Flash Memory and RAM: “Random-access” memory

Flash Memory

It is an extension of EEPROM. It has the same floating gate principle and same write ability and storage permanence. It can be erased at a faster rate i.e. large blocks of memory erased at once, rather than one word at a time. The blocks are typically several thousand bytes large.

• Writes to single words may be slower.
– Entire block must be read, word updated, then entire block written back.
• Used with embedded systems storing large data items in nonvolatile memory.
– e.g., digital cameras, TV set-top boxes, cell phones.

RAM: “Random-access” memory




• Typically volatile memory
– bits are not held without power supply.

• Read and written to easily by embedded system during execution

• Internal structure more complex than ROM
– a word consists of several memory cells, each storing 1 bit
– each input and output data line connects to each cell in its column
– rd/wr connected to every cell
– when row is enabled by decoder, each cell has logic that stores input data bit when rd/wr indicates write or outputs stored bit when rd/wr indicates read



Basic types of RAM

• SRAM: Static RAM
– Memory cell uses flip-flop to store bit.
– Requires 6 transistors.
– Holds data as long as power supplied.

• DRAM: Dynamic RAM
– Memory cell uses MOS transistor and capacitor to store bit.
– More compact than SRAM.
– “Refresh” required due to capacitor leak.

• word’s cells refreshed when read
– Typical refresh rate 15.625 microsec.
– Slower to access than SRAM.



Ram variations


• PSRAM: Pseudo-static RAM
– DRAM with built-in memory refresh controller.
– Popular low-cost high-density alternative to SRAM.

• NVRAM: Nonvolatile RAM
– Holds data after external power removed.
– Battery-backed RAM.

• SRAM with own permanently connected battery

• writes as fast as reads

• no limit on number of writes unlike nonvolatile ROM-based memory
– SRAM with EEPROM or flash stores complete RAM contents on EEPROM or flash before power.

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