Digital design principles and practices
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Digital Design: Principles and Practices. Chapter 9 Memory, CPLDs, and FPGAs. 9.1 Read-Only Memory. Read-Only Memory (ROM). A read-only memory ( ROM ) is a combinational circuit with n inputs and b outputs.

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Digital Design: Principles and Practices

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Digital Design:Principles and Practices

Chapter 9

Memory, CPLDs, and FPGAs

9.1 Read-Only Memory

Read-Only Memory (ROM)

  • A read-only memory (ROM) is a combinational circuit with n inputs and b outputs.

    • The inputs are called address inputs and are traditionally named A0, A1, …, An-1.

    • The outputs are called data outputs and are typically named D0, D1, …, Db-1.

Read-Only Memory (ROM)

  • A ROM “stores” the truth table of an n-input, b-output combinational logic function.

Read-Only Memory (ROM)

  • A ROM is a combinational circuit

    • Not really a memory

    • Information is “stored” when a ROM is manufactured or programmed.

  • ROM is nonvolatile memory; that is, its contents are preserved even if no power is applied.

Nonvolatile Memory

  • ROM: hardwired during fabrication

  • PROM (programmable ROM): can be programmed once only

    • fuse

  • EPROM (Erasable PROM): can be erased by UV light, and can be re-programmed

    • Floating gate

  • EEPROM (Electrically Erasable PROM): can be erased with on-chip circuitry

    • Floating gate

  • Flash memory: a variant of EEPROM that erases entire blocks rather than individual bits

    • Floating gate

Categories of Memory Arrays

Programmable ROMs

  • ROM has in practice become synonymous with nonvolatile, not read-only memory.

  • Programming/writing speeds are generally slower than read speeds.

  • Four type of nonvolatile memories:

    • PROM (Programmable ROM)

    • EPROM (Erasable Programmable ROM)

    • EEPROM (Electrically Erasable Programmable ROM)

    • Flash memories

  • PROMs us fuses while EPROMs, EEPROMs, and Flash use charge stored on a floating gate.

Floating Gate nMOS Transistor

Floating Gate nMOS Transistor

  • The floating gate is a good conductor, but it is not attached to anything.

  • Applying a high voltage to the upper gate causes electrons to jump through the thin oxide onto the floating gate.

  • Injecting the electrons induces a negative voltage on the floating gate, effectively increasing the threshold voltage (Vt) of the transistor to the point that it is always OFF.

  • EPROM: knock off the electrons off the floating gate by UV light

  • EEPROM and Flash can be erased electrically.

9.2 Read/Write Memory

Read/Write Memory

  • The name read/write memory (RWM) is given to memory arrays in which we can store and retrieve information at any time.

  • Random-Access Memory (RAM)

    • Static RAM (SRAM)

    • Dynamic RAM (DRAM)

9.3 Static RAM

Basic Structure of a 2n x b RAM

  • CS: Chip Select

  • OE: Output Enable

  • WE: Write Enable

  • Read: CS and OE are asserted

  • Write: CS and WE are asserted

Internal Structure of an 8 x 4 SRAM

Functional Behavior of an SRAM Cell

12-Transistor SRAM Cell

6-Transistor (6T) SRAM Cell

Stick Diagram of 6T SRAM Cell

Layout of 6T SRAM Cell

Only poly and diff layers are shown.

9.4 Dynamic RAM



  • A DRAM cell contains a transistor and a capacitor.

  • A basic DRAM cell is substantially smaller than a SRAM cell, but the cell must be periodically read and refreshed so that its contents do not leak away.

  • One a read, the bitline is first precharged to VDD/2. When the wordline rises, the capacitor shares its charge with the bitline, causing a voltage change △V that can be sensed. The read process disturbs the cell contents at x, so the cell must be rewritten after each read. [Figure 11.26]

    • Sense amplifier

DRAM Cell – Read Operation

DRAM’s Capacitor

DRAM’s Capacitor

  • A large cell capacitance (Ccell) is important to provide a reasonable voltage swing. It also is necessary to retain the contents of the cell for an acceptably long time.

  • Specialized DRAM processes are required for manufacturing trench capacitors.

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