CSET 4650 Field Programmable Logic Devices

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CSET 4650 Field Programmable Logic Devices
Introduction to PLDs

• Dan Solarek

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Objectives

• Review of Digital Systems
• History of PLDs
• Types of PLDs
• Terminology

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Some Basics

• a combinational logic circuit is one where the
  next output depends only on the current input

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Some Basics

• a sequential logic circuit is one where the next
  output depends not only on the current input but
  also on the sequence of past inputs

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Some Basics

• the design of a sequential logic circuit includes
  the design of a combinational logic circuit

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Purpose of this Course

• This course is about logic circuit implementation

Karnaugh Map
X1 X2 Q1 Q2 Y 0 0 0 0 0 0 0 0 1
0 0 0 1 0 0 0 0 1 1 0 0 1
0 0 1 0 1 0 1 1 0 1 1 0
x 0 1 1 1 1 1 0 0 0 0 1 0 0
1 1 1 0 1 0 x 1 0 1 1 1 1
1 0 0 0 1 1 0 1 1 1 1 1 0
1 1 1 1 1 1
specification
circuit
?
function
truth table
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Logic Circuit Implementation

• there are several options

Memory
SSI Gates
MSI ICs
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Logic Circuit Implementation

• there are several options

U19-251641-02PLA
Programmable Devices
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Electronic Components
Programmable Logic Devices (PLDs)
Gate Arrays
Cell-Based ICs
Full Custom ICs
SPLDs (PALs)
FPGAs
Acronyms SPLD Simple Prog. Logic Device PAL
Prog. Array of Logic CPLD Complex PLD FPGA
Field Prog. Gate Array ASIC Application
Specific IC

• Common Resources
• Configurable Logic Blocks (CLB)
• Memory Look-Up Table (LUT)
• AND-OR planes
• Simple gates
• Input / Output Blocks (IOB)
• Bidirectional, latches, inverters,
  pullup/pulldowns
• Interconnect or Routing
• Local, internal feedback, and global

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How Do You Make a Programmable Circuit?

• One time programmable
• Fuses (destroy internal links with current)
• Anti-fuses (grow internal links)
• PROM
• Reprogrammable
• EPROM
• EEPROM
• Flash
• SRAM - volatile

non-volatile
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Programmable ROM (PROM)

• First ones had fusible links
• High voltage would blow out links
• Fast to program
• Single use

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UV EPROM

• Erasable PROM
• Common technologies used UV light to erase
  complete device
• Took about 10 minutes
• Holds state as charge in very well insulated
  areas of the chip
• Nonvolatile for several (10?) years

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EEPROM

• Electrically Erasable PROM
• Similar technology to UV EPROM
• Erased in blocks by higher voltage
• Programming slower than reading
• Some called flash memory
• Digital cameras, MP3 players, BIOS
• Limited life
• Some support individual word write, some block
• One on Xess board has 5 blocks
• Has a boot block that is carefully protected

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Details of ROM

• Memory that is permanent
• k address lines
• 2k items
• n bits

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Simple PLDs
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Programmable Logic Device

• A programmable logic device or PLD is an
  electronic component used to build digital
  circuits.
• Unlike a logic gate, which has a fixed function,
  a PLD has an undefined function at the time of
  manufacture.
• Before the PLD can perform in a circuit it must
  be programmed.

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Programmable Array Logic (PAL)

• The first programmable logic devices were
  produced by the Advanced Micro Devices (AMD)
  corporation.
• The devices were called PALs, for programmable
  array logic.
• The PLD business split from AMD under the name
  Vantis, and was acquired by Lattice Semiconductor
  in 1999.

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Early PALs

• The programmable array contains logic gates,
  themselves fixed in function, with programmable
  interconnections between them.
• The array has a number of inputs and outputs, and
  can create any Boolean function of a selection of
  the inputs at any of its outputs.
• A single PAL can replace a circuit containing a
  large number, perhaps a few hundred, of fixed
  logic gates.

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Early PALs

• In a PAL the logic gates are arranged as a
  sum-of-products array.
• In Boolean terms, this means a number of AND
  gates whose outputs feed into a large OR gate
  that drives one output.
• By selecting which inputs drive each AND gate,
  and which AND gates drive the OR gate, any
  Boolean function can be created.

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Early PALs

• A PAL is programmed by fitting it into a machine
  called a PAL programmer.
• PAL programmers are usually general-purpose
  machines that can program all types of PLDs from
  all manufacturers.
• A PAL may be programmed only once.

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Early PALs

• The PAL programmer must be supplied with a
  description of the PAL's desired configuration.
• This is usually in the form of a computer text
  file with a standard format defined by the Joint
  Electron Device Engineering Council (JEDEC).
• JEDEC files can be hand-typed by the design
  engineer or, more commonly, produced by a
  computer program similar to the language
  compilers used by software engineers.

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Generic Array Logic (GAL)

• An innovation of the PAL was the generic array
  logic device, or GAL, invented by Lattice
  Semiconductor Inc.
• This device has the same logical properties as
  the PAL but can be erased and reprogrammed.
• The GAL is very useful in the prototyping stage
  of a design, when any bugs in the logic can be
  corrected by reprogramming.
• GALs are programmed and reprogrammed using a PAL
  programmer.

