Memory Chips and Memory Interfacing (Chapter 7)

• Two types of memory:
  • Main memory:
    • Semiconductor
    • Mostly volatile
  • Secondary memory:
    • Magnetic; Opticalfloppy, hard disk; CD-ROM
    • Permanent

Main Memory Technologies

• PROM, SRAM, DRAM, & flash memory

• The ideal memory chip:4 attributes:
  • Access time: “time to R/W data to the chip” (0 ns)
  • Non-volatile
  • Very dense
  • Low power consumption:
    • Long battery life, small power supply

Comparing Memory Technologies

ROM Types

• Mask programmable ROMfig. 7.1: n-inputs, m-outputs. (2**n x m)
  • Three parts:
    • Address decoder
    • Programmable memory array
    • Output buffer

• Field programmable ROM:fig. 7.2: blown fuse => programs “1”
  • Fusible link: one time programmable (OTP)

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ROM Types (Cont.)

• UV-light erasable PROM (EPROM)fig. 7.3 (a)
  • More expensive w.R.To PROMquartz-window package
  • Programming: trap electric charge
  • Storage period: about 20 years
  • Blank EPROM: storing “1’s”
  • Ultra-violet light bulk erase
  • Fig. 7.4: EPROM programmer

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ROM Types (Cont.)

• Electrically erasable PROM (EEPROM)fig. 7.3 (b)

• Advantages w.R.To UV EPROM:
  • No need to remove from PCB to program
  • Byte erasure possible (bulk erasure also.)
  • Cheaper package (no quartz window)

• 10,000 to 1000,000 R/W cycles

• Slow writing time (25 ms typical)

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ROM Types (Cont.)Flash Memory

• Similar to EEPROM: in circuit write

• No byte erasure: block & bulk erasure

• Cheaper than EEPROM; Simpler cell:
  • More reliable
  • More density; Example:Intel’s 28F020: 256K bytes = 2 M bits

PCMCIA Credit-card-size Flash-memory Devices

• Personal computer memory card international association

• Silicon hard disks:40 MB non-volatile memory

• Examples: fig. 7.5:modemEthernet adapter

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The Intel 28F020 Flash Memory

• Fig. 7.6 (data sheet):
  • Typical erase time: 2s (bulk erasure only)
  • Single byte write time = 10 micro-second
  • 4 seconds to write to all 256 bytes
  • 100,000 erase/program cycles
  • Read access time = 70 ns
  • Vcc = 5.0V and Vpp = 12.0V (programming)
  • Icc = 10 ma (similar DRAMS: 65-90 ma)
  • Programming: table 7.1

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Intel’s 28F400B (W/ Boot Block)

• Fig. 7.7 (data sheet):
  • 256K x 16-bits or 512K x 8-bits
  • Seven memory blocks:
    • 1 16K boot block can be protected [RP]
    • 2 16K 8K parameter blocks
    • 1 96K main block
    • 3 384K 128K main blocks
    • Sum=512k

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Applications for Flash Memory

• BIOS: easy upgrade, even remotely

• Network I/O address & interrupt number

• Laser printers fonts

• Automotive diagnostics

• Robotics (stored path for robot arm)

• Space applications

• Smart cards, digital cameras

• USB memory cards (up to 1 GB!)

Recent!

Static RAM (SRAM)

• Basic cell: flip-flop (fig. 7.8)

• 6-transistor:
  • Q1, Q2: cross-coupled flip flop
  • Q3, Q4: act as pull-up resistors
  • Q5, Q6: switches

• Applications:
  • Cache memory
  • E.G. 7C1024-15 SRAM: 128K, 15 ns

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Dynamic RAM (DRAM)

• Storage cell: capacitor and transistor (fig. 7.9)

• High density (w.R.To SRAM)

• Requires refresh (once every 1 to 16 ms)

• Simms: single in-line memory modulesfig. 7.11 (30- and 72-pin)

• DIMMs: dual in-line memory modules168-pins. Fig. 7.14, 7.15

• RIMMs: Rambus in-line memory modules16 bits @ 400 MHz serial data, fig. 7-16

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Memory OrganizationChip Organization

• 16 Mb memory:
  • 16M x 1 (16 M bits)
  • 4M x 4 (4 M nibbles)
  • 2M x 8 (2M Bytes)
  • 1M x 16 (1M words: 8086)

• SRAM and ROM: Byte-wide, ... x 8

• The Micron MT46V8M8 SDRAM: Fig. 7.10
  • 2M x 8 x 4 (banks): 4 banks of 2MB each

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Example 7.1

• Using 64K x 8 SRAM, find the minimum # of chips for each processor and the memory size

• Processor data bus min#chips cap.8088 8-bits 1 64K 8086 16-bits 2 128K

• 80386 32-bits 4 256K80486 32-bits 4 256K

• Pentium 64-bits 8 512K

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Memory Module Organization

• Locations x width=> table 7-4
  • FPM: fast page mode
  • EDO: extended data out

• 9-bits: error detection/correction:two schemes: parity & error check code (ECC) => useful for servers => increases mean time between failure (MTBF) => better reliability!

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Memory Access Time

• DRAM: 60 ns => memory access timeolder memory chips: > 150 ns access time => O.K. With 5 MHz processor clock

• SDRAM: PC100 => 100 MHz external sys busPC133 => 133 MHz external system bus

• Ram-bus: 400, 600 MHz  => serial channel, 16-bits wide!

Memory Part Numbers

• WWW XX YYYY-ZZ: (table 7.5)
  • WWW: manufacturer code
  • XX: type of memory (SRAM, DRAM, EPROM…)
  • YYYY: total bit storage
  • ZZ: access time (ns)

• E.G. MC27256-10: Motorola semiconductor:
  • 27=EPROM, 256K bits, 100ns (EPROM: slow)
  • XX=41 or 51: DRAM; =62 or 7C: SRAM ?!

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