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COMP541

Memories - I

Montek Singh

Oct {8, 15}, 2014

Topics Lab 8

Briefly discuss RAM specification in Verilog

Overview of Memory TypesRead-Only Memory (ROM): PROMs, FLASH, etc.Random-Access Memory (RAM)

Static todayDynamic next

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Verilog for RAM (Lab 8)module ram_module #( parameter Abits = 4, // Number of bits in address parameter Dbits = 4, // Number of bits in data parameter Nloc = 16 // Number of memory locations)( input clock, input wr, // WriteEnable: if wr==1, data is written into mem input [Abits-1 : 0] addr, // Address for specifying location input [Dbits-1 : 0] din, // Data for writing (if wr==1) output [Dbits-1 : 0] dout // Data read from memory (all the time) );

reg [Dbits-1 : 0] mem [Nloc-1 : 0];// The actual registers where data is stored// Memory write: only when wr==1 and clock tick

always @(posedge clock) if(wr) mem[addr] <= din;

assign dout = mem[addr]; // Memory read all the time, no clock involved

endmodule

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Types of Memory Many dimensions

Read Only vs. Read/Write (or write seldom)Volatile vs. Non-VolatileRequires refresh or not

Look at ROM first to examine interface

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Non-Volatile Memory Technologies Mask (old) ROM

read-only memory Fuses (old) PROM

programmable read-only memory Erasable EPROM

erasable programmable read-only memory Electrically erasable EEPROM

electrically-erasable programmable read-only memory today called FLASH!

used everywhere!

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Details of ROM Memory that is permanent

k address lines2k itemsn bits

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Notional View of Internals

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Programmed Truth Table

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Resulting Programming

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Mask ROMs Oldest technology Originally “mask” used as last step in

manufacturingSpecify metal layer (connections)Used for volume applicationsLong turnaroundUsed for applications such as embedded systems and,

in the old days, boot ROM

but cheap to mass produce!

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Programmable ROM (PROM) Early ones had fusible links

High voltage would blow out linksFast to programSingle use

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UV EPROM Erasable PROM

Common technologies used UV light to erase complete device

Took about 10 minutesHolds state as charge in very well insulated areas of

the chipNonvolatile for several (10?) years

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EEPROM Electrically Erasable PROM

Similar technology to UV EPROMErased in blocks by higher voltageProgramming is slower than reading

Today’s flavor is called “flash memory”Digital cameras, MP3 players, BIOSLimited lifeSome support individual word write, some block

Our boards have it:A flash memory chip on our Nexys boardsHas a “boot block” that is carefully protectedWe will learn to use it in upcoming labs

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How Flash Works Special transistor with floating gate This is part of device surrounded by insulation

So charge placed there can stay for yearsAside: some newer devices store multiple bits of info

in a cell

Interested in this? If so, we can cover in more detail w/ transistors

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Read/Write Memories Flash is obviously writeable

But not meant to be written rapidly (say at CPU rates)And often writing must be by entire blocks (disk

replacement)

For frequent writing, use RAM

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Random Access Memories So called because it takes same amount of

time to address any particular locationNot entirely true for modern DRAMs, but somewhat

true…

First look at asynchronous static RAM reading and writing typically controlled by

“handshakes”clock may still be present, but actions controlled by

handshake signals

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Simple View of RAM Typical parameters:

some word size nsome capacity 2k

k bits of address line

Need a line to specify reading or writing typically only one wire needed

sometimes two separate ones

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Example: 1K x 16 memory RAM comes in variety of

sizes from 1-bit widemain issue is no. of pins

available on chip

Memory size often specified in bytesThis would be 2KB memory10 address lines (=1K

locations)16 data lines (=2

bytes/location)

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Writing Sequence of steps

Set up address linesSet up data linesActivate write line (e.g., maybe a positive edge)

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Reading Steps

Setup address linesActivate read lineData available soon

for asynchronous memory: after simply a specified amount of time

for synchronous memory: after a clock edge

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Chip Select Enable:

Usually a line to enable the chipWhy?

