COMP541
Memories - I
Montek Singh
Oct {8, 15}, 2014
1
Topics
 Lab 8
 Briefly discuss RAM specification in Verilog
 Overview of Memory Types
 Read-Only Memory (ROM): PROMs, FLASH, etc.
 Random-Access Memory (RAM)
 Static today
 Dynamic next
2
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-Volatile
 Requires refresh or not
 Look at ROM first to examine interface
4
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!
5
Details of ROM
 Memory that is permanent
 k address lines
 2k items
 n 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 manufacturing
 Specify metal layer (connections)
 Used for volume applications
 Long turnaround
 Used 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 links
 Fast to program
 Single use
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UV EPROM
 Erasable PROM
 Common technologies used UV light to erase complete device
 Took about 10 minutes
 Holds state as charge in very well insulated areas of the chip
 Nonvolatile for several (10?) years
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EEPROM
 Electrically Erasable PROM
 Similar technology to UV EPROM
 Erased in blocks by higher voltage
 Programming is slower than reading
 Today’s flavor is called “flash memory”
 Digital cameras, MP3 players, BIOS
 Limited life
 Some support individual word write, some block
 Our boards have it:
 A flash memory chip on our Nexys boards
 Has a “boot block” that is carefully protected
 We will learn to use it in upcoming labs
1
How Flash Works
 Special transistor with floating gate
 This is part of device surrounded by insulation
 So charge placed there can stay for years
 Aside: 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
1
Random Access Memories
 So called because it takes same amount of time to
address any particular location
 Not 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 n
 some 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 wide
 main issue is no. of pins available
on chip
 Memory size often specified in
bytes
 This would be 2KB memory
 10 address lines (=1K locations)
 16 data lines (=2 bytes/location)
1
Writing
 Sequence of steps
 Set up address lines
 Set up data lines
 Activate write line (e.g., maybe a positive edge)
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Reading
 Steps
 Setup address lines
 Activate read line
 Data 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 chip
 Why?
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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. DRAM
 DRAM:
 DRAM stores charge in capacitor
 Disappears after short period of time
 Must be refreshed
 Small size
 Higher storage density  larger capacities
 SRAM:
 SRAM easier to use
 Uses transistors (think of it as latch)
 Faster
 More expensive per bit
 Smaller sizes
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Structure of SRAM
 Internally, each bit stored in a “latch”
 One memory cell per bit
 Cell consists of a few transistors
 Not really a latch made of NANDs/NORs, but logically equivalent
 Behaves like an SR latch
 Control logic
 also 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 equivalent
 Behaves like an SR latch
 e.g., a simpler 6-transistor memory cell
 wordline  Select
 (bitline, bitline’)  (B, B’) as well as (C, C’)
bitline
bitline
wordline
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Example: A Simple Organization
 Note:
 In reality, more complex
 Only one word-line is “on” at a time
2:4
Decoder
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Address
wordline3
2
10
01
00
bitline2
wordline2
wordline1
wordline0
bitline1
stored
bit = 0
stored
bit = 1
stored
bit = 0
stored
bit = 1
stored
bit = 0
stored
bit = 0
stored
bit = 1
stored
bit = 1
stored
bit = 0
stored
bit = 0
stored
bit = 1
stored
bit = 1
Data2
Data1
bitline0
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
2
Let’s look at a single bit cell
bitline
wordline
stored
bit
Example:
bitline = Z
bitline = 0
wordline = 1
wordline = 0
stored
bit = 0
stored
bit = 0
bitline = Z
bitline = 1
wordline = 1
wordline = 0
stored
bit = 1
(a)
stored
bit = 1
(b)
Bit Slices and Modules
 Entire column of cells
 called a bit slice
 basically a 1-bit wide memory!
 Module
 module refers to a single chip of
memory
 1-bit wide memory chips are
quite common!
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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 latch
 but much smaller and faster!
bitline
wordline
stored
bit
bitline
wordline
bitline
16 X 1 RAM “Chip”
 Now shows
address decoder
 selects
appropriate
location
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Row/Column Layout
 For larger RAMs:
 decoder becomes pretty big
 also run into chip layout issues
 Typically:
 larger memories use “2D” matrix layout
 see next slide
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16 X 1 RAM as 4 X 4 Array
 Two decoders
 Row
 Column
 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 pins
 Spot the difference!
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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 nanoseconds
 say 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 chips
 Expensive ones can go a bit bigger
 Energy/power
 SRAMs 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 RAM
 our building block
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Larger
 Let’s build a larger
memory
 256K X 8
 Decoder for high-order 2 bits
 Selects chip
 Look at selection logic
 Address ranges
 Tri-state outputs
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Summary
 Today we looked at:
 Quick look at non-volatile memory
 Static RAM
 Next topic:
 Dynamic RAM
 Complex, largest, cheap
 Much more design effort to use
 Talk about memories for lab
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