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UNIT 6 - MEMORY TYPES

UNIT 6 - MEMORY TYPES. Six-Transistor Static RAM Cell. DYNAMIC RAM MEMORIES TYPES OF RAM ---DYNAMIC RAM (CAPACITOR TYPE STRUCTURE) COMPARED WITH: ---STATIC RAM (FLIP-FLOP TYPE STRUCTURE). ADVANTAGES OF DYNAMIC RAM

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UNIT 6 - MEMORY TYPES

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  1. UNIT 6 - MEMORY TYPES

  2. Six-Transistor Static RAM Cell

  3. DYNAMIC RAM MEMORIES TYPES OF RAM ---DYNAMIC RAM (CAPACITOR TYPE STRUCTURE) COMPARED WITH: ---STATIC RAM (FLIP-FLOP TYPE STRUCTURE)

  4. ADVANTAGES OF DYNAMIC RAM • 1. HIGHER DENSITY (APPROX. ¼ THE PHYSICAL SIZE OF STATIC RAM; USES LESS TRANSISTORS PER BIT: 1 OR 3. VS. 6) • 2. LOWER POWER CONSUMPTION (< .05 MILLIWATT/BIT FOR DYNAMIC RAM VS. .2 MILLIWATT/BIT FOR STATIC RAM, EXCEPT FOR CMOS STATIC RAM) • 3. LESS EXPENSIVE PER BIT THAN STATIC RAM. (HOWEVER, NEED FOR “REFRESH CIRCUITRY” ADDS TO COST)

  5. DISADVANTAGES OF DYNAMIC RAM • 1. POSSIBLE DISADVANTAGE: DYNAMIC RAM IS OFTEN ORGANIZED AS 2N X 1 • 2. NEED FOR REFRESH CYCLES (SIMILAR TO “DUMMY READS”) • ---COST • ---COMPLEXITY • ---MAY SLOW DOWN THE PROCESSOR SOMEWHAT (FROM 1% FOR A HARDWARE APPROACH TO 10% FOR A SOFTWARE APPROACH).

  6. Typical One-Transistor DRAM Cell

  7. TYPICAL % TIME REQUIRED FOR DRAM REFRESH CYCLES CONSIDER A 256K X 1 DRAM CHIP - 18 BIT ADDRESS INPUT - 9 BIT ROW ADDRESS (NO. OF ROW = 29 = 512) - 9 BIT COLUMN ADDRESS (NO. COLUMNS = 28 = 512) ASSUME A 2 MILLISEC MAXIMUM REFRESH INTERNAL ASSUME A 100 NANOSECOND MEMORY CYCLE TIME FOR READ, WRITE, OR REFRESH TOTAL TIME TO REFRESH ALL 512 ROWS: 512 X 100 X10-9 SEC = .0512 X 10-3 SEC THIS MUST BE DONE EVERY 2 MILLISECONDS

  8. ==> PERCENTAGE OF TIME THAT MUST BE USED FOR MEMORY REFRESH IS .0512 MILLISECONDS = 2.56 % 2 MILLISECONDS

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