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Programmable Electrically Erasable Logic (PEEL)

• A PEEL is a device similar to a GAL and was
  introduced by the Integrated Circuit Technology
  (ICT) corporation.
• As the name implies, it differs from a GAL or PAL
  in that it is electrically erasable
  (reprogrammable)

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Complex Programmable Logic Devices (CPLDs)

• Xilinx devices that are cheaper and have fewer
  gates than FPGAs
• Meant for interfacing rather than heavy
  computation
• Built-in flash memory
• Instead of FPGA, which needs external
• Xess bd. has XC9572XL part
• Approx 2-7 in ones qty. (vs. 15-20 for the
  Spartan2 on the board). Larger qty much lower.
• 1600 gates, 72 registers

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Complex PLDs

• PALs and GALs are available only in small sizes,
  equivalent to a few hundred logic gates.
• For bigger logic circuits, complex PLDs or CPLDs
  can be used.
• These contain the equivalent of several PALs
  linked by programmable interconnections, all in
  one integrated circuit.
• CPLDs can replace thousands, or even hundreds of
  thousands, of logic gates.

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Complex PLDs

• Some CPLDs are programmed using a PAL programmer,
  but this method becomes inconvenient for devices
  with hundreds of pins.
• A second method of programming is to solder the
  device to its printed circuit board, then feed it
  with a serial data stream from a personal
  computer.
• The CPLD contains a circuit that decodes the data
  stream and configures the CPLD to perform its
  specified logic function.

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Complex PLDs

• Each manufacturer has a proprietary name for its
  CPLD programming system.
• For example, Lattice calls it "in-system
  programming".
• However, these proprietary systems are beginning
  to give way to a standard from the Joint Test
  Action Group (JTAG).

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Field Programmable Logic Devices (FPGAs)

• While PALs were busy developing into GALs and
  CPLDs, a separate stream of development was
  happening.
• This type of device is based on gate-array
  technology and is called the field-programmable
  gate array (FPGA).

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Field Programmable Logic Devices (FPGAs)

• Gate arrays are non-programmable devices that can
  be manufactured more cheaply than other types of
  IC, because they contain a standard grid of logic
  gates whose interconnections are specified by the
  customer.
• When a customer orders a new type of chip, the
  manufacturer does not have to design it from
  scratch, but can just take a standard gate array
  and modify it to the customer's requirement.

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Field Programmable Logic Devices (FPGAs)

• FPGAs use a similar grid of logic gates, but the
  programming is done by the customer, not by the
  manufacturer.
• The term "field-programmable" may be obscure to
  some, but the "field" is just an engineering term
  for the world outside the factory where customers
  live.

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Field Programmable Logic Devices (FPGAs)

• FPGAs are usually programmed after being soldered
  down to the circuit board, in the same way as
  larger CPLDs.
• In most larger FPGAs the configuration is
  volatile, and must be re-loaded into the device
  whenever power is applied or different
  functionality is required.

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Field Programmable Logic Devices (FPGAs)

• FPGAs and CPLDs are often equally good choices
  for a particular task.
• Sometimes the decision is more an economic one
  than a technical one, or may depend on the
  engineer's personal preference and history.

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PLD Device Density and VLSI Technology
note Xilinx Virtex-II Pro XC2VP100 (9/16/2003)
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How PLDs Remember Their Configuration

• A PLD is a combination of a logic device and a
  memory device.
• The memory is used to store the pattern that was
  given to the chip during programming.
• Most of the methods for storing data in an
  integrated circuit have been adapted for use in
  PLDs.

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How PLDs Remember Their Configuration

• The methods for storing data in an integrated
  circuit include
• silicon antifuses
• static RAM (SRAM)
• flash memory
• EPROM cells

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How PLDs Remember Their Configuration - Antifuses

• Silicon antifuses are the storage elements used
  in the PAL, the first type of PLD.
• These are connections that are made by applying a
  voltage across a modified area of silicon inside
  the chip.
• They are called antifuses because they work in
  the opposite way to normal fuses, which begin
  life as connections until they are broken by an
  electric current.

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How PLDs Remember Their Configuration - SRAM

• SRAM, or static RAM, is a volatile type of
  memory, meaning that its contents are lost each
  time the power is switched off.
• SRAM-based PLDs therefore have to be programmed
  every time the circuit is switched on.
• This is usually done automatically by another
  part of the circuit.

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How PLDs Remember Their Configuration - Flash

• Flash memory is non-volatile, retaining its
  contents even when the power is switched off.
• It can be erased and reprogrammed as required.
• This fact makes flash memory useful for PLD
  memory.

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How PLDs Remember Their Configuration - EPROM

• An EPROM cell is a MOS (metal-oxide-semiconductor)
  transistor that can be switched on by trapping
  an electric charge permanently on its gate
  electrode.
• This is done by a PAL programmer.
• The charge remains for many years and can only be
  removed by exposing the chip to strong
  ultraviolet light in a device called an EPROM
  eraser.

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PLD Programming Languages

• PLD programming languages
• PALASM
• ABEL
• CUPL

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Hardware Description Languages

• The languages used as source code for logic
  compilers are called hardware description
  languages, or HDLs.
• Examples of HDLS include
• VHDL
• Verilog HDL
• AHDL

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Programmable Logic Solutions

• No high development cost barriers
• Recovered time for authoring and innovating
• SW improvements reduce design iterations
• No lengthy prototyping cycle
• Ability to remotely upgrade any networked system
• Ultimate flexibility to manage rapid change