programmable logic arrays, programmable logic designs,

Charles Kime & Thomas Kaminski
© 2004 Pearson Education, Inc.
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Chapter 3 – Combinational
Logic Design
Part 2 – Programmable Implementation
Technologies
Logic and Computer Design Fundamentals

Chapter 3 - Part 2 2
Overview
 Part 1 - Implementation Technology and Logic Design
• Design Concepts and Automation
 Fundamental concepts of design and computer-aided design techniques
• The Design Space
 Technology parameters for gates, positive and negative logic and design
tradeoffs
• Design Procedure
 The major design steps: specification, formulation, optimization,
technology mapping, and verification
• Technology Mapping
 Mapping from AND, OR, and NOT to other gate types
• Verification
 Does the designed circuit meet the specifications?
 Part 2 - Programmable Implementation Technologies
• Read-Only Memories, Programmable Logic Arrays, Programmable
Array Logic
 Technology mapping to programmable logic devices

Overview
 Why programmable logic?
 Programmable logic technologies
 Read-Only Memory (ROM)
 Programmable Logic Array (PLA)
 Programmable Array Logic (PAL)
 VLSI Programmable Logic Devices -
covered in VLSI Programmable Logic Devices reading
supplement

Why Programmable Logic?
 Facts:
• It is most economical to produce an IC in large
volumes
• Many designs required only small volumes of ICs
 Need an IC that can be:
• Produced in large volumes
• Handle many designs required in small volumes
 A programmable logic part can be:
• made in large volumes
• programmed to implement large numbers of
different low-volume designs

Programmable Logic - Additional Advantages
 Many programmable logic devices are field-
programmable, i. e., can be programmed outside of the
manufacturing environment
 Most programmable logic devices are erasable and
reprogrammable.
• Allows “updating” a device or correction of errors
• Allows reuse the device for a different design - the ultimate in re-
usability!
• Ideal for course laboratories
 Programmable logic devices can be used to prototype
design that will be implemented for sale in regular ICs.
• Complete Intel Pentium designs were actually prototype with
specialized systems based on large numbers of VLSI
programmable devices!

Programming Technologies
 Programming technologies are used to:
• Control connections
• Build lookup tables
• Control transistor switching
 The technologies
• Control connections
 Mask programming
 Fuse
 Antifuse
 Single-bit storage element

Programming Technologies
 The technologies (continued)
 Storage elements (as in a memory)
• Transistor Switching Control
 Stored charge on a floating transistor gate
• Erasable
• Electrically erasable
• Flash (as in Flash Memory)

Technology Characteristics
 Permanent - Cannot be erased and reprogrammed
 Mask programming
 Fuse
 Antifuse
 Reprogrammable
• Volatile - Programming lost if chip power lost
 Single-bit storage element
• Non-Volatile
 Erasable
 Electrically erasable
 Flash (as in Flash Memory)
• Transistor Switching Control
 Stored charge on a floating transistor gate
• Erasable
• Electrically erasable
• Flash (as in Flash Memory)

Programmable Configurations
 Read Only Memory (ROM) - a fixed array of AND
gates and a programmable array of OR gates
 Programmable Array Logic (PAL)
- a
programmable array of AND gates feeding a fixed
array of OR gates.
 Programmable Logic Array (PLA) - a programmable
array of AND gates feeding a programmable array
of OR gates.
 Complex Programmable Logic Device (CPLD) /Field-
Programmable Gate Array (FPGA) - complex enough
to be called “architectures” - See VLSI Programmable
Logic Devices reading supplement
PAL is a registered trademark of Lattice Semiconductor Corp.

