Multi-height RTL2GDS flow for Mixed Signal SoC

This project describes how the PNR of an analog IP, 2:1 analog multiplexer is carried out by opensource EDA tools. It also discusses the steps to modify the current IP layouts inorder to ensure its acceptance by the EDA tools.

Table of Contents

• Mixed Signal SoC
• RTL2GDS flow of mixed signal SoC
• The OpenROAD Project
• OpenROAD tools installation
• Inputs required for physical design
• Obtaining IP
• Obtaining verilog files
• Getting LEF file
• Limitations of current IP layouts
• Resolving the pin issue
  • For the labels on metal layers:
  • For the labels on polysilicon layers:
• Other importanat fields of LEF file
• Writing LIB file
• Verilog files
• Experiments with Openlane and sky130
  • Installation
  • Adding a new project
  • Setting up the new project
  • Adding input files
    • Verilog files
    • LEFs
  • The Flow
    • Setting up flow
    • The LEF file for macro
    • Synthesis
    • Floorplanning
    • IO Placement
    • Placement
    • Generation of Power Delivery Network
    • DRC Cleaning
    • Final Layout Generation
  • Notes and Tips
• Future Work
• Acknowledgement
• Contact Information

Mixed Signal SoC

Mixed signal SoC is a chip which contains both analog and digital blocks. The designers are adding more analog circuitry and increasing their complexities day by day. Not only that, they also contain digital control logic. As the process nodes shrink, the demand for integration grows. A divide and conquer approach is followed, where the analog and digital structures were dealt with separately. Usually, an analog IP (Intellectual Property) is bought as black- box.

RTL2GDS flow of mixed signal SoC

To implement a RTL-to-GDS flow for mixed signal SoC, there is need to establish communication between the analog and digital blocks. For this integration to happen, hierarchical level of abstraction with either analog or digital as top level is required. In order to carry out this task, OpenROAD project can be utilized.

The OpenROAD Project

In order to carry out this task, OpenROAD project can be utilized. OpenROAD, is a ‘no human in loop’ hardware compiler which translates source code (RTL) to layouts (GDS). The time required for the translation is less than 24 hours. It is a DARPA initiative, an open source project. RTL2GDS flow of OpenROAD is used only for digital SoC. Hence, with OpenROAD, we can establish digital as our top level hierarchy

For more details visit The OpenROAD Project

OpenROAD tools installation

For detailed installation steps, go to Steps to install OpenROAD tools.pdf

Inputs required for physical design

The main inputs required to carry out RTL2GDS flow are:

• LIB file
• LEF file
• top level verilog file

Obtaining IP

The analog multiplexer for OSU018 is available on the following github page: https://github.com/prithivjp/avsdmux2x1_3v3 This page includes the spice files and magic layout files. The magic file can be seen here

From this, build a layout using sky130.tech using the Magic Layout Tool. The modified layout can he seen here

Obtaining verilog files

The verilog file for analog multiplexer can be procured from Efabless PicoRV-32 Github page

To get the repository:

git clone https://github.com/efabless/raven-picorv32.git

Getting LEF file

The LEF file can be obtained from magic layout synthesis tool from mag file .

The IP uses sky130 as the technology node. So it is essential that the tech file for sky130 is downloaded in the machine. The tech file is present in this repository as sky130.tech.

From the terminal type the following :

magic -T `/sky130A.tech ~/AMUX2_3V.mag

A layout window and a tkcon window will open. In the tkcon window, type the following command to dump out the LEF file

lef write AMUX2_3V.lef

Limitations of current IP layouts

On opening the LEF file, it is observed that it is incomplete. The file does not contain the pin descriptions. On observing the layout from the layout window, we can see that the pins are not present. Only labels are present in the layout. And the magic tool is not able to identify these labels. Some fields which must be present in a LEF file appear to be missing.

This incomplete LEF file would not be accepted by the PnR tools.

Resolving the pin issue

To convert the labels into pins, a command called port can be used in magic.

For the labels on metal layers:

• Select the area under which the label is present.

