Lecture 1
Design Hierarchy
Chapter 1
Digital System Design Flow
1. Register-Transfer Levl (RTL)
– e.g. VHDL/Verilog
2. Gate Level Design
3. Circuit Level Design
4. Physical Layout
Verilog
• Include a set of 26 predefined functional
models of common combinational logic gates
called primitives.
• Primitives
– The most basic functional objects that can be used
to compose a design
– Are built into the language by means of internal
truth tables
– Examples: and, nand, or, nor, xor, xnor
More on Primitives
• 3-input nand primitive
– Input signal a, b, and c
– Output signal y
• Each primitive has ports (corresponding to
hardware pins and terminals)
– The output port(s) of a primitive must be first in
the list, followed by the primitive’s input ports.
Instantiated Primitives
• Instantiated Primitives (nor, and,nand) are
connected by wires.
• A wire is a data-type which is used to establish
connectivity in a design, just as a physical wire
establishes connectivity between gates.
Example: a Full Adder
• Binary Addition
• Gate-Level Synthesis
• Verilog Representation
Binary Addition (1)
Binary Addition (2)
Derivation of ∑
B
A
∑
0
0
0
1
0
1
0
1
1
1
1
0
Question: What primitive best implements ∑?
• Inputs: A, B
• Outputs: xor (∑, A, B)
Derivation of Carry Out
B
A
Co
0
0
0
1
0
0
0
1
0
1
1
1
Question: What primitive best implements Co?
• Inputs: A, B
• Outputs: and (Co, A, B)
A Half Adder
A half adder is useful for adding LSB.
Limitation of a Half Adder
A half-adder does not account for carry-in.
Truth Table of ∑ of a Full Adder
Cin+B+A=Cin+∑HA=Cin XOR ∑HA
Cin
B
A
∑
0
0
0
0
0
0
1
1
0
1
0
1
0
1
1
0
1
0
0
1
1
0
1
0
1
1
0
0
1
1
1
1
Identical to
∑ of a Half
Adder
Truth Table of Co of a Full Adder
Cin
B
A
Co
0
0
0
0
0
0
1
0
0
1
0
0
0
1
1
1
1
0
0
0
1
0
1
1
1
1
0
1
1
1
1
1
Identical to
∑ of a Half
Adder
Use a Half Adder
with Cin and ∑HA to
generate Co
Schematic of a Full Adder
A 3 bit parallel adder
Gate Level vs. Verilog Model
of a Full Adder
Explanation
• The keywords module and endmodule
encapsulate the text that describes the
module
• The module name is Add_full
• Module Ports are
– Input a, b, c_in
– Output c_out, sum
• Module instances: Add_half, or
Nested Module
• Add_half is a child module of Add_full
Gate Level Design
• Basic Gates
– AND, NAND,OR, NOR, XOR, XNOR,NOT
• Universal Gates
– NAND Gates
– NOR Gates
• Multiple Inputs Logic Gates
NAND Based Logic Gates
NOR Based Logic Gates
Multiple Inputs Logic Gates
Circuit Level
Physical Design
• Floor Planning
– Estimates of the area of major units in the chip
and defines their relative placements.
– Estimate wire lengths and wring congestions.
– Challenge: estimate the size of each unit without
proceeding through a detailed design of the chip.
• Layout
• Design Verification
• Tapeout
A Sample Floor Plan
λ= ½ of minimum
channel length
A Sample Layout
Layout of an Inverter
In a 0.6 um process
4/2=1.2 um/0.6 um.
Design Verification
• LVS (Layout vs. Schematic) checks that
transistors in a layout are connected in the
same way as in the circuit schematic.
• DRC (Design Rule Checkers) verify that the
layout satisfies design rules.
• ERC (Electrical Rule Checkers) scan for
problems such as noise or premature
wearout.
Tapeout
• Tapeout gets its name from the old practice of
writing a specifications of masks to a magnetic
tape.
• GDS
• Foundries:
– TSMC
– UMC
– IBM
Fabricated Chip
IC Decapsulation
Cross Section
IC Chip
Wire Bond
Silver- Epoxy
Dielectric
Top Metal Trace
Via
package material
(plastic)
Ball
Bottom Metal Trace
Low Cost Package
1
7
12
•Red: Top layer trace
•Green: Via
•Blue: Bottom layer
trace
Package Parasitics