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Chapter 16
Semiconductor, Magnetic and Optical Memory
William KleitzDigital Electronics with VHDL, Quartus® II Version
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Memory Concepts
• Memory locations have memory addresses
• Data are the memory contents
• 8 bits known as a byte
• See Figure 16-2 - Logic Diagram
• See Figure 16-3 - Timing Requirements
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Figure 16-2
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Figure 16-3
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Static RAMs
• Random-Access Memory
• Read/Write Memory
• Temporary storage of data
• User can access data at any location randomly
• CD player or Hard Disk
• Static or DynamicWilliam KleitzDigital Electronics with VHDL, Quartus® II Version
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Static RAMs
• Static– use flip-flops as basic storage elements
• Dynamic– use capacitors as basic storage elements– need additional refresh circuitry– can be densely packed– lower cost per bit
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Static RAMs
• The 2147H Static MOS RAM– 4096 memory locations
• 4K = 4 x 1024
– each location can contain 1 bit– 4096 unique addresses
• needs 212 = 4096 address lines
– A0 to A5 identify rows
– A6 to A11 identify columnsWilliam KleitzDigital Electronics with VHDL, Quartus® II Version
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Static RAMs
• The 2147H Static MOS RAM– row and column circuitry pinpoint the memory
cell– Row Select– Column Select– uses three-state buffers– See Figure 16-6
• read cycle
• write cycle
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Figure 16-6
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Static RAMs
• Memory Expansion– using multiple chips to get more memory capacity– See Figure 16-7 - eight 4K chips
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Figure 16-7
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Dynamic RAMs
• Require more support circuitry
• More difficult to use
• Less expensive per bit
• Higher density, minimizing circuit-board area
• Usually multiplex address lines
• Capacitor refreshed during refresh cycleWilliam KleitzDigital Electronics with VHDL, Quartus® II Version
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Dynamic RAMs
• Refresh cycle timing– usually every 2 ms or sooner
• Dynamic RAM Controllers– developed to simplify the tasks– Intel 3243– See Figure 16-12
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Figure 16-12
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Read-Only Memories• Store data on a permanent basis
• Nonvolatile
• EPROM– erasable-programmable-read-only memory
• Stores– operating systems– table look-ups– language compilers
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Read-Only Memories
• Mask ROMs– one-time fee to design a unique mask– very inexpensive after one-time fee
• Fusible-Link PROMs– avoid one-time fee– every memory cell has a fusible link– burned open to permanently store data– PROM programmer or MDS
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Read-Only Memories
• EPROMs– can change the memory contents– expose an open window to ultraviolet light– slowest erasure time
• EEPROMs– non-volatile– erased while still in circuit– individual bits erased
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Read-Only Memories• Flash Memory
– faster access times– erase entire blocks quickly– digital cameras and PDAs
• Floating-gate MOSFET used– charge remains on gate for 10 years
• OTP (one-time-programming)
• Timing requirements must be metWilliam KleitzDigital Electronics with VHDL, Quartus® II Version
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Read-Only Memories
• See Table 16-4– Summary of Semiconductor Memory
• See Figure 16-19– read cycle– write cycle
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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Figure 16-19
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Memory Expansion and Address Decoding Applications
• Address Decoding– to identify which IC is to be read or written to– See Figure 16-20
• 16K-byte EPROM (4 x 4K)
• A PROM Look-Up Table– See Application 16-1
• A Digital LCD Thermometer– See Application 16-2
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Figure 16-20
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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William KleitzDigital Electronics with VHDL, Quartus® II Version
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Figure 16-23
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Magnetic and Optical Storage
• Electro-mechanical in nature
• Non-volatile
• Magnetic– north-south or south-north polarities
• Optical– pits and lands read by a laser system
• Slower and bulkier but less expensive and higher storage capacities
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Magnetic and Optical Storage
• Magnetic Memory; The Floppy Disk and Hard Disk– magnetizable medium– rigid plastic jacket– Floppy
