ASIC 120: Digital Systems and Standard-Cell ASIC Design

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ASIC 120 Digital Systems and Standard-Cell ASIC
Design

• Tutorial 1 Introduction to Digital Circuits
• October 11, 2005

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Outline

• Digital Systems
• Combinational Logic
• NOT, AND, OR, XOR, NAND, etc.
• mux, half-adder, full-adder
• Sequential Logic
• flip-flop/register, shift register, counter
• state machines

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Digital Systems

• Analog vs. Digital
• continuously varying vs. discrete
• imprecise vs. precise
• 0..1 vs. 0 or 1
• Digital systems excel at
• repetitive calculations
• large amounts of data
• reproducible results

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The Big Picture
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The Big Picture
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Combinational and Sequential Logic

• We can break a digital system into two types of
  logic
• Combinational
• computation happens in a linear fashion
• Sequential
• computation involves a feedback loop (memory)

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Combinational Logic NOT
Truth Table
Input
Output
A X
0 1
1 0
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Combinational Logic AND
A B X
0 0 0
0 1 0
1 0 0
1 1 1
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Combinational Logic OR
A B X
0 0 0
0 1 1
1 0 1
1 1 1
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Combinational Logic XOR
A B X
0 0 0
0 1 1
1 0 1
1 1 0
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Combinational Logic NAND
A B X
0 0 1
0 1 1
1 0 1
1 1 0
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Combinational Logic NOR, XNOR
A B X
0 0 1
0 1 0
1 0 0
1 1 0
A B X
0 0 1
0 1 0
1 0 0
1 1 1
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Building Combinational Circuits
A B C X
0 0 0 0
0 1 0 0
1 0 0 1
1 1 0 1
0 0 1 0
0 1 1 1
1 0 1 0
1 1 1 1
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Combinational Logic MUX(multiplexer)
A B C X
0 0 0 0
0 1 0 0
1 0 0 1
1 1 0 1
0 0 1 0
0 1 1 1
1 0 1 0
1 1 1 1
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Half Adder
A B S C
0 0 0 0
0 1 1 0
1 0 1 0
1 1 0 1
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Full Adder
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Full Adder
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What Ive Skipped

• Gates with more than two inputs
• Boolean algebra
• Karnaugh maps
• Quine-McCluskey method
• Binary arithmetic, base conversions
• Practical digital circuits have more than 0s and
  1s
• Transmission gates, tri-state buffers

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Sequential Logic
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Basic Feedback Element SR latch
S R Q Qnext
0 0 0 0
0 0 1 1
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 N/A
1 1 1 N/A
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D Flip-Flop or Register
D Clk Q Qnext
0 0 0 0
0 0 1 1
1 0 0 0
1 0 1 1
0 0?1 0 0
0 0?1 1 0
1 0?1 0 1
1 0?1 1 1
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Synchronous vs. Asynchronous

• Synchronous
• circuit operation governed by a clock
• currently more popular and practical
• Asynchronous
• circuit operation independent of a clock
• potentially faster than synchronous
• lower power consumption
• difficult to design

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Sequential Constructs

• Shift register
• Counter
• State Machines

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State Machines

• Useful abstract constructs for more complex
  sequential logic

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What Ive Skipped

• Other flip-flops (RS, T, JK)
• Other interesting sequential circuits (barrel
  shifters, etc.)

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Hardware Description Languages (HDLs)

• HDL describes in text a digital circuit
• Examples
• VHDL (we will look at this next time)
• Verilog
• AHDL
• JHDL

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Hardware Description Languages (HDLs)

• schematics are useful for
• drawing high level diagrams
• manually working out simple pieces of logic
• HDLs are useful for
• describing complex digital systems
• HDLs are not...
• software programming languages (C, Java,
  assembly, etc.)

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Summary

• Digital Systems
• Combinational Logic
• NOT, AND, OR, XOR, NAND, etc.
• mux, half-adder, full-adder
• Sequential Logic
• flip-flop/register, shift register, counter
• state machines

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UW ASIC Design Team

• www.asic.uwaterloo.ca
• reference material
• Bryce Leungs tutorials (UW ASIC website)
• Michael Goldsmiths tutorials (UW ASIC website)
• your course notes
• my contact info
• Jeff Wentworth, jswentwo_at_engmail.uwaterloo.ca