Transcript Flip-Flops and Related Devices

Memory Devices
Wen-Hung Liao, Ph.D.
Introduction
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Main memory vs. auxiliary memory
Memory Terminology
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Memory cell: a device used to store a single bit (0 or
1). Examples: FF, charged capacitor, a single spot on a
magnetic disk or tape.
Memory word: a group of bits (cells) in a memory that
represents instructions or data of some type.
Byte: a special term used for a group of 8 bits.
Capacity: a way of specifying how many bits of data
can be stored in a particular memory device. Example:
4096 20-bit words = 4K x20
Density: another term for capacity. Often with
reference to space.
Memory Terminology(cont’d)
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Address: A number that identifies the location of a
word in memory. (Figure 11-2)
Read operation: the operation whereby the binary
word stored in a specific memory location is sensed
and then transferred to another device.
Write operation: the operation whereby a new word is
placed into a particular memory location.
Access time: amount of time required to perform a
read operation.
Volatile memory: any type of memory that requires the
application of electrical power in order to store
information.
Memory Terminology (cont’d)
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Random Access Memory (RAM): memory in which
the actual physical location of memory word has no
effect on how long it takes to read from or write into
that location.
Sequential Access Memory: a type of memory in
which the access time is not constant but varies
depending on the address location.
Read/Write Memory: any memory that can be read
from and written into with equal ease.
Read-Only Memory(ROM)
Memory Terminology (cont’d)
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Static memory devices: semiconductor memory
devices in which the stored data will remain
permanently stored as long as power is applied.
Dynamic memory devices: data need to be
periodically refreshed.
Main memory: also referred to as the computer’s
working memory.
Auxiliary memory: also referred to as mass storage.
Always nonvolatile.
General Memory Operation
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Select the address in memory that is being accessed
for a read or write operation.
Select either a read or a write operation to be
performed.
Supply the input data to be stored in memory during a
write operation.
Hold the output data coming from memory during a
read operation.
Enable (or disable) the memory so that it will (or will
not) respond to the address inputs and read/write
command.
Address Inputs
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N words  log2 N address inputs
The R/W’ input
Memory Enable: Chip Enable, Chip Select
CPU-Memory Connections
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Address Bus: a unidirectional bus that carries the binary outputs from the
CPU to the memory IC to select one memory location.
Data Bus: a bi-directional bus that carries data between the CPU and the
memory IC.
Control bus: carries control signals from the CPU to the memory IC.
Read-Only Memory
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Designed to hold data that either are
permanent or will not change frequently.
During normal operation, no data can be
written into a ROM, but data can be read from
ROM.
The process of entering data is called
programming or burning-in the ROM.
All ROMs are nonvolatile.
ROM Block Diagram
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Figure 11-6 shows a 16x8 ROM.
4 address inputs, 8 data outputs.
CS: Chip Select.
The Read operation.
ROM Architecture
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Figure 11-7: architecture of a 16x8 ROM.
Register array
Row decoder
Column decoder
Output buffers.
ROM Timing
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tACC: access time, time interval between the application of a
ROM’s input and the appearance of the data outputs during a
read operation.
tOE: output enable time, the delay between the CS input and the
valid data output.
Types of ROMs
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Mask-Programmed ROM: cannot be reprogrammed
(Figure 11-9).
Programmable ROMs (PROMs): Figure 11-11.
Erasable Programmable ROM (EPROM): use UV lights
to erase all cells at the same time. (15-20 minutes.)
Vpp: programming voltage. Figure 11-12.
Electrically Erasable PROM (EEPROM): Figure 11-13,
allows rapid in-circuit erasure and reprogramming of
individual bytes, suffer from low density and higher cost.
CD-ROM
Mask-Programmed ROM
Programmable ROMs
EPROM
EEPROM
Flash Memory
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Figure 11-14 shows the trade-offs for the
various semiconductor nonvolatile memories.
Flash Memory (cont’d)
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Flash memory aims to provide in-circuit
electrical erasability, high-speed access, high
density, low cost.
Erase mode: bulk erase, sector erase.
The 28F256A CMOS flash memory IC: Figure
11-15.
Figure 11-16: functional diagram of the
28F256A chip.
28F256A IC
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Read command, Set-up Erase/Erase command, Erase-verify
command, Set-up Program/Program command, Program-verify
command
ROM Applications
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Firmware
Bootstrap memory
Data tables
Data converter
Function generator
Auxiliary storage: flash memory.
Semiconductor RAM
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When the term RAM is used with
semiconductors memories, it is usually taken to
mean read/write memory as opposed to ROM.
RAM is used for temporary storage of
programs and data.
RAM is volatile.
Standby mode saves power.
RAM Architecture
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Consisting of a number of registers, each
storing a single data word, and each having a
unique address.
Read operation
Write operation
Chip Select
Common input/output pins
Internal Organization of a 64x4 RAM
Static RAM (SRAM)
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Stores data as long as power is applied.
Static RAM timing
Read cycle (Figure 11-22a)
Write cycle (Figure 11-22b)
Actual SRAM chip: MCM6264 CMOS 8Kx8
Read Cycle
Write Cycle
Dynamic RAM (DRAM)
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Needs to be refreshed every 2, 4,or 8 ms.
DRAM structures and operation (Figure 1125,26)
Address multiplexing
DRAM read cycle (Figure 11-30)
DRAM write cycle (Figure 11-31)
Cell Arrangement in a 16Kx1 DRAM
Dynamic Memory Cell
•WRITE operation: SW1,SW2 closed
•READ operation: all closed except SW1
Address Multiplexing
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16Kx1 DRAM is obsolete. (has 14 address
inputs)
4Mx1 DRAM would require 22 address lines.
To reduce the number of pins on high-capacity
DRAM, address multiplexing is utilized.
High-order bits  row address
Low-order bits column address
TMS44100 4Mx1 DRAM
RAS/CAS Timing
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Row address strobe/column address strobe
Address Bus
DRAM Read Cycle
DRAM Write Cycle
DRAM Refreshing
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DRAM chips are designed so that whenever a
read operation is performed on a cell, all of the
cells in that row will be refreshed.
Two refresh modes:
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Burst refresh: normal memory operation is
suspended, and each row of the DRAM is refreshed
in succession until all rows have been refreshed.
Distributed refresh: row refreshing in interspersed
with the normal operation.
Expanding Word Size and Capacity
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Expanding word size: connecting two 16x4
RAMs for a 16x8 module. (Figure 11-34)
Expanding capacity: connecting two 16x4 chips
for a 32x4 memory (Figure 11-36)
Expanding Word Size
Expanding Capacity
Special Memory Functions
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Power-down storage
Cache memory
First-in, First-out memory (linear buffers)
Circular buffers