Download - EE121 John Wakerly Lecture #14
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EE121 John Wakerly Lecture #14
Read-only memories (review)
Static read/write memories
Dynamic read/write memories
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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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Typical commercial EEPROMs
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Microprocessor EPROM application
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ROM control and I/O signals
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ROM timing
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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 array
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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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SRAM devices
• Similar to ROM packages
28-pin DIPs 32-pin DIPs
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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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DRAM (Dynamic RAMs)
• SRAMs typically use six transistors per bit of storage.
• DRAMs use only onetransistor per bit:
• 1/0 = capacitorcharged/discharged
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DRAM read operations
– Precharge bit line to VDD/2.
– Take the word line HIGH.– Detect whether current flows into or out of the cell.– Note: cell contents are destroyed by the read!– Must write the bit value back after reading.
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DRAM write operations
– Take the word line HIGH.– Set the bit line LOW or HIGH to store 0 or 1.– Take the word line LOW.
– Note: The stored charge for a 1 will eventually leak off.
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DRAM charge leakage
• Typical devices require each cell to be refreshed once every 4 to 64 mS.
• During “suspended” operation, notebook computers use power mainly for DRAM refresh.
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DRAM-chip internal organization
64K x 1DRAM
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RAS/CAS operation
• Row Address Strobe, Column Address Strobe– n address bits are provided in two steps using n/2
pins, referenced to the falling edges of RAS_L and CAS_L
– Traditional method of DRAM operation for 20 years.– Now being supplanted by synchronous, clocked
interfaces in SDRAM (synchronous DRAM).
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DRAM read timing
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DRAM refresh timing
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DRAM write timing
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Next Time
• CPLDs• FPGAs