8x4 Barrel Shifter using NMOS Pass Transistor Logic

• This project focuses on design of a 8x4 right Barrel Shifter using NMOS pass transistor logic using Google Skywater (sky130nm) Technology node with operating voltage of 1.8V .The project is build using Open Source Tools like Sky130PDK and eSim and Ngspice. Refer following website for more details on Barrel Shifter:https://en.wikipedia.org/wiki/Barrel_shifter

Table of Contents

1. Introduction
2. Installation of the tools
3. Creation of project
   • eSim Window
   • eSim Schematic Window
4. Reference Circuit
5. Reference Waveform
6. Methodology
7. Schematic
8. Circuit Schematic
9. Netlist
10. Initial Transitent Analysis
11. Waveforms
    • Input Waveforms
    • Output Waveforms
12. References
13. Contributor
14. Acknowledgments

Introduction

Barrel Shifter

• Barrel Shifter is a digital circuit that can shift a data word by a specified number of bits without the use of any sequential logic, only pure combinational logic, i.e. it inherently provides a binary operation important function to optimize the RISC processor, so it is used for rotating and transferring the data either in left or right direction. This shifter is useful in lots of signal processing ICs. The Arithmetic and the Logical Shifters additionally can be changed by the Barrel Shifter Because with the rotation of the data it also supply the utility the data right, left change all mathemetically or logically.A purpose of this project is to design CMOS using NMOS Pass Transistor logic.CMOS based 8 bit barrel shifter has implemented in this project using eSim and Ngspice tools using skywater 130nm PDKs Tech lib files

Installation of the Tools

• eSim : Link
• Ngspice : Link

• Ngspice gets installed alongwith eSim. If any other version ids to be installed refer: http://ngspice.sourceforge.net/download.html

• Sky130 PDK : Link

  • After Downloading eSim extract it choose the Directory to save the eSim Workspace

• Opensource Tools used

1. eSim: eSim (previously known as Oscad / FreeEDA) is a free/libre and open source EDA tool for circuit design, simulation, analysis and PCB design developed by FOSSEE, IIT Bombay. It is an integrated tool built using free/libre and open source software such as KiCad, Ngspice and GHDL. eSim is released under GPL. eSim offers similar capabilities and ease of use as any equivalent proprietary software for schematic creation, simulation and PCB design, without having to pay a huge amount of money to procure licenses. Hence it can be an affordable alternative to educational institutions and SMEs. It can serve as an alternative to commercially available/licensed software tools like OrCAD, Xpedition and HSPICE. For more info refer: https://esim.fossee.in/home

2. Ngspice: Ngspice is the open source spice simulator for electric and electronic circuits. Ngspice offers a wealth of device models for active, passive, analog, and digital elements. Model parameters are provided by the semiconductor manufacturers. The user add her circuits as a netlist, and the output is one or more graphs of currents, voltages and other electrical quantities or is saved in a data file. For more info refer: http://ngspice.sourceforge.net/

3. Skywater Pdk: The SkyWater Open Source PDK is a collaboration between Google and SkyWater Technology Foundry to provide a fully open source Process Design Kit and related resources, which can be used to create manufacturable designs at SkyWater’s facility. As of May 2020, this repository is targeting the SKY130 process node. If the SKY130 process node release is successful then in the future more advanced technology nodes may become available. For more info refer: https://github.com/google/skywater-pdk, https://skywater-pdk.readthedocs.io/en/latest/

Google SkyWater PDK

The SkyWater Open Source PDK is a collaboration between Google and SkyWater Technology Foundry to provide a fully open source Process Design Kit and related resources, which can be used to create manufacturable designs at SkyWater’s facility.

More details of SkyWater Open Source PDK can be found here.

Skywater Pdk Installation(Ubuntu)

Open the terminal and follow these steps:

git clone git://opencircuitdesign.com/open_pdks
cd open_pdks
./configure --enable-sky130-pdk
make
sudo make install

Cloning

• Clone this repository using the following commands:

$ sudo apt install -y git
$ git clone https://github.com/iaakash47/Barrel_shifter.git

Follow these steps for Sky130 download and implementaion :

1. Download sky130 from this link mentioned above and unzip it.
2. Save the .cir.out file in the sky_fd_pr folder as .cir file.
3. Open with notepad and add the path .lib "models/sky130.lib.spice" tt at the top.
4. Replace with CMOSP, mos_p with sky130_fd_pr_pfet_01v8 and CMOSN, mos_n with sky130_fd_pr_nfet_01v8.
5. To replace inductor, capacitor, resistor do it this way, for Ex : L1 out gnd 1m by x1 out gnd mid 0 sky130_fd_pr__ind_03_90.