• In the tkcon window type

  port make

  To verify if the port is made

  port name

• Similarly, carry out the same process for other labels

For the labels on polysilicon layers:

• Type g to enable the grid option
• delete the label on poly layer by selecting the area where label is present and typing the following in tkcon window

  erase labels

• Connect a polycont layer on the poly on one side
• To the polycont, connect a locali layer .
• Remove DRC errors if any.
• Create a label on locali layer , by selecting a point on the layer and in tkcon window, typing

  label 'name_of_label'

• Continue the same process for turning a label into port as mentioned above.

Now, dump out the LEF file again by using

lef write AMUX2_3V.lef

All the pins and their descriptions can now be seen.

A section called OBS must also be seen at the end of the file. If it is not visible, it means that the label on polysilicon is not converted into port properly.

Other important fields of LEF file

In addition to converting pins from labels, there are certain other lines included in the LEF file, such that it can be accepted by the openlane tool:

• CLASS CORE

This line can be added using the following command in the tkcon window:

 property LEFclass CORE

• ORIGIN 0.000 0.000

The layout must start from the origin (0,0) In order to get this:

1. first find out the current co-ordinates of origin by: selecting the whole layout and type the following in tkcon window

box

From this, llx and lly are X and Y co-ordinates respectively.

2. setting X co-ordinate to 0:

move origin right 'llx' um

3. setting Y co-ordinate to 0:

move origin bottom -`lly`um

4. checking if the origin has shifted to (0,0): first find out the current co-ordinates of origin by:

box

Now, the llx and lly should have the value of 0.

• SITE unithddbl

To set this, type the following from tkcon window:

 property LEFsite unithddbl

• SIZE

The height of the macro must be either 2.72 um or 5.444 um in order to fit into the rails ( for fd_sc_hd) . In order to acheive this, first find the current height of the macro by selecting the entire macro and typing the following in tkcon window:

box

Then, find the value by which it should be scaled to get height=5.44 um. Save the magic file.

Next, add the following in the .mag file:

magscale 10075 1000

Reload the file in magic. Check the height of the macro now by typing the following in tkcon window:

box

The height should be 5.44 um now. Remove the magscale line from .mag file and save the file.

• DIRECTION

Select the part which contains the pin and type the following in tkcon window:

1. For Power and Ground pins:

port class inout

2. For Input pins:

port class input

3. For Output pins:

port class output

• USE

Select the part which contains the pin and type the following in tkcon window:

1. Power pin:

port use power

2. Ground pin:

port use ground

3. Other pins:

port use signal

• Ultimately, after configuring all the lines for LEF, create a LEF file by typing the following in tkcon window:

lef write AMUX2_3V.lef

Writing LIB file

LIB file can be got by using a perl script, which converts verilog file to LIB file. The script is taken from the website VLSI Professional Network. To view the script, go to verilog_to_lib.pl

The verilog file is obtained from the efabless github page. But, the names of the pins defined in the verilog file and the layout and LEF file obtained above may be different. Therefore, change the pin names in the verilog file accordingly and then obtain the LIB file by using the perl script by typing the following on terminal

perl verilog_to_lib.pl AMUX2_3V AMUX2_3V

The modified verilog file and the LIB file can be viewed from AMUX2_3V.v and AMUX2_3V.lib directory.

Verilog files

• The top level verilog file design_mux.v contains instantiation of the AMUX2_3V macro and other circuitry.
• [AMUX2_3V.v]((https://github.com/praharshapm/vsdmixedsignalflow/blob/master/Verilog/AMUX2_3V.v) is the verilog file for AMUX2_3V macro.
• raven_spi.v and spi_slave.v are the files to be included for the design. They can be obtained from this repository

Experiments with Openlane and sky130

OpenLANE is an automated RTL to GDSII flow based on several components including OpenROAD, Yosys, Magic, Netgen, Fault and custom methodology scripts for design exploration and optimization. It can be used for PNR on sky130 PDK.

Installation

Openlane along with sky130 PDK can be downloaded and installed by following the steps in this repository.

