RAM Basics
Anselmo Lastra
The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Topics
VGA timing project
Deadline Thursday
Class time change
Semester project topics
RAMs
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Class Time
Preference for keeping TTh
Unfortunately, no open time for
all
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Projects
Some have project already
Polygon pipeline
Stopping points: Gouraud, texturing, etc.
Ray caster/tracer
Similar possible milestones
Can share common parts
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Simple Hdw View of RAM
Some capacity 2k
k bits of address lines
Often multiplexed
Maybe have read line, clock, chip
select
Have a write enable line
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Reading
Setup address lines
Activate enable, read/write line
Data available after specified amt of time
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Writing
Setup address lines
Setup data lines
Activate write line
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Static vs Dynamic RAM
SRAM vs DRAM
DRAM stores charge in what’s essentially
capacitor
Disappears over short period of time
Must be refreshed (rewritten/recharged)
SRAM easier to use
Faster
More expensive per bit
Smaller sizes
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Structure of SRAM
Control logic
One memory cell per bit
Cell consists of one or more transistors
Not really a latch made of logic
Logic equivalent
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Bit Slice
Cells connected to form
1 bit position
Word Select gates one
latch from address lines
Note it selects Reads
also
B (and B not) set by
R/W, Data In and
BitSelect
Funny thing here when
you write. What is it?
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Bit Slice can Become Module
Basically bit slice
is a X1 memory
Next
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16 X 1 RAM
Now
shows
decoder
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Tri-State
Have three states: H, L, and Hi-Z
High impedance
Behaves like no output connection if in
Hi-Z state
Allows connecting multiple outputs
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Multiplexed with Hi-Z
Normal behavior is blue
area
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Row/Column
If RAM gets large, there is a large
decoder
Also run into chip layout issues
Larger memories usually “2D” in
a matrix layout
Next Slide
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16 X 1 as 4 X 4 Array
Two
decoders
Row
Column
Address just
broken up
Not visible
from outside
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Change to 4 X 2 RAM
Minor change
in logic
Also pinouts
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Realistic Sizes
Imagine 256K memory as 32K X 8
One column layout would need
15-bit decoder with 32K outputs!
Can make a square layout with 9bit row and 6-bit column decoders
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SRAM Performance
Current ones have cycle times in
low nanoseconds (say 2.5ns)
Used as cache (typically offchip
secondary cache)
Sizes up to 8Mbit or so for fast
chips
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Using SRAM on Spartan II
Recall block SRAM available on
chip
11 4Kb blocks
Configured in many ways (table)
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Using from Verilog
Instantiate a block (here called R1)
RAMB4_S8_S8 R1 (.DOA (data_a),
.DOB (data_b),
.ADDRA (addr_a),
.ADDRB (addr_a),
.CLKA (clk),
.CLKB (clk),
.DIA (data_in),
.DIB (data_in),
.ENA (ena),
.ENB (enb),
.RSTA (rsta),
.RSTB (rstb),
.WEA
(wea),
.WEB
(web));
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Can Initialize
Have to do it two ways, one for
simulator, another for hardware
//synthesis attribute INIT_00 of R1 is
"08192A3B4C5... total of 256 bits (64 hex
characters)..."
//synthesis attribute INIT_01 of R1 is
"08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4C5D6E7F0
8192A3B4C5D6E7F“
// Up to INIT_0F
Above is for hardware (next
software)
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For Simulation
//synopsys translate_off
defparam R1.INIT_00 =
64'h08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4
C5D6E7F08192A3B4C5D6E7F; // 256-bit hex value
defparam R1.INIT_01 =
64'h08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4
C5D6E7F08192A3B4C5D6E7F; // 256-bit hex value
...
