ENGR ECE 212 11

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ENGR / ECE 21211 14 April, 2003

• All slides except those with blue backgrounds
• are taken from John Wakerlys
• Stanford EE 121, Digital Design Laboratory

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Read-Only Memories

• Program storage
• Boot ROM for personal computers
• Complete application storage for embedded systems.

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Two-dimensional decoding
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Larger example, 32Kx8 ROM
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ROM control and I/O signals
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ROM timing
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Programmable ROMs

• PROM
• Programmable ROM
• EPROM
• Erasable PROM
• Erased by exposing to UV light
• EEPROM
• Electrically EPROM
• Data writes are slow and limited

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Read/Write Memories

• a.k.a. RAM (Random Access Memory)
• Volatility
• Most RAMs lose their memory when power is removed
• NVRAM RAM battery
• Or use EEPROM
• SRAM (Static RAM)
• Memory behaves like latches or flip-flops
• DRAM (Dynamic Memory)
• Memory lasts only for a few milliseconds
• Must refresh locations by reading or writing

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SRAM
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SRAM operation

• Individual bits are D latches, not
  edge-triggered D flip-flops.
• Fewer transistors per cell.
• Implications for write operations
• Address must be stable before writing cell.
• Data must be stable before ending a write.

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SRAM control lines

• Chip select
• Output enable
• Write enable

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SRAM read timing

• Similar to ROM read timing

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SRAM write timing

• Address must be stable before and after
  write-enable is asserted.
• Data is latched on trailing edge of (WE CS).

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Bidirectional data in and out pins

• Use the same data pins for reads and writes
• Especially common on wide devices
• Makes sense when used with microprocessor buses
  (also bidirectional)

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Synchronous SRAMs

• Use latch-type SRAM cells internally
• Put registers in front of address and control
  (and maybe data) for easier interfacing with
  synchronous systems at high speeds
• E.g., Pentium cache RAMs

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Some real life examples

• 64-bit (16?4) RAM
• Intels specification for 133 Mhz SDRAM
• IBM SRAM
• 64k bit EEPROM

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Memory today

• Dells Learn More
• DDR SDRAM
• Double Data Rate Synchronous DRAM
• Why PC2100?
• 8 bytes x 266M/sec 2,128MB/sec
• Registered, Buffered, ECC,
• RDRAM
• Rambus DRAM
• Why 1066
• Runs at 1066 Mhz
• With 16, 32, or 64 bits per clock cycle

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With it in April 2003

• Flash for consumer electronics
• Cameras, cell phones, sneaker-net
• Examples include
• Compact Flash
• PCMCIA card format
• SmartMedia
• Once SSFCD (Solid State Floppy Disk Card)
• Memory Stick
• MultiMediaCard
• Secure Digital Card
• XD Picture Card
• Olympus and Fuji

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How Stuff Works

• ROM
• RAM
• Flash memory
• Computer memory