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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

Digital Design:Principles and Practices

Chapter 9

Memory, CPLDs, and FPGAs

read only memory rom
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 rom1
Read-Only Memory (ROM)
  • A ROM “stores” the truth table of an n-input, b-output combinational logic function.
read only memory rom2
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
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
programmable roms
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 transistor1
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.
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)
basic structure of a 2 n x b 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
layout of 6t sram cell
Layout of 6T SRAM Cell

Only poly and diff layers are shown.

dram cell1
DRAM Cell
  • 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 s capacitor1
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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