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Understanding Semiconductor Memory Types

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Learn about different types of semiconductor memory including SRAM, DRAM, EPROM, ROM, and more. Explore memory organization, design, and advancements in DRAM technology.

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Understanding Semiconductor Memory Types

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  1. Chapter 5Internal Memory

  2. Semiconductor Memory Types Today’s technology: 1 Gigabit / sq in In R&D: 100 Gigabits / sq in

  3. Semiconductor Memory (SRAM)

  4. Semiconductor Memory (DRAM) 16Mbit DRAM

  5. Semiconductor memory (EPROM)

  6. Static RAM (SRAM) • Desired for main memory • Used in cache • Basically an array of flip-flops • Simple to interface and control • Fast • Relatively low density - complex • Relatively expensive

  7. Static RAM model

  8. 1K X 4 SRAM (Part Number 2114N)

  9. 1K X 4 SRAM (Part Number 2114N)

  10. 1K X 4 SRAM (Part Number 2114N)

  11. Memory Organization • A 16Mbit chip can be organised as 1M of 16 bit words • A bit per chip system has 16 lots of 1Mbit chip with bit 1 of each word in chip 1 and so on • A 16Mbit chip can be organised as a 2048 x 2048 x 4bit array • Reduces number of address pins • Multiplex row address and column address • 11 pins to address (211=2048) • Adding one more pin doubles range of values so x4 capacity

  12. Memory Design – 1K x 4 A[00:09]    D[03:00] Addr Block Select 

  13. Memory Design – 1K x 8 D[07:04] D[03:00] A[00:09]  A[00:09]    D[07:04]   D[03:00] Addr BlockSelect => Addr Block Select =>

  14. Memory Design - 2k x 8 D[07:04] D[03:00] Block 00 Block 01

  15. Memory Design - 4k x 8 D[07:04] D[03:00] Block 00 Block 01 Block 10 Block 11

  16. 22 x 3 Memory word select word WE input bits address write enable address decoder output bits

  17. 2 BIT Decoder (2 to 4)

  18. 2 to 4 Bit Decoder

  19. 3 to 8 Bit Decoder

  20. 2 to 1 MUX

  21. 4 to 1 MUX

  22. 8 to 1 MUX

  23. 16 to 1 MUX ?

  24. Register

  25. 22 x 3 Memory word select word WE input bits address write enable address decoder output bits

  26. 24 x 8 Memory ?

  27. Dynamic RAM (DRAM) • Used in main memory • Bits stored as charge in capacitors Essentially analog device Charges leak • Need refreshing even when powered Need refresh circuits • Higher density (more bits per chip) • Slower than Static RAM • Less expensive

  28. Dynamic RAM model

  29. Read Only Memory (ROM) • Permanent storage • Nonvolatile • Microprogramming (will address later) • Library subroutines • Systems programs (BIOS) • Function tables • Controllers

  30. Types of ROM • ROM: Written during manufacture • Very expensive for small runs • PROM: Programmable (once) • Needs special equipment to program • Read “mostly” • EPROM: Erasable Programmable • Erased by UV (All of chip!) • Flash memory • Whole blocks of memory stored/changed electrically • EEPROM: Electrically Erasable • Takes much longer to write than read (lower density)

  31. EPROM

  32. Semiconductor Memory 16Mbit DRAM

  33. 256kByte Module Organisation (256K x 1)

  34. Typical 16 Mb DRAM (4M x 4)

  35. 1MByte Module Organization (1Meg x 8 bits)

  36. Refreshing • Refresh circuit is included on the chip • Count through rows • Read & Write back • Chip must be disabled during refresh • Takes time • Occurs asynchronously • Slows down apparent performance

  37. Improvements in memory RAM – continually gets denser. DRAM – Several improvements: SDRAM – synchronous DRAM DDR-SDRAM - doubles transfer speed RDRAM – asynchronous one transfer per clock cycle

  38. Comparison of improved DRAM Conventional DRAM – 40 to 100 MB/S transfer rate?

  39. Synchronous DRAM (SDRAM) • Access is synchronized with an external clock • Address is presented to RAM • RAM finds data (CPU waits in conventional DRAM) • Since SDRAM moves data in time with system clock, CPU knows when data will be ready • CPU does not have to wait, it can do something else • Burst mode allows SDRAM to set up stream of data and fire it out in block • DDR-SDRAM sends data twice per clock cycle (leading & trailing edge)

  40. SDRAM Read Timing

  41. SDRAM

  42. DDR SDRAM • SDRAM can only send data once per clock • Double-data-rate SDRAM can send data twice per clock cycle • Rising edge and falling edge

  43. RAMBUS • Adopted by Intel for Pentium & Itanium • Main competitor to SDRAM • Separate bus (hence the name RAMBUS) • maximum 12 centimeter length bus ! • Bus addresses up to 320 RDRAM chips • at 1.6Gbps • Asynchronous block protocol • Precise control signal timing • 480ns access time

  44. RAMBUS Diagram

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