CompE 460 Real-Time and Embedded Systems

CompE 460Real-Time and Embedded Systems

Lecture 5 – Memory Technologies

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Agenda

• Prayer/Thoughts• The CPU-Memory Interface• The Memory Subsystem and Technologies

• Volatile Memory• DRAM• SDRAM

SRAM• DDR RAM

• Non-Volatile Memory• ROM• EEPROM• Flash


Memory

• What are some different kinds of memory?• Volatile

• SRAM: Static Random Access Memory • DRAM: Dynamic Random Access Memory • SDRAM – Synchronous DRAM• DDR SDAM – Dual data rate ram• RDRAM – Rambus DRAM• VideoRAM – Dual Port RAM

• Non-Volatile• Read Only Memories

• ROM• PROM• EPROM• EEPROM

• Flash• MRAM – Magnetic RAM• FRAM – FerroElectric RAM• Phase Change Memory – Based on electrically induced phase change of

chalcogenide materials (has both crystaline and non-crystaline states)


Memory Cont

In a PC, what do you normally use?

• Volatile main memory

• Memory Modules

• Non-Volatile BIOS

•EEPROM/Flash


• Memory subsystems generally consist of multiple chips

• Both volatile and non-volatile• And/or multiple chips for each

• Depending on system, each chip provides few bytes (e.g., 1 4) per access (like in a PC system), or each provides all bytes for an access (like in most embedded systems)

• Bytes from multiple chips are accessed in parallel to fetch words

• Memory controller decodes/translates address and control signals

Memory Subsystem Components


Volatile Memory

• Forgets memory when power is turned off• Usually volatile is much faster than non-volatile memory• Many types of volatile memory – Usually comes in 2 flavors

• Asynchronous• SRAM: Static Random Access Memory

– upside: fast and no refresh required – downside: not so dense and not so cheap – often used for caches

• DRAM: Dynamic Random Access Memory – upside: very dense (1 transistor per bit) and inexpensive – downside: requires refresh and often not the fastest access times – often used for main memories

• Synchronous• SDRAM – Synchronous DRAM – refers to the fact that this DRAM is

tied to a common clock with the uproc, so all data is read/write on a particular clock edge (either rising or falling)

• DDRAM – Dual data rate ram – data is read/written on each clock edge (both rising and falling)


Asynch Mem - Static versus Dynamic RAM

• What is a D flip-flop?• Static RAM uses Flipflops for each storage

element

• What is a capacitor• Dynamic Ram uses capacitors for each storage

elementD Q

CLK


Storage Basics

• RAM chips don't store whole bytes, but rather they store individual bits in a grid, which you can address one bit at a time


CPU Async Memory Interface

• CPU Asynch Memory Interface usually consists of: • uni directional address bus • bi directional data bus • read control line • write control line • ready control line • size (byte, word) control line

• Memory access involves a memory bus transaction • read:

(1) set address, read and size, (2) copy data when ready is set by memory

• write:(1) set address, data, write and size, (2) done when ready is set

address bus

data bus

CPU MemoryReadWriteReadysize


Storage Basics Cont

• Row and Column decoders are simple 1 of X decoders (from CompE 224)

• Example: • contains 8 16x1 bit

decoders

address bus

data bus

CPU MemoryReadWriteReadySize

1-of-16decoder

1 0 1 1 0 0 1 01 0 0 0 0 0 0 1

0 1 0 1 0 0 1 1

address

00000001

1111

16x8-bit memory array

D7 D6 D5 D4 D3 D2 D1 D0


SRAM Organization and Operations

(a) Address lines/decodersto select a row and a column

(b) Chip Select(c) Read/Write enable(d) Data in/out


SRAM Memory Timing for Read

Accesses • Address and chip select signals are provided tAA before data

is available

• Outputs reflect new data

2147H2147H High-Speed 4096x1-bit static RAM

A11-A0

DinWE CS

Dout

tRC = Read cycle time tAA = Address access time tACS = Chip select access time tHZ = Chip deselections to high Z out

old address

highimpedance

undef Data Valid

tRC

tAA

tACS

tHz

new addressAddressA11-A0

CS

WE

Dout

Address Bus


SRAM Memory Timing for Write

Accesses • Address and data must be stable tS time-units

before write enable signal falls

2147H2147H High-Speed 4096X1-bit static RAM

A11-A0

DinWE CS

Din

tS = Signal setup timetRC = Read cycle time tAA = Address access time tACS = Chip select access time tHZ = Chip deselections to high Z out

old address

old data new data

tWC

tAA

tACS

tHz

new addressAddressA11-A0

CS

WE

Din

tS

Address Bus


DRAM Organization and Operations

(a) Address lines/decodersto select a row and a column

(b) Chip Select (not shown)(c) Read/Write enable(d) Data in/out(e) Refresh counters


DRAM Memory Access

• DRAM Memory is arranged in a XY grid pattern of rows and columns (like SRAM)

• First, the row address is sent to the memory chip and latched, then the column address is sent in a similar fashion

• This row and column-addressing scheme (called multiplexing) allows a large memory address to use fewer pins

• BF533 uses this for DRAM memory maps

• The charge stored in the chosen memory cell is amplified using the sense amplifier and then routed to the output pin

• Read/Write is controlled using the read/write logic


How DRAM Works


Non-Volatile Memory

• ROM – Read Only Memory – come from factory programmed

• PROM – Programmable ROM – one time use – programmed with programmer

• EPROM – Erasable PROM – can use multiple times – need special eraser to reuse them

• EEPROM – Electrically erasable PROM – Typically slow reading/writing – Can be used “in circuit” – generally small memory footprints

• Flash – Have to write a block of data – writing is slow, reading fast – will wear out after ~10k to 100k writes


Flash Memory


Flash Technology - Example

• This is an 8 Mbit, 3.0 volt-only Flash memory organized as 1,048,576 bytes or 524,288 words.


Flash Technology

• Two cautions for flash memory• Flash is usually spec’d as Mb not MB• Reading from flash is same as RAM• Writing to flash requires a sector write

Backup


DRAM Performance Specs

• Important DRAM Performance Considerations • Random access time: time required to read any random

single cell • Fast Page Cycle time: time required for page mode access

read/write to memory location on the most recently accessed page (no need to repeat RAS in this case)

• Extended Data Out (EDO): allows setup of next address while current data access is maintained

• SDRAM Burst Mode: Synchronous DRAMs use a self incrementing counter and a mode register to determine the column address sequence after the first memory location accessed on a page effective for applications that usually require streams of data from one or more pages on the DRAM

• Required refresh rate: minimum rate of refreshes

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CompE 460 Real-Time and Embedded Systems