Note: For more details go to the cells folder in sky_fd_pr. 
Open the specific component folder which you want to use. 
Then open the test folder and check the SPICE file. 
The SPICE file is an example of implementation of that component. 
You will get to know how to use the component in your ckt.

Creation of project

• Click on the New Project and save the file name without any space
• Project will be created

eSim Window

eSim Schematic Window

To Run the ckt using ngspice:

1. Replace with sky130nm cells and save this .cir file.
2. Paste the .cir file in gedit document where the sky130_fd_pr folder is present
3. To Run the ngspice waveform,save the .cir file and replace with filename.spice
4. Open Terminal in the saved Directory
5. And run the following command(ngspice filename.spice)

• For reference to download the tools and get an exposure to simulation using eSim, ngspice and Sky130, you may want to check the course : Link

Reference Circuit

CMOS transmission gates may be used in place of the simple pass transistor switches.In this project, a 8x4 right barrel shifter circuit using NMOS pass transistor logic is implemented in eSim with skywater 130nm technology. The shifter circuit takes 8 inputs bits and shifts them according to 5 control shift bits and generates 4 output bits

• Reference Circuit of Barrel Shifter is as shown below

Reference Waveform

• Reference Waveform of Barrel Shifter is as shown below

Methodology

Prelayout Simulation in eSim and Ngspice

Refer following manual to know how to operate eSim: https://static.fossee.in/esim/manuals/eSim_Manual_2020_August.pdf

Schematic

• Construct the "8 * 4 Barrel Shifter using NMOS" as shown in the figure below using eSim

Circuit Schematic

• 8 * 4 Barrel Shifter using NMOS

• Once every step is followed perfectly open the Netlist that is generated and make the necessary changes to add the Sky130 models
• The Netlist generated initially is as shown below and replace the components with skywater130nm cells

Editing the Netlist

a) To execute the .cir file, follow these steps:
Step 1: Open the .cir file (barrel_shifter.cir in my case) with notepad 
Step 2: Make sure the path .lib "sky130_fd_pr/models/sky130.lib.spice" tt at the top.
Step 3: Make sure that all mosfets are replace with sky130_fd_pr__nfet_01v8
Step 4: Delete any / added if any 
Step 5: Prefix x with all Transistors starting with M. Do this for all transistors 
e.g Before M1, after xM1
Step 6: Comment with * if any ports are added by defaults 
e.g *U1  /input_In0 PORT	
*U9  /Out_0 PORT	
Step 7: Next Start adding Inputs sources, supply voltage, etc (Vdd was considered as 1.8V)
Step 8: Add .tran statement with initial step time and final stop time 
eg. .tran 0.4ns 40ns 
Step8: Add .control command and run command to run the plot 
Step9: Add the voltage or current input or output quantities to be plotted 
eg. plot V(input_In0) 
plot V(Out_0) 
Step 10: Last step is to add .endc followed by .end command 

• Now Run the circuit with ngspice.

To Run the ckt using ngspice:

1. Replace with sky130nm cells and save this .cir file.
2. Paste the .cir file in gedit document where the sky130_fd_pr folder is present
3. To Run the ngspice waveform,save the .cir file and replace with filename.spice
4. Open Terminal in the saved Directory
5. And run the following command(ngspice filename.spice)

• For reference to download the tools and get an exposure to simulation using eSim, ngspice and Sky130, you may want to check the course : Link

Netlist

* /home/user/eSim-Workspace/Barrel_shifter/bareelshifter.cir

* EESchema Netlist Version 1.1 (Spice format) creation date: Mon Jan 31 16:56:06 2022

.lib "sky130_fd_pr/models/sky130.lib.spice" tt

* Sheet Name: /
xM16 Out_4 GND input_in3 GND sky130_fd_pr__nfet_01v8 
xM17 Out_4 GND input_in4 GND sky130_fd_pr__nfet_01v8 	
xM18 Out_4 GND input_in5 GND sky130_fd_pr__nfet_01v8 			
xM19 Out_4 GND input_in6 GND sky130_fd_pr__nfet_01v8 	  
xM20 Out_4 Vdd input_in7 GND sky130_fd_pr__nfet_01v8 	
xM11 Out_3 GND input_in2 GND sky130_fd_pr__nfet_01v8 		
xM12 Out_3 GND input_in3 GND sky130_fd_pr__nfet_01v8 		
xM13 Out_3 GND input_in4 GND sky130_fd_pr__nfet_01v8 		
xM14 Out_3 GND input_in5 GND sky130_fd_pr__nfet_01v8 	
xM15 Out_3 Vdd input_in6 GND sky130_fd_pr__nfet_01v8 		
xM6  Out_2 GND input_in1 GND sky130_fd_pr__nfet_01v8 		
xM7  Out_2 GND input_in2 GND sky130_fd_pr__nfet_01v8 
xM8  Out_2 GND input_in3 GND sky130_fd_pr__nfet_01v8 
xM9  Out_2 GND input_in4 GND sky130_fd_pr__nfet_01v8 
xM10 Out_2 Vdd input_in5 GND sky130_fd_pr__nfet_01v8		
xM1  Out_1 GND input_in0 GND sky130_fd_pr__nfet_01v8 	
xM2  Out_1 GND input_in1 GND sky130_fd_pr__nfet_01v8 			
xM3  Out_1 GND input_in2 GND sky130_fd_pr__nfet_01v8 		
xM4  Out_1 GND input_in3 GND sky130_fd_pr__nfet_01v8 	
xM5  Out_1 Vdd input_in4 GND sky130_fd_pr__nfet_01v8 			
*U1 input_in0 PORT
*U2 input_in1 PORT
*U3 input_in2 PORT
*U4 input_in3 PORT
*U5 Vdd PORT
*U6 input_in4 PORT
*U7 input_in5 PORT
*U8 input_in6 PORT
*U9 input_in7 PORT
*U10 Out_3 PORT
*U11 Out_2 PORT
*U12 Out_1 PORT
*U13 Out_4 PORT		

V1 input_in4 0 PULSE(0 1.8 1n 10p 10p 10n 20n)
V2 input_in5 0 PULSE(0 1.8 7n 10p 10p 10n 20n)
V3 input_in6 0 PULSE(0 1.8 3n 10p 10p 10n 20n)
V4 input_in7 0 PULSE(0 1.8 9n 10p 10p 10n 20n)
VDD Vdd 0 1.5V
V6 input_in0 0 PULSE(0 1.8 4n 10p 10p 10n 20n)
V7 input_in1 0 PULSE(0 1.8 8n 10p 10p 10n 20n)
V8 input_in2 0 PULSE(0 1.8 12n 10p 10p 10n 20n)
V9 input_in3 0 PULSE(0 1.8 16n 10p 10p 10n 20n)

.tran 0.4ns 40ns 

.control
run
plot V(input_in0)
plot V(input_in1) 
plot V(input_in2) 
plot V(input_in3) 
plot V(input_in4) 
plot V(input_in5) 
plot V(input_in6) 
plot V(input_in7)
plot V(Out_1)   
plot V(Out_2)
plot V(Out_3) 
plot V(Out_4)  


.endc
.end

• Save the above ngspice subcircuit spice file in a folder where the sky_fd_pr_ folder is present

• Run the spice file with following Terminal Command as shown

Initial Transitent Analysis

Waveforms

• Input Waveforms (8 Inputs)
• Output Waveforms (4 Outputs)

Input Waveforms

Output Waveforms

References

• Realization of 8 x 4 barrel shifter with 4-bit binary to gray
• Design of Various 4 Bit Shifters using CMOS

Contributor

Aakash.K
Contact:[email protected]

Kunal Ghosh, Co-founder, VSD Corp. Pvt. Ltd. - [email protected]

Acknowledgments

Kunal Ghosh, Co-founder, VSD Corp. Pvt. Ltd. - [email protected]