CCS vs. NLDM: A Comparison of Delay Models

Introduction to Delay Models

• CCS stands for Composit Current Sourse Model, and NLDM stands for Non-Linear Delay Model.

• Both CCS & NLDM are delay models used in timing analyze.

Key Differences in Driver Modeling

• NLDM uses a voltage source for driver modeling

• CCS uses a current source for driver modeling

Advantages of CCS over NLDM

• The issues with NLDM modeling is that, when the drive resistance RD becomes much less than Znet(network load impedance), then ideal condition arises i.e Vout=Vin.

• Which is impossible in practical conditions.

• So with NLDM modeling parameters like the cell delay calculation, skew calculation will be inaccurate.

• That is the reason why we prefer CCS to NLDM

Physical Cells :TAP CELLS, TIE CELLS, ENDCAP CELLS, DECAP CELLS

Tap Cells (Well Taps) : These library cells connect the power and ground connections to the substrate and n­wells, respectively. 

By placing well taps at regular intervals throughout the design, the n­well potential is held constant for proper electrical functioning. The placer places the cells in accordance with the specified distances and automatically snaps them to legal positions (which are the core sites).

All the cells discussed in these post are called Physical only cells, as these cells are not important with respect to Funtional Design. These were invented to Complete the Design properly. These cells does not have any timing constains. There are different types of physical cells. Let us see the usage of different cells

TIE CELLS: Tie High and Tie Low cells are used to connect the gate of the transistor to either power or ground. In Lower technology nodes, if the gate is connected to  power/ground the transistor might be turned on/off dure to power or ground bounce. These cells are part of standard cell library. The cells which required Vdd connected to Tie high cells. The cells which require Vss/Gnd connected to Tie low cells. These cells are basically present in the ".lib"

• Floating nets or unused nets need to be tied with some constant value (0 or 1), can be achieved using Tie cells
• Using connect_tie_cells command, we can inser a tie cell in the design during the PNR

TIE HIGH CELLS:

Using these cells we directly connect the VDD to the gate transistor, now we connect the O/P of these cells to the transistor gate if any fluctuations in VDD due to ESD then PMOS circuit pull it back to stable state. PMOS should be ON always, the I/P of the PMOS transistor is coming from the O/P of NMOS transistor and For NOMS gate and drain are shorted and NMOS will be in saturation mode which act as a pull down circuit and always gives a low voltage at the gate of PMOS. Now PMOS will on and gives stable high output and this output is connected to the gate of transistor

ENDCAP CELL: These Library cells do not have signal connectivity. They connect only to power and ground rails once power rails created in design. They also ensure that gaps do not occur between the well and implant layers. This prevents DRC violations by satisfying well tie off requirements for the core rows. Each end of the core row, left and right can have only one ENDCAP cell

Hower you can specify a list of different END CAPS for inserting horizontal ENDACP lines, which terminate the top and bottom boundaries of object such as macros. A core row can be fragmented (contains gaps), since row donot intersect objects such as power domains. For this, the tool places ENDCAP cells on both ends of the unfragmented segment.

    

• To Protect the gate of a standard cell placed near the boundary from damage during manufacturing
• To Avoid the base layer DRC(Nwell and Implant Layer) at the boundary
• To make the proper alignment with other block
• Some standard cell library has END CAP cell which serve as Decap cell as well

DeCAP CELLS: These are temporary capacitors added in the design between power and ground rails to counter functional failures due to dynamic IR Drop. Dynamic IR Drop happens at the active edge of the clock at which a high % of sequential and Digital elements switch . Due to this simultaneous switching a high current is drawn from the power grid for a small duration. if the power source is far away from a flop, then there are high chances of this flop to move into metastable state due to IR Drop. To overcome this DECAPS are added. At the active edge of clock when the current requirement is high, these DECAPS discharges and provide boost to the power grid. One major disadvantage in usage of DECAPS is that these add leakage current in the circuit. DECAPS are placed as fillers. The closer they are to the flops, the better it is.