Chapter 3 - Part 2
ROM, PAL and PLA Configurations
(a) Programmable read-only memory (PROM)
Inputs
Fixed
AND array
(decoder)
Programmable
OR array
Outputs
Programmable
Connections
(b) Programmable array logic (PAL) device
Inputs Programmable
AND array
Fixed
OR array
Outputs
Programmable
Connections
(c) Programmable logic array (PLA) device
Inputs Programmable
OR array
Outputs
Programmable
Connections
Programmable
Connections
Programmable
AND array

Chapter 3 - Part 2
Read Only Memory
 Read Only Memories (ROM) or Programmable Read Only Memories
(PROM) have:
• N input lines,
• M output lines, and
• 2N
decoded minterms.
 Fixed AND array with 2N
outputs implementing all N-literal minterms.
 Programmable OR Array with M outputs lines to form up to M sum of
minterm expressions.
 A program for a ROM or PROM is simply a multiple-output truth table
• If a 1 entry, a connection is made to the corresponding minterm for the
corresponding output
• If a 0, no connection is made
 Can be viewed as a memory with the inputs as addresses of data (output
values), hence ROM or PROM names!

Chapter 3 - Part 2
 Example: A 8 X 4 ROM (N = 3 input lines, M= 4 output lines)
 The fixed "AND" array is a
“decoder” with 3 inputs and 8
outputs implementing minterms.
 The programmable "OR“
array uses a single line to
represent all inputs to an
OR gate. An “X” in the
array corresponds to attaching the
minterm to the OR
 Read Example: For input (A2,A1,A0)
= 011, output is (F3,F2,F1,F0 ) = 0011.
 What are functions F3, F2 , F1 and F0 in terms of (A2, A1, A0)?
Read Only Memory Example
D7
D6
D5
D4
D3
D2
D1
D0
A2
A1
A0
A
B
C
F0
F1
F2
F3
X X
X
X
X
X
X
X
X
X

Chapter 3 - Part 2
Programmable Array Logic (PAL)
 The PAL is the opposite of the ROM, having a
programmable set of ANDs combined with fixed ORs.
 Disadvantage
• ROM guaranteed to implement any M functions of N
inputs. PAL may have too few inputs to the OR gates.
 Advantages
• For given internal complexity, a PAL can have larger N and M
• Some PALs have outputs that can be complemented, adding
POS functions
• No multilevel circuit implementations in ROM (without external
connections from output to input). PAL has
outputs from OR terms as internal inputs to all AND
terms, making implementation of multi-level circuits easier.

Chapter 3 - Part 2
Programmable Array Logic Example
 4-input, 3-output PAL
with fixed, 3-input OR
terms
 What are the equations
for F1 through F4?
F1 = +
F2 = B + AC + AB
F3 =
F4 =
0 9
1 2 3 4 5 6 7 8
AND gates inputs
0 9
Product
term
1
2
3
4
5
6
7
8
9
10
11
12
F1
F2
F3
F4
I3 5C
I2 5B
I15 A
1 2 3 4 5 6 7 8
I4
X X
X X
X X X
X X
X
X
X
X
X
X
X X
X
X X
B C
A
A C

Chapter 3 - Part 2
Programmable Logic Array (PLA)
 Compared to a ROM and a PAL, a PLA is the most
flexible having a programmable set of ANDs combined
with a programmable set of ORs.
 Advantages
• A PLA can have large N and M permitting implementation of
equations that are impractical for a ROM (because of the number
of inputs, N, required
• A PLA has all of its product terms connectable to all outputs,
overcoming the problem of the limited inputs to the PAL Ors
• Some PLAs have outputs that can be complemented, adding POS
functions
 Disadvantage
• Often, the product term count limits the application of a PLA.
Two-level multiple-output optimization reduces the number of
product terms in an implementation, helping to fit it into a PLA.

Chapter 3 - Part 2
Programmable Logic Array Example
 3-input, 3-output PLA
with 4 product terms
 What are the equations for F1 and F2?
 Could the PLA implement the
functions without the XOR gates?
Fuse intact
Fuse blown
1
F1
F2
X
A
B
C
C C B B A A 0
1
2
3
4
X
X
X
X X
X
X
X
X
X
X
X
X
X A B
A C
B C
A B
X

Chapter 3 - Part 2
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