RISC-V based SOC Design with PLL using Open-Source EDA Tools

Table of Contents

1. Introduction
   
2. Required Tools
3. Problem Statement
   
4. PLL Specs
   
5. Installation & Runs
   
6. Methodology
   • Rvmyth simulation
   • Designing avsdpll
   • Integration of rvmyth and avsdpll
7. OpenLANE and sky130 Installation
   
8. Synthesis in yosys
   
   • Pre-Synthesis
   • Post-Synthesis
9. References
10. Acknowledgements

Introduction

The aim of this repository is to integrate rvmyth (RISC-V CPU core developed by VSD) with a PLL (Phase Locked Loop) as a clock multiplier using open-source EDA tools. The whole project focuses on Mixed Signal Physical Design.

Required Tools

iverilog and gtkwave

Problem Statement

Integration of rvmyth with avsd_pll_1v8.

PLL Specs

Installation & Runs

Install iverilog and gtkwave

Methodology

The rvmyth is a digital block whereas the pll is an ananlog block. To achieve the integration firstly we need to generate a verilog block for the PLL and take the output of the PLL and give it as an input to the rvmyth. This will create the interface then the PNR flow must be implemented.

• Rvmyth simulation

  Simulate rvmyth using the following steps:

    git clone https://github.com/kunalg123/rvmyth/ 
    cd rvmyth 
    iverilog mythcore_test.v tb_mythcore_test.v 
    ./a.out
    gtkwave tb_mythcore_test.vcd 
  

• Designing avsdpll

Design a PLL as a clock multiplier using verilog and test the functionality.

  iverilog avsd_pll_1v8.v pll_tb.v
  ./a.out
  gtkwave test.vcd

• Integration of rvmyth and avsdpll

Now integrate both rvmyth and avsdpll using a top level testbench and test it to verify.

 iverilog rvmyth_pll.v rvmyth_pll_tb.v
 ./a.out
 gtkwave test1.vcd

OpenLANE and sky130 Installation

OpenLANE is an automated RTL to GDSII flow based on several components including OpenROAD, Yosys, Magic, Netgen, Fault,SPEF-Extractor and custom methodology scripts for design exploration and optimization (https://github.com/efabless/openlane).

The OpenLANE and sky130 installation can be done by referring this repository https://github.com/nickson-jose/openlane_build_script.

Once installation is done the next step is synthesis.

Synthesis in yosys

In OpenLANE the RTL synthesis, technology mapping and timing reports are performed by different tools.

• RTL synthesis by yosys.
• Technology mapping by abc.
• Timing reports are generated by OpenSTA.
  

Verilog file (.v) of the avsdpll and its LIB (.lib) file. To generate the LIB file run the below perl script.
perl verilog_to_lib.pl avsdpll.v avsdpll

To perform synthesys in yosys

• Just type yosys in linux shell and follow the script.
  

read_verilog rvmyth_pll.v 
read_liberty -lib avsd_pll_1v8.lib 
read_liberty -lib sky130_fd_sc_hd__tt_025C_1v80.lib 
synth -top rvmyth_pll_interface 
dfflibmap -liberty sky130_fd_sc_hd__tt_025C_1v80.lib 
opt 
abc -liberty sky130_fd_sc_hd__tt_025C_1v80.lib -script +strash;scorr;ifraig;retime;{D};strash;dch,-f;map,-M,1,{D} 
flatten 
setundef -zero 
clean -purge 
rename -enumerate
stat 
write_verilog -noattr rvmyth_pll.synth.v 

The synthesized netlist is rvmyth_pll.synth.v

The snapshot of the synthesized netlist.

Now, with the generated netist simulate it in iverilog and check the results in gtkwave to compare the pre-synth and post-synth simulations. To achieve that first a GLS testbench file should be written in verilog to compare the results. The pre-synth and post-synth simulation output must be same.

Pre-Synthesis simulation

Use the following commands for pre-synth simulation output.

    iverilog rvmyth_pll.v rvmyth_pll_tb.v
    ./a.out
    gtkwave test1.vcd

Post-Synthesis simulation

Use the following commands for post-synth simulation output.

    iverilog gls.v 
    ./a.out
    gtkwave gls.vcd 

References

1. https://github.com/vsdip/avsdpll_1v8_sky130_ss
2. https://github.com/vsdip/vsdmixedsignalflow
3. https://github.com/nickson-jose/vsdstdcelldesign
4. https://gitlab.com/gab13c/openlane-workshop

Acknowledgements

Mr.Kunal Ghosh Co-founder of VLSI System Design Pvt. Ltd.
Shivani Shah, IIIT Bangalore.
Mili Anand, M.Tech VLSI Design, Indira Gandhi Delhi Technical University for Women (IGDTUW).