digital design: principles and practices chapter 9 memory, cplds, and fpgas
TRANSCRIPT
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.
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Read-Only Memory (ROM)
• A ROM “stores” the truth table of an n-input, b-output combinational logic function.
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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.
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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
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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.
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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.
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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)
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Basic Structure of a 2n x b RAM
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• CS: Chip Select• OE: Output Enable• WE: Write Enable
• Read: CS and OE are asserted• Write: CS and WE are asserted
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
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