• 300 rpm• two read/write heads (one each side)• 1.44 MB• removable• transfer rates of 45KB/sec
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Magnetic and Optical Storage
• Magnetic Memory; The Floppy Disk and Hard Disk– Hard Disk
• not removable
• rigid platters
• sealed unit
• several two-sided platters
• one read/write head for each platter surface
• thousands of rpms
• Gigabytes of storage capacityWilliam KleitzDigital Electronics with VHDL, Quartus® II Version
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Magnetic and Optical Storage
• Magnetic Memory; The Floppy Disk and Hard Disk– Hard Disk
• controlled internal environment
• bits closely packed
• concentric circles called tracks (cylinders)
• 20,000 tracks per inch
• 300K bits per inch on each track
• transfer rates of 30 MB/secWilliam KleitzDigital Electronics with VHDL, Quartus® II Version
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Magnetic and Optical Storage
• Magnetic Memory; The Floppy Disk and Hard Disk– Removable Hard Disks
• Zip disk– 300 rpm
– 100 MB
• Jaz cartridge– two rigid platters
– 2 GB
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Magnetic and Optical Storage
• Optical Memory– CD
• not as fast as hard disks
• removable
• 650 MB
• aluminum alloy coating
• rigid polycarbonate wafer
• pits = 1 lands = 0
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Magnetic and Optical Storage
• Optical Memory– CD
• one track starting at center and spiraling outward
• 16,000 tracks per inch
• thin plastic coating to protect
• land reflects light, pit does not
– CD-R• photosensitive dye on reflective gold layer
• laser super heats spot and it will not reflect
• cannot be erased or re-writtenWilliam KleitzDigital Electronics with VHDL, Quartus® II Version
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Magnetic and Optical Storage
• Optical Memory– CD-RW
• silver alloy crystalline structure
• laser superheats to amorphous state (non-reflective)
• laser can reheat at lower level to turn back into crystalline state
• reflective and non-reflective areas
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Summary
• A simple 16-byte memory circuit can be constructed from 15\6 octal D flip-flops and a decoder. This circuit would have 16 memory locations (addresses) selectable by the decoder, with 1 byte (8 bits) of data at each location.
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Summary
• Static RAM (random-access memory) ICs are also called read/write memory. They are used for the temporary storage of data and program instructions in microprocessor-based systems.
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Summary
• A typical RAM IC is the 2114A. It is organized as 1K x 4, which means that it has 1K locations, with 4 bits of data at each location. (1K is actually an abbreviation for 1024.) An example of a higher-density RAM IC is the 6206, which is organized as 32K x 8.
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Summary
• Dynamic RAMs are less expensive per bit and have a much higher density than static RAMs. Their basic storage element is an internal capacitor at each memory cell. External circuitry is required to refresh the charge on all capacitors every 2 ms or less.
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Summary
• Dynamic RAMs generally multiplex their address bus. This mean that the high-order address bits share the same pins as the low-order address bits. They are demultiplexed by the RAS and CAS (Row Address Strobe and Column Address Strobe) control signals.
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Summary
• Read-only memory (ROM) is used to store data on a permanent basis. It is nonvolatile, which means that it does not lose its memory contents when power is removed.
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Summary
• Three common ROMs are (1) the mask ROM, which is programmed once by a masking process by the manufacturer; (2) the fusible-link programmable ROM (PROM), which is programmed once by the user; and (3) the erasable-programmable ROM (EPROM), which is programmable and UV-erasable by the user.
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Summary• Memory expansion in microprocessor
systems is accomplished by using octal or hexadecimal decoders as address decoders to select the appropriate memory IC.
• The Electrically-Erasable PROM (EEPROM) and Flash memory use a floating-gate MOSFET for their primary storage element. A charge on the floating gate represents the stored data.
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Summary• Magnetic storage like the floppy or hard
disk use magnetized particles to represent the stored 1 or 0. Individual data bits are read and written using an electro-magnetic read/write head.
• Optical memory like the CD or DVD use a laser beam to reflect light off of a rigid platter. The CD or DVD platter will either have a non-reflective pit to represent a 1 or a non-pit (land) to represent a 0.
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