10-08-2013, 04:56 PM
Modeling Flash Memories for IC Designs
[attachment=57081]
Dynamic Random Access Memory
each data bit is stored in a separate capacitive element in the Integrated Circuit
Because capacitors leak their charge the memory must be periodically refreshed, thus the name Dynamic or Volatile
requires one transistor and one capacitor per bit
simple, allowing high packaging densities
refresh rate dependent on implementation
8 usec – 64 msec
includes DDR and SDRAM
SRAM
Static Random Access Memory
Static in the sense that it does not need refreshing like DRAM, but is still considered volatile (loses its memory when power is removed)
requires six transistors (multi ported memory may require 8, 10 or more transistors per bit)
lower packaging density than DRAM
easier to interface with than DRAM because of simplicity and no need to refresh
ROM
Read-only memory
mask ROM – non-volatile, permanently programed in the manufacturing process
PROM – Programmable Read-Only Memory
Programmable once then unalterable, non-volatile
essentially a fuse matrix in which programming is done by blowing the fuses
EPROM – Erasable Programmable Read-Only Memory
semi-permanent in that the memory can be erased (by exposing to ultrviolet light) and reprogrammed, considered to be non-volatile
EEPROM – Electronically Erasable Programmable Read-only
non-volatile, parallel and serial access.
serial access via SPI, I2C, 1-wire
Flash
Not an Acronym, just a name
stores one bit per cell, cells are floating gate MOSFETs
two types, NAND and NOR
NOR – used like traditional memory, execute-in-place memory (DRAM, SRAM)
NAND – accessed like a block device (disk), used for memory sticks, flash drived, MMC, CF
finite number of writes, unlimited reads
need wear leveling and bad block management
NAND vs. NOR Flash
NAND denser than NOR because of smaller cell size
preferred for data memory storage (flash drives)
NAND is about 2x denser than NOR because of layout efficiencies due to series transistors (less metal contacts)
NAND slower read time because of stacked cell arrangement
NOR preferred as program memory storage for microcontrollers because of faster access time
NAND is always block read/block write; NOR allows read of individual memory locations
NAND programming/erase is faster than NOR programming/erase
[attachment=57081]
Dynamic Random Access Memory
each data bit is stored in a separate capacitive element in the Integrated Circuit
Because capacitors leak their charge the memory must be periodically refreshed, thus the name Dynamic or Volatile
requires one transistor and one capacitor per bit
simple, allowing high packaging densities
refresh rate dependent on implementation
8 usec – 64 msec
includes DDR and SDRAM
SRAM
Static Random Access Memory
Static in the sense that it does not need refreshing like DRAM, but is still considered volatile (loses its memory when power is removed)
requires six transistors (multi ported memory may require 8, 10 or more transistors per bit)
lower packaging density than DRAM
easier to interface with than DRAM because of simplicity and no need to refresh
ROM
Read-only memory
mask ROM – non-volatile, permanently programed in the manufacturing process
PROM – Programmable Read-Only Memory
Programmable once then unalterable, non-volatile
essentially a fuse matrix in which programming is done by blowing the fuses
EPROM – Erasable Programmable Read-Only Memory
semi-permanent in that the memory can be erased (by exposing to ultrviolet light) and reprogrammed, considered to be non-volatile
EEPROM – Electronically Erasable Programmable Read-only
non-volatile, parallel and serial access.
serial access via SPI, I2C, 1-wire
Flash
Not an Acronym, just a name
stores one bit per cell, cells are floating gate MOSFETs
two types, NAND and NOR
NOR – used like traditional memory, execute-in-place memory (DRAM, SRAM)
NAND – accessed like a block device (disk), used for memory sticks, flash drived, MMC, CF
finite number of writes, unlimited reads
need wear leveling and bad block management
NAND vs. NOR Flash
NAND denser than NOR because of smaller cell size
preferred for data memory storage (flash drives)
NAND is about 2x denser than NOR because of layout efficiencies due to series transistors (less metal contacts)
NAND slower read time because of stacked cell arrangement
NOR preferred as program memory storage for microcontrollers because of faster access time
NAND is always block read/block write; NOR allows read of individual memory locations
NAND programming/erase is faster than NOR programming/erase