Memory Types

• Two basic types:
• ROM : Read-only memory
• RAM : Read-Write memory

 

• Four commonly used memories:
• ROM
• Flash (EEPROM)
• Static RAM (SRAM)
• Dynamic RAM (DRAM)

 

• Generic pin configuration:

Memory Chips

• The number of address pins is related to the number of memory locations .
• Common sizes today are 1K to 256M locations.
• Therefore, between 10 and 28 address pins are present.

 

• The data pins are typically bi-directional in read-write memories.
• The number of data pins is related to the size of the memory location .
• For example, an 8-bit wide (byte-wide) memory device has 8 data pins.
• Catalog listing of 1K X 8 indicate a byte addressable 8K memory.

 

• Each memory device has at least one chip select ( CS ) or chip enable ( CE ) or select ( S ) pin that enables the memory device.
• This enables read and/or write operations.
• If more than one are present, then all must be 0 in order to perform a read or write.

Memory Chips

• Each memory device has at least one control pin.
• For ROMs, an output enable ( OE ) or gate ( G ) is present.
• The OE pin enables and disables a set of tristate buffers.
• For RAMs, a read-write ( R/W ) or write enable ( WE ) and read enable (OE ) are present.
• For dual control pin devices, it must be hold true that both are not 0 at the same time.

 

• ROM:
• Non-volatile memory: Maintains its state when powered down.
• There are several forms:
• ROM : Factory programmed, cannot be changed. Older style.
• PROM : Programmable Read-Only Memory.
• Field programmable but only once. Older style.
• EPROM : Erasable Programmable Read-Only Memory.
• Reprogramming requires up to 20 minutes of high-intensity UV light exposure.

Memory Chips

• ROMs (cont):
• Flash EEPROM : Electrically Erasable Programmable ROM.
• Also called EAROM (Electrically Alterable ROM) and NOVRAM (NOn-Volatile RAM).
• Writing is much slower than a normal RAM.

 

• Used to store setup information, e.g. video card, on computer systems.
• Can be used to replace EPROM for BIOS memory.

EPROMs

• Intel 2716 EPROM (2K X 8):

EPROMs

• 2716 Timing diagram:

• Sample of the data sheet for the 2716 A.C. Characteristics.

Symbol	Parameter	Limits			Unit	Test Condition
		Min	Typ.	Max
tACC1	Addr. to Output Delay		250	450	ns	PD/PGM= CS =VIL
tOH	Addr. to Output Hold	0			ns	PD/PGM= CS =VIL
tDF	Chip Deselect to Output Float	0		100	ns	PD/PGM=VIL
...	...	...	...	...	...	...

• This EPROM requires a wait state for use with the 8086 ( 460ns constraint).

SRAMs

• TI TMS 4016 SRAM (2K X 8):

• Virtually identical to the EPROM with respect to the pinout.
• However, access time is faster (250ns).
• See the timing diagrams and data sheets in text.
• SRAMs used for caches have access times as low as 10ns .

DRAMs

• DRAM:
• SRAMs are limited in size (up to about 128K X 8).
• DRAMs are available in much larger sizes, e.g., 64M X 1.

 

• DRAMs MUST be refreshed (rewritten) every 2 to 4 ms
• Since they store their value on an integrated capacitor that loses charge over time.

 

• This refresh is performed by a special circuit in the DRAM which refreshes the entire memory using 256 reads.
• Refresh also occurs on a normal read, write or during a special refresh cycle.
• More on this later.

 

• The large storage capacity of DRAMs make it impractical to add the required number of address pins.
• Instead, the address pins are multiplexed .

DRAMs

• TI TMS4464 DRAM (64K X 4):

• The TMS4464 can store a total of 256K bits of data.

 

• It has 64K addressable locations which means it needs 16 address inputs, but it has only 8 .
• The row address (A ₀ through A ₇ ) are placed on the address pins and strobed into a set of internal latches.
• The column addres (A ₈ through A ₁₅ ) is then strobed in using CAS.

DRAMs

• TI TMS4464 DRAM (64K X 4) Timing Diagram:

• CAS also performs the function of the chip select input.

DRAMs

• Larger DRAMs are available which are organized as 1M X 1 , 4M X 1 , 16M X 1 , 64M X 1 (with 256M X 1 available soon).
• DRAMs are typically placed on SIMM (Single In-line Memory Modules) boards.
• 30-pin SIMMs come in 1M X 8 , 1M X 9 (parity), 4M X 8 , 4M X 9 .
• 72-pin SIMMs come in 1 / 2 / 3 / 8 / 16M X 32 or 1M X 36 (parity).

DRAMs

• Pentiums have a 64-bit wide data bus.
• The 30-pin and 72-pin SIMMs are not used on these systems.
• Rather, 64-bit DIMMs ( Dual In-line Memory Modules) are the standard.
• These organize the memory 64-bits wide.
• The board has DRAMs mounted on both sides and is 168 pins.

 

• Sizes include 2M X 64 ( 16M ), 4M X 64 ( 32M ), 8M X 64 ( 64M ) and 16M X 64 ( 128M ).

 

• The DIMM module is available in DRAM , EDO and SDRAM (and NVRAM ) with and without an EPROM.

 

• The EPROM provides information abou the size and speed of the memory device for PNP applications.