DeCAP Cells are typically poly gate transistors where source and drain are connected to the ground rail, and gate is connected to the power rail

When there is an instantaneous switching activity the charge required moves from intrinsic and extrinsic local charge reservoirs as oppose to voltage sources. Extrinsic capacitances are DECAP cells placed in the Design. Intrinsic capacitances are those present naturally in the circuit, such as the grid capacitance, the variable capacitance inside nearby logic, and the neighborhood loading capacitance exposed when the P or N channel are open

One drawback of DECAP cells is that they are very leaky, so the more decap cells the more leakage. Another drawback, which many designers ignore , is the interaction of the decap cells with the package RLC network. Since the die is essentially a capacitor with very small R & L, the package is a hug RL network, the more decap cells placed the more chance of tuning the circuit into its resonance frequency. That would cause a trouble, since both the VDD and GND will be oscillating. Few Design were failed because the DECAP cells placed near high activity clock buffers. Most recommended option is a decap optimazation flow where the tool will study charge requirements at every moment in time and figure out how much decap to place at any node. This should be done while taking package models into a account to ensure resonance frequency is not hit

 

   

What is Verilog

 What is Verilog?

Verilog HDL is a hardware description language used to design and document electronic systems. Verilog HDL allows designers to design at various levels of abstraction. 

A brief history 

Verilog HDL originated at Automated Integrated Design Systems (later renamed as Gateway  Design Automation) in 1985. The company was privately held at that time by Dr. Prabhu  Goel, the inventor of the PODEM test generation algorithm. Verilog HDL was designed by Phil   Moorby, who was later to become the Chief Designer for Verilog-XL and the first Corporate Fellow at Cadence Design Systems. Gateway Design Automation grew rapidly with the success of Verilog-XL and was finally  acquired by Cadence Design Systems, San Jose, CA in 1989. 

Verilog was invented as simulation language. Use of Verilog for synthesis was a complete  afterthought 

Cadence Design Systems decided to open the language to the public in 1990, and thus OVI  (Open Verilog International) was born. Till that time, Verilog HDL was a proprietary language, being the property of Cadence Design Systems. When OVI was formed in 1991, a number of small companies began working on Verilog  simulators. The first of these came to market in 1992, and now there are mature Verilog  simulators available from several sources. 

As a result, the Verilog market has grown substantially. The market for Verilog related tools in  1994 was well over $75m, making it the most commercially significant hardware description language on the market. 

An IEEE working group was established in 1993 under the Design Automation Sub-Committee  to produce the IEEE Verilog standard 1364. Verilog became IEEE Standard 1364 in 1995.

The Verilog Standard was revised in 2001 and it became IEEE Standard 1364-2001

WHAT IS SYNTHESIS IN DIGITAL DESIGN

 Synthesis

Synthesis: It is a process to map and optimizing higher level HDL description to technology cells (gates, flip flops etc.)

Synthesis Flow Diagram:

HDL Description: This is description of design in Verilog. One has to use subset of constructs as synthesis tools does not support all of them.

Technology Library: This file contains functional description and other information related to area and speed for all the cells of particular technology.

Here "technology" means information about particular process for particular vendor. For example Company X, Standard Cell, 0.18 micron, Y Process, Z Type.

Constraints: This optional file contains information about physical expectations from design. For example speed and area.

Netlist: A netlist is a text file description of a physical connection of components.

Reports: This optional output file contains physical performance of design in terms of speed and area.

Schematic: Some tools provide the facility to view netlist in terms of schematics for better understanding of design and to match the results with the expectations.

Simple example:
Following trivial example explains the Synthesis process. In this example only always procedural statement is used.

module test (out, in1, in2); // behavioral description
  input in1, in2;
  output out;
  reg out;
  reg temp;                 // temporary register

  always@(in1 or in2) begin
    temp = ~in2;
    out = ~in1 ^ temp;  // I am trying to have exor with inverted 
  end                   // inputs
endmodule

after synthesis one gets following "netlist" in verilog. Note that XOR2 is module picked up from technology library. It will be different for different libraries.

module add ( out , in1 , in2 );  // netlist

    output out ;
    input in1 ;
    input in2 ;

    XOR2   instance_name (.Y (out ),.A (in1 ),.B (in2 ) );

endmodule

VERILOG CODE FOR D FLIP FLOP

The Verilog beginners need examples of simple building blocks to learn coding techniques. Now  we will go through different implementation of D FLIP FLOP

=========================================================================

1.Simple D FLIP FLOP 

module dff (data, clock, q);
    // port list
    input   data, clock;
    output  q;

    // reg / wire declaration for outputs / inouts     
    reg     q;

    // logic begins here
    always @(posedge clock) 
        q <= data;
endmodule

========================================================================

2. D Type Flip-flop with asynchronous reset

module dff_async (data, clock, reset, q);