Adding a new project

In the designs directory create a folder with the name of the project

 cd designs
 mkdir design_mux

setting up the new project

To set-up the project, run the following : Go to the ~/openlane_working_dir/openlane and execute the following:

export PDK_ROOT=<absolute path to where skywater-pdk and open_pdks reside>

docker run -it -v $(pwd):/openLANE_flow -v $PDK_ROOT:$PDK_ROOT -e PDK_ROOT=$PDK_ROOT -u $(id -u $USER):$(id -g $USER) openlane:rc2

 ./flow.tcl -design design_mux -init_design_config

This will create `config.tcl` file with default settings. The final `config.tcl` file can be seen from [here](https://github.com/praharshapm/vsdmixedsignalflow/blob/master/openlane/config.tcl). This file also contains the reasons for some configurations as comments.

The information about configuration veriables can be found here

Adding input files

Verilog files

The input files are to added under ~/designs/design_mux/src directory. The srcfolder consists of verilog files for the design which may be found here

• design_mux.v is the name of the top level verilog file.
• AMUX2_3V.v is the verilog file of the macro.
• raven_spi.v and spi_slave.v are the files to be included in the design.

LEFs

LEF file obtained must be included in ~/designs/design_mux/src/lef directory

The flow

To harden a macro, the automated flow for Openlane cannot by used. Instead an interactive script should be used.

Go to the ~/openlane_working_dir/openlane and execute the following:

export PDK_ROOT=<absolute path to where skywater-pdk and open_pdks reside>

docker run -it -v $(pwd):/openLANE_flow -v $PDK_ROOT:$PDK_ROOT -e PDK_ROOT=$PDK_ROOT -u $(id -u $USER):$(id -g $USER) openlane:rc2

A bash window will open. In the bash window, the interactive flow is executed.

 ./flow.tcl -design design_mux -interactive

Setting up flow

The commands to be executed are also present in a script here:

package require openlane 0.9

prep -design design_mux -overwrite

The LEF file for macro

set lefs 	 [glob $::env(DESIGN_DIR)/src/lef/*.lef]

add_lefs -src $lefs

Synthesis

run_synthesis

The output files can be found here

Floorplanning

init_floorplan_or

After floorplanning, the layout can be viewed in magic using the merged LEF and DEF file produced. The DEF file can be found here

magic -T ~/sky130A.tech lef read ~/merged.lef def read design_mux.floorplan.def

IO Placement

place_io

Placement

global_placement_or

detailed_placement

tap_decap_or

detailed_placement

After final placement, the layout can be viewed in magic using merged LEF and DEF file. The DEF file can be found here

magic -T ~/sky130A.tech lef read ~/merged.lef def read design_mux.placement.def

Generation of Power Delivery Network(PDN)

gen_pdn

Routing

run_routing

After routing, the layout can be viewed in magic using merged LEF and DEF file. The DEF file can be found [here] (https://github.com/praharshapm/vsdmixedsignalflow/tree/master/openlane/results/routing)

magic -T ~/sky130A.tech lef read ~/merged.lef def read design_mux.def

DRC Cleaning

run_magic_drc

The output file can be found here

magic -T ~/sky130A.tech design_mux.drc.mag

Final Layout generation

run_magic

The final layout output is in the form of design_mux.mag. The files can be found here

magic -T ~/sky130A.tech design_mux.mag

Notes and Tips

• You may have to update RePlace to the latest version if you wish to run placement for low instance count designs.
• If, the error still persists in placement, check if following block of code in ~scripts/openroad/or_replace.tcl is commented. If not, comment it.

global_placement \
 	-density $::env(PL_TARGET_DENSITY) \
 	-verbose 3

• The verilog files must not contain the power and ground pins.
• Add the macro LEF both in the config.tcl file and in the interactive flow.

Future Work

• To include custom LIB for macro and include timing constraints.
• To perform PNR on macro of triple-height or more

Acknowledgement

• Kunal Ghosh, Director, VSD Corp. Pvt. Ltd
• Nickson Jose, VLSI Engineer
• Openlane team, Efabless corporation
• Prithivi Raj K, National Institute of Technology Tiruchirapalli

Contact Information

• Praharsha Mahurkar, BE Electronics and Telecommunication, Maharashtra Institute of Technology, Pune, praharshapm@gmail.com
• Kunal Ghosh, Director, VSD Corp. Pvt. Ltd. kunalpghosh@gmail.com