Double Data Rate SDRAM – The Next Generation
description
Transcript of Double Data Rate SDRAM – The Next Generation
Double Data Rate SDRAM – The Next Generation
An overview of the industry roadmap for main system memory technology, and details on DDR which represents the latest state of the art for SDRAM. We will cover:
• The industry standards process for product definition
• The evolution of main memories
• Comparing DDR to SDRAM
• DDR configuration options & applications
• Design tricks for DDR systems
• What’s next for main memory?
The JEDEC Standards Process
The JEDEC Standards Process
•JEDEC is a non-profit standards organization
•265 member companies from all over the world
•Suppliers & users and even competitors
•Working together to expand the market
How standards get done
• Any company presents a market need
• Interested companies form a Task Group
• TG does development, submits ballot to committee
• Feedback from voting incorporated into spec
• The new standard is published
• Task Group reforms as needed for enhancements
Industry Evolution from SDRAM to DDR
Main Memory DRAM Evolution
320MB/s400MB/s
1000MB/s
2100MB/s
MainstreamMemories
FPEDO
SDRAMDDR
Simple,incrementalsteps
DDR II4800MB/s
Cost remains constant
• The top three factors driving memory evolution1.Cost
2.Cost
3.Cost
• The price of memory has remained essentially constant
• Each incremental enhancement must “come for free”
• “Free” means similar evolution of costs:– Direct: die size, packaging, testers, licensing– Indirect: PCB complexity, heat sinks, support components– 2x indirect: dummy continuity boards
What is DDR?
•Internally, DDR is an SDRAM with ping pong registers
•Data is posted on rising and falling edges of the clock
•Commands still sampled on rising edge
How Different is DDR?
•Simple upgrade from SDRAM designs–Same PCB characteristics: 60 6 –Same RAS/CAS command set
•A few evolutionary improvements–Low voltage swing I/O–Differential clocks–Source synchronous data strobe
DDR low voltage signaling
•SSTL_2 low voltage swing inputs–2.5V I/O with 1.25V reference voltage–Low voltage swing with termination–Rail to rail if unterminated
DDR
DDR Differential Clocks
•Route differential clocks on adjacent traces
•Timing is relative to crosspoint
•Helps insure 50% duty cycle
DDR Read Timing – Data delivered on both edges of CK
•Data valid on rising & falling edges
•Data Strobe “DQS” travels with data
•DLL aligns data to clock edges
Emphasis on “Matched”
•DM/DQS loading identical to DQ
•Route as independent 8 bit buses
DQ/DQS
DM
VREF
VREF
VREF
VREF
Disable
CONTROLLER DDR SDRAM
Combining 8 bit buses into internal bus width
•Each byte samples using DQS as input strobe
•Input buffers capture one odd and one even byte
•Commit to FIFO on controller clock
DQDM
DQS
8
DQDM
DQS
8
DQDM
DQS
8
Internal Memory FIFO
Clocked in memory time domain
Clocked in controller time domain
DDR Configuration Options for Different Applications
DDR Configurations
TSOP
SO-DIMM
DIMM
TQFP
DDR Configurations, Chips
66 pin TSOP-II–Inexpensive high volume plastic package–Compatible pinout for X4, X8, X16–64Mb to 512Mb; 1Gb in development
100 pin TQFP–Inexpensive high volume plastic package–X32 configuration–64Mb and 128Mb
DDR Configurations, Modules
Desktop & Server184 pins, 5.25” long
X64 or X72 (ECC)
64MB to 2GB
Mobile & Small Form Factor 200 pins, 2.7” long
X64 or X72 (ECC)
32MB to 512MB
DDR Unbuffered DIMM
• Least expensive module• Limits number of loads supportable• Address bus hits all DDR SDRAMs• Fastest access time
Data Data Data DataAddress
DDRSDRAM
DDRSDRAM
DDRSDRAM
DDRSDRAM
DDR Registered DIMM
• Doubles density of each module orhalves number of address buses needed
• Address bus latched before going to DDR SDRAMs• Access time increased by one clock
Data Data Data DataAddress
Register
DDRSDRAM
DDRSDRAM
DDRSDRAM
DDRSDRAM
DDR Tips and Tricks for Power Management
Power Management
RelativePower
CPU Clocks of Latency**
Active on 100% 0 * 5 = 0
Inactive on 3 * 5 = 15
Active off 1 * 5 = 5
Inactive off 0.2% 4 * 5 = 20
Sleep 0.4% 200*5 = 1000
PowerState*
12%
4%
* Not industry standard terms – simplified for brevity**Assumes 5 CPU clocks per memory clock
Op
enP
age
Clo
sed
Pag
e
Power: DDR vs SDRAM
0
0