07. schematic capture
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ENG SC757 - Advanced Microprocessor Design
Babak Kia
Adjunct Professor
Boston University
College of Engineering
Email: bkia -at- bu.edu
Schematic Capture
Schematic Capture
The Schematic Capture tool is part of a suite ofComputer Aided Design (CAD) tools which assist
engineers in the design of modern circuits
The CAD tool can be used to “capture” a virtual
representation of a digital circuit on a computer,which can then be analyzed and simulated before
an actual circuit board is created
There are many varieties of Schematic Capture
CAD tools, allowing designers to create anythingfrom complex VLSI circuitry which can besimulated to fully, to Microprocessor based
boards, which cannot!
The Goal
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The Block Diagram
Every schematicbegins with a blockdiagram
The Block Diagramgives and architecturaloverview and showsthe data paths andinterrelationshipsbetween componentsof a system
Example of a Schematic
Example of a Schematic
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Example of a Schematic
Example of a Schematic
Example of a Schematic
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Example of a Schematic
Example of a Schematic
Example of a Schematic
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Example of a Schematic
Example of a Schematic
Example of a Schematic
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Example of a Schematic
Example of a Schematic
Example of a Schematic
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Requirements
There are many Schematic Capture CAD tools
available to designers, providing:• Good user-interface, allowing the designer to
seamlessly connect and modify differentcomponents
• Good library support, which not only allows forcreation of complex libraries, but also their usage ina hierarchical fashion
• Electrical Rule Check
• No two outputs are connected together
• Inputs aren’t left floating
• Fan-in and Fan-out limitations
• Netlist generation and management
• Enabling import/export to other modules such assimulators, or other CAD tools
Simulation
Schematic Capture tools also allow forsimulation of the electrical components
However, with a microprocessor baseddesign, there is no way of simulating thebehavior of the circuit, in particular thatof the microprocessor
Instead, we design as carefully aspossible, and hope for the best!
Most designs go through at least onephase of redesign. This is why we firstmake a prototype of the system and then
address all the changes in the productionrun of the board
Creating Libraries
Many CAD packages come equipped with a widearray of component libraries, which includeeverything from resistors and capacitors, tointegrated circuits
Chances are, however, that you will need to createa library for most of your ICs, including themicroprocessor from scratch
Creating libraries is time consuming, but ofcrucial importance, since it is the building blocksof your schematic
Components in a library must be checked and re-checked to ensure pins aren’t mislabeled and thatthere are no other errors
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Component
A component in a library is
simply a symbolicrepresentation of an
integrated circuit
The picture to the right shows
the 68098GT8microcontroller’s pinouts
This image can be
represented in one of twoways as a component in a
schematic library
Pin-to-pin Representation
Grouped by Functionality
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Pin Properties
Each pin of the component
has a set of parameters thatneeds to be set at the time
of creating the librarycomponent
This is important later onwhen we run the ElectricalRule Check (ERC) to ensure
that no two outputs areconnected to each other,
and Power and Ground pinsaren’t shorted
Naming conventions
Pin polarities, and theirI/O characteristics areoutlined in the processorspec. For example,
• The reset signal RST# isactive low, and is
designated as such on theschematic with the ‘#’ sign
• TXD is an output, although
this property is not visible
on the schematic
Starting your schematic design
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Adding components
Annotating the Schematic
Annotating the schematic assigns aunique number to every componentwhich can then be used to create aBill Of Materials (BOM) which is thenfurther used for manufacturing andcomponent placement
R P R es is t or Pac k
R Resistors
C Capacitors
L Inductors and ferrite beads
D Diodes
Q Transistor
Y Crystal
OSC Oscillator
H Header
JP Jumper
J Connector
RL Relay
U Other components
Annotated Schematic
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Electrical Rule Check
The ERC is performed to ensure that a design
meets electrical rule guidelines and that a validnetlist can be generated
The ERC checks both for electrical errors such astwo output pins being connected together, and for
drafting errors such as duplicate designators
Once an ERC has completed successfully, anetlist is generated.
A Netlist is a text file listing all the connectionsbetween components. It is used to transfer the
design between different CAD tools, and alsoused to import the design into a PCB document
Bill of Materials
The Bill of Materials (BOM)is an ordered list of partnumbers along with theirdesignators
It is the complete list of allthe items that go intomaking the finishedproduct, including the PCBboard itself, casing, etc.
It includes but is not limitedto manufacturer’s partnumber, quantity,description, and assemblynotes for each component
Portions of this power point presentation mayhave been taken from relevant users and technical manuals. Original content Copyright © 2005 – Babak Kia