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Timing: Writing

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Timing: Reading

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Static vs. Dynamic RAM Different internal implementations: SRAM vs.

DRAMDRAM:

DRAM stores charge in capacitorDisappears after short period of timeMust be refreshedSmall sizeHigher storage density larger capacities

SRAM:SRAM easier to useUses transistors (think of it as latch)FasterMore expensive per bitSmaller sizes

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Structure of SRAM Internally, each bit stored in a “latch”

One memory cell per bitCell consists of a few transistorsNot really a latch made of NANDs/NORs, but logically

equivalentBehaves like an SR latch

Control logicalso need extra logic around the latch to make it work like

a memory cell

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Structure of SRAM Several optimized circuits often used

replace a full-fledged SR latch with something simpler, smaller, faster…Not really a latch made of NANDs/NORs, but logically

equivalentBehaves like an SR latch

e.g., a simpler 6-transistor memory cellwordline Select(bitline, bitline’) (B, B’) as well as (C, C’)

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wordline

bitline bitline

Example: A Simple Organization Note:

In reality, more complexOnly one word-line is “on” at a time

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wordline311

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2:4Decoder

Address

01

00

storedbit = 0

wordline2

wordline1

wordline0

storedbit = 1

storedbit = 0

storedbit = 1

storedbit = 0

storedbit = 0

storedbit = 1

storedbit = 1

storedbit = 0

storedbit = 0

storedbit = 1

storedbit = 1

bitline2 bitline1 bitline0

Data2 Data1 Data0

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Zoom in: A single bit slice Operation:

Cells connected to form 1 bit position (column)

Word Select enables one latch from address lines

only this cell is writable only this cell is read

B (and B’) set by: Read/Write’ Data In Bit Select

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Let’s look at a single bit cell

stored bit

wordline

bitline

Example:

stored bit = 0

wordline = 1

stored bit = 1

stored bit = 0

stored bit = 1

bitline =

(a) (b)

wordline = 1

wordline = 0

wordline = 0

bitline =

bitline =

bitline =0

1

Z

Z

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Bit Slices and Modules Entire column of cells

called a bit slicebasically a 1-bit wide

memory! Module

module refers to a single chip of memory

1-bit wide memory chips are quite common!

Inside an SRAM Bit Cell Actual implementation does not use a real SR

latch!a tinier approximation is used logically behaves very much like an SR latchbut much smaller and faster!

stored bit

wordline

bitline

wordline

bitline bitline

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16 X 1 RAM “Chip”

Now shows address decoder selects

appropriate location

Row/Column Layout For larger RAMs:

decoder becomes pretty bigalso run into chip layout issues

Typically: larger memories use “2D” matrix layoutsee next slide

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16 X 1 RAM as 4 X 4 Array

Two decodersRowColumn

Address just broken up

Not visible from outside on SRAMs

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Not the same as 8 X 2 RAM! Minor change in

logic and pinsSpot the difference!

Spot the difference!

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Realistic Sizes Example: 256Kb memory organized 32K X 8

Single-column layout would need 15-bit decoder with 32K outputs!

Better organization:A 2D (i.e., square) layout with:

9-bit row and 6-bit column decoders

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SRAM Performance Latency and Throughput important

Current ones have cycle times in low nanosecondssay 1-2ns (top-end ones even lower)

Used as cache (typically on-chip or off-chip secondary cache)

Sizes up to 8Mbit or so for fast chipsExpensive ones can go a bit bigger

Energy/powerSRAMs also better for low power vs. DRAMs

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Wider Memory What if you don’t have enough bit width?

use multiple chips and side-by-side

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Larger/Wider Memories Made up from sets of

chips Consider a 64K by 8

RAMour building block

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Larger Let’s build a larger

memory256K X 8Decoder for high-order 2

bitsSelects chipLook at selection logicAddress ranges

Tri-state outputs

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Summary Today we looked at:

Quick look at non-volatile memoryStatic RAM

Next topic:Dynamic RAM

Complex, largest, cheapMuch more design effort to use

Talk about memories for lab

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