q<= 1'b0;
    else              q<= d_in;
end

asynchronous set dff

always@(posedge clk or posedge async_set) begin
    if(async_set)  q<= 1'b1;
    else           q<= d_in;
end

asynchronous-synchronous reset dff

always@(posedge clk or posedge async_reset) begin
    if(async_reset)     q<= 1'b0;
    else if(sync_reset) q<= 1'b0;
    else                q<= d_in;
end

synchronous reset dff

always@(posedge clk) beign
    if(sync_reset)  q<= 1'b0;
    else            q<= d_in;
 end

Note

• Asynchronous reset pin is directly synthesized on the flip flop.
• Synchronous reset signal is passed through a combinational logic to the flip flop.

Optimization in constant multiplier circuit

module mul2(
  output wire y[3:0],
  input wire in[2:0]
);
    assign y= a * 2;
endmodule
// there is an interesting optimiztion taking place here instead of generating any hardware for the logic.

Note

• there are only wire components in the circuit. Also note the comment by the synthesis tool(yosys) while mapping.

• netlist
• logical description

Similar optimization

module mul8(
    output wire y[5:0],
    input wire in[2:0]
    );
        assign y= a * 9;
endmodule

Note

• here replication of input takes place.

• netlist
• logical description

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Day 03

Combinational and Sequential Optimization

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Introduction to optimizations

command to perform optimization

$ yosys > opt_clean -purge        // all optimizations are done with this

Combinational logic optimization

• case 1

module opt_check(                             

  output wire y,
  input wire a,
  input wire b
);
    assign y= a ? b : 0;
endmodule

• expected optimization by tool.

• result obtained.

• Case 2

module opt_check(
  output wire y,
  input wire a,
  input wire b
);
    assign y= a ? 1 : b;
endmodule

• expected optimization by tool.

• result obtained.