defparam R1.INIT_0F =
64'h08192A3B4C5D6E7F08192A3B4C5D6E7F08192A3B4
C5D6E7F08192A3B4C5D6E7F; // up to INIT_0F
//synopsys translate_on
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Look at Test Code
My RAM loading example from
undergrad class
http://www.cs.unc.edu/~lastra/comp190/Assignments/block_ram_C.txt
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Dynamic RAM
Capacitor can hold charge
Transistor acts as gate
No charge is a 0
Can add charge to store a 1
Then open switch (disconnect)
Can read by closing switch
Explanation next
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Precharge and Sense Amps
You’ll see “precharge time”
B is precharged to ½ V
Charge/no-charge on C will
increase or decrease voltage
Sense amps detect this
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DRAM Characteristics
Destructive Read
When cell read, charge removed
Must be restored after a read
Refresh
Also, there’s steady leakage
Charge must be restored periodically
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DRAM Logical Diagram
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DRAM Read Signaling
Lower pin count by using same pins for
row and column addresses
Delay until
data
available
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DRAM Write Timing
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DRAM Refresh
Many strategies w/ logic on chip
Here a row counter
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CAS Before RAS
Set column address
Apply CAS first (opposite of RW)
Then toggle RAS enough times to
cycle through row addresses
On-board refresh counter applies
the row addresses
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Timing
Say need to refresh every 64ms
Distributed refresh
Spread refresh out evenly over 64ms
Say on a 4Mx4 DRAM, refresh every
64ms/4096=15.6 us
Total time spent is 0.25ms, but spread
Burst refresh
Same 0.25ms, but all at once
May not be good in a computer system
Refresh takes 1 % or so of total time
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Larger/Wider Memories
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Bidirectional Lines
One set of data pins
Used as input for write
As output for read
Tri-state
Makes sense because don’t need
both at once
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Page Mode DRAM
DRAMs made to read & write blocks
Example
Assert RAS, leave asserted
Assert CAS multiple times to read sequence
of data
Similar for writes
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Synchronous DRAM (SDRAM)
Common type in PCs late-90s
Burst transfers
Multiple banks
Pipelined
Start read in one bank after another
Come back and read the resulting values
one after another
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SDRAM on Xess Board
Relatively small at 128Mbits
2M X 4 banks X 16 bits
Refresh every 64ms
Supports pipelining
Bidirectional data lines
Detailed info in a few slides
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DDR DRAM
Double Data Rate SDRAM
Transfers data on both edges of
the clock
Currently popular
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RAMBUS DRAM (RDRAM)
Another attempt to alleviate
pinout limits
Many (16-32) banks per chip
Made to be read/written in
packets
Up to 400MHz bus speeds
But DDR doing very well also
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DRAM Controllers
Very common to have circuit that
controls memory
Handles banks
Handles refresh
Multiplexes column and row
addresses
RAS and CAS timing
Northbridge on PC chip set
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Next: Specifics on Our Chip
Protocol for reading/writing
Activate row first
Then read/write with column
Initialization
Setting parameters
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Block Diagram
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Activate Row
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Read (Select column)
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Burst Reads
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Read with Autoprecharge
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Read w/o Autoprecharge
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Random Reads
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Alternating Banks
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Single Write
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Initializing
See P.9 of Micron datasheet
Just NOP commands for 100 us
Precharge all banks
Two Auto Refresh commands
Then load mode register
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Mode Register
Several operating modes of
SDRAM
1,2,4,8 or
full page
2 or 3
Burst or single
0
Order or
accesses
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DRAM Links
DRAM on XSA-100 board
http://www.hynix.co.kr/datasheet/pdf/dram/(2)HY57V281620A(L)T-I.PDF
Low-Tech RAM description
http://www.arstechnica.com/paedia/r/ram_guide/ram_guide.part1-1.html
Datasheets
http://www.hynix.co.kr/datasheet/pdf/dram/(2)HY57V281620A(L)T-I.PDF
http://download.micron.com/pdf/datasheets/dram/128msdram_f.pdf
http://www.infineon.com/cmc_upload/documents/018/329/hb39s128CT.pdf
Verilog model
http://download.micron.com/downloads/models/verilog/sdram/sdr/128meg/
mt48lc8m16a2.zip
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Assignment
Try Block RAM
Maybe to scan small stamps
Or as character/sprite device
Make DRAM controller to refresh
screen
Deadline – end of next week
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Next Time
Thursday 9/11 read Kurt Akeley,
"Reality Engine Graphics",
SIGGRAPH 93
Link is
http://doi.acm.org/10.1145/166117.166131
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