vsdip / vsdsram

An SRAM IP Uniquely designed with open source tools. Static RAM is a type of random-access memory that uses latching circuitry (flip-flop) to store each bit. The size of SRAM specs is 32kbit/4k bytes with 1.8v. A 6T SRAM pairs up with two access transistors for read, write state and cross coupled inverter to hold/regenerate the state.

vsdsram

An SRAM IP Uniquely designed with open source tools.

Static RAM is a type of random-access memory that uses latching circuitry (flip-flop) to store each bit.

The Size of SRAM Specs is 32kbit/4k Bytes with 1.8v.

To get insight of mem4kBytesOr32kbits design Specifications, check this.

Block Symbol and pins

Circuit Diagram of SRAM and Its Read Write Analysis

A 6T sram pairs up with two access transistors for read, write state and cross coupled inverter to hold/regenerate the state.

During write operation i.e., to write Q=0 while initial Q=vdd or 1, when the voltage at node Q reaches to a threshold voltage wherein PMOS M5 gets ON and the voltage at node Qbar starts to rise and the regenerative action of the cross-coupled inverter will force the write Q = 0. To write logic 0, Bitlline should be logic 0 and BLbar is complemented to 1. Wordline is high.

During Read operation i.e., to read Q=vdd or 1, when the voltage at node Qbar reaches to a threshold voltage wherein NMOS M3 gets ON and the voltage at node Q starts to fall and cross-coupled inverter state will force to flip the bit in a cell. Wordline is high.

Cell Ratio and Pull-up Ratio Analysis

Cell Ratio is the ratio between the sizes of the driver transistor to access transistor.

From the circuit diagram reprsentation(transistor labels); For OSU180nm Technology.

CR = ((W3/L3)/(W2/L2))=((W4/L4)/(W6/L6))=((0.36u/0.18u)/(0.36u/0.18u))=1

Pull-up Ratio is the ratio between load transistor to the access transistor.

PR = ((W1/L1)/(W2/L2))=((W5/L5)/(W6/L6))=((0.9u/0.18u)/(0.36u/0.18u))=2.5

Upcoming Future Works

The Circuit design of 6T SRAM alongwith Precharge, Sense amplifier is designed solely in opensouce tool and its characterstics is analyzed. As a post layout stage, the design's layout is drawn and spice netlist is extracted. As a part of upcoming work the next step is post layout simulation, verifying it with pre layout. The32kbit and PnR (Place and Route) flow and lastly GDS to be released.

IP USAGE

The user of this IP has to install Ngspice for Circuit simulation and Magic for Layout.

Getting Started with NGspice: An open Source Circuit Simulator

For ubuntu linux

To Install Ngspice, Open terminal and Type

sudo apt-get install -y ngspice

To Clone the IP Repository

1. Download the Zip file or use git clone https://github.com/Anushar123/vsdsram

2. Change the path or directory using cd vsdsram/Circuit-Inv to view the circuit.

3. Change the path or directory using cd vsdsram/Netlist to run the simulation.

To run PreLayout Simulations

1. Go to the path or directory using cd vsdsram/Netlist

2. To simulate transient Analysis type :ngspice Sram.cir.out

   Or

   Type ngspice in terminal

   ngspice 1 -> source Sram.cir.out

To View Simulation Plots:

ngspice 1 -> plot q

ngspice 1 -> plot qbar

ngspice 1 -> plot q qbar

ngspice 1 -> plot bl

ngspice 1 -> plot blbar

ngspice 1 -> plot bl blbar

To simulate dc Analysis

Type :ngspice sramdc.cir.out

ngspice 1 -> plot bl q

To exit from Ngspice Shell type:

ngspice 1 -> exit

Getting Started with Magic Layout

Magic is a venerable VLSI layout tool, written in the 1980's at Berkeley by John Ousterhout, now famous primarily for writing the scripting interpreter language Tcl. Magic has remained popular with universities and small companies.

For ubuntu linux

Download Magic from this link

Open Terminal and type below commands. 
cd Downloads/
tar xzf magic-7.5.232.tgz
cd magic-7.5.232/
sudo apt-get install m4
sudo apt-get install tcl-dev
sudo apt-get install tk-dev
sudo apt-get install blt
sudo apt-get install freeglut3
sudo apt-get install libglut3
sudo apt-get install libgl1-mesa-dev
sudo apt-get install libglu1-mesa-dev
./configure
sudo apt-get install csh
sudo make
sudo make install

Or

Install Qflow & It will install all necessary open source tools including magic.

Download qflow from this link

Open Terminal and type below commands. 
cd Downloads/
tar xzf qflow-1.4.83.tgz
sudo apt-get update
sudo apt-get install qflow

To View Magic Layouts

1. Open the repository and change directory to cd vsdsram/Layout

2. To view the layout, type on terminal : magic sram6.mag or magic -T SCN6M_SUBM.10.tech sram6.mag

3. Similarly for precharge and senseamplifier: magic precharge6.mag & magic senseamplifier6.mag

4. To extract the spice netlist : Go to Tkcon window and type these commands.

extract all

ext2spice rthresh 0 cthresh 0 To include Parasitics in spice netlist

ext2spice

Note: IP is yet under process and yet to release as a final view.

But incase if any difficulties with respect to IP Usage so far, contact the administrator or report as github Issue.

Author

Anusha R

Contact Information

• Anusha.R Mtech (VLSI Design and Embedded Systems), Bangalore. [e-mail]
• Kunalghosh Director VSD Corp.Pvt.ltd [e-mail]
• PHILIPP GUHRING Software Architect at LibreSilicon Association [e-mail]
• Dr. GAURAV TRIVEDI Co-Principal Investigator,ECIT Academy and Associative Professor, EEE Department, IIT Guwahati [e-mail]