    // port list
    input   data, clock, reset;
    output  q;

    // reg / wire declaration for outputs / inouts
    reg     q;

    // reg / wire declaration for internal signals

    // logic begins here
    always @(posedge clock or negedge reset)
        if(reset == 1'b0)
            q <= 1'b0;
        else 
            q <= data;
endmodule

=======================================================================

3. D Type Flip-flop with Synchronous reset

module dff_sync (data, clock, reset, q);
    // port list
    input   data, clock, reset;
    output  q;

    // reg / wire declaration for outputs / inouts
    reg     q;

    // reg / wire declaration for internal signals

    // logic begins here
    always @(posedge clock) 
        if(reset == 1'b0)
            q <= 1'b0;
        else 
            q <= data;
endmodule

================================================================================

4.D Type Flip-flop with asynchronous reset and clock enable

module dff_cke (data, clock, reset, cke, q);
    // port list
    input   data, clock, reset, cke;
    output  q;

    // reg / wire declaration for outputs / inouts
    reg     q;

    // logic begins here
    always @(posedge clock or negedge reset) 
        if (reset == 0)
            q <= 1'b0;
        else if (cke == 1'b1)
            q <= data;
endmodule

Vlsi Design Styles in Digital Design

Digital Design can be implemented by various design styles. And depending on the market requirement different design styles are used.

• Programmable Logic Design
  • Field Programmable Gate Array (FPGA)
  • Gate Array
• Standard Cell (semi custom design)
• Full Custom Design

• Field Programmable Gate Array (FPGA):
  • Using VHDL or verilog
  • Implementation
    • Placement and Routing
    • BitStream Generation
    • Analyse timing, view layout, simulations etc
• Gate Array: Gate Array design implementation is done with metal design and processing. The implementation requires two-step manufacturing process
  • First phase, which is based on standard masks, results in an array of uncommitted transistors on each GA chips
  • These uncommitted chips can be customized later, which is completed by defining the metal interconnects between the transistor of the array
  • In this chip utilization factor is higher than that of FPGA
  • Chip speed is higher
• Standard Cell or Semi Custom Design:
  • The standard-cells based design is often called semi custom design.
  • The cells are pre-designed for general use and the same cells are utilized in many different chip designs. 
• Full Custom Design
• Full custom design involves creating IC where each individual transistors architecture and interconnections are specified. Designers manually place transistors, resistors,capacitors and other components at the transistor level

STANDARD CELLS IN DIGITAL DESIGN/VLSI

Standard cell are well defined cells which are used in Digital Design more frequently. To name few AND, NOR, NAND, XOR ,etc belongs to standard cell family. All the standard cells from one library will have equal drive strength  and  equal height. Standard cell Architecture is defined based on  cell height which is determined on the basis of the number of trackes , beta ratio, pitch and transistor widths. To attain the similarity amoung the cells and aboid the alignment issues ,standard cells are designed with fixed height

The height of a standard cell can be calculated by considering number of tracks required for power rail, ground rail, I/O pins and routing. Often the standard cells are available in single height and double height. The Double height cells are the high density cells and are used for ultra high speed operations 

STANDARD CELL DESIGN METHODOLOGY

• VDD and GND should be of same height and parallel. Both the power rails used metal M1
• make sure within the cell all the PMOS should occupy top and all NMOS should occupy bottom of the Layout
• Preferred Practice:  Diffusion layer for all the transistor in a row
• All the gates include the gate and substrate

 Layout for any schematic can be drawn in many ways. Layout of INVERTER can be drawn in two different ways.

In the Fig 2 was preferred layout as all the PMOS will be in one level and all the NOMS will be at one level, and also the poly gates are drawn vertical and these are common to nmos and pmos transistors. 

One more layout example with NAND GATE

There are many reasons for choosing the FIG 2 and FIG 3 as most preferred Layout

• Save the Design Area: Both the nwell and pwell are in the same level for all the standard cell, so make a common well which saves lots of areas 
• Easy Placement for APR tool: All the standard cells have the same height and easily can be fit into the standard cell row so make it easy for APR (Automatic Place and Route) to place them. They also have power rails in the same location for all the standard cells, so power rails can also be abutted easily 
• Easy to Route: All the pins of standard cells are in the intersection of horizontal and vertical tracks, So it becomes easy to route them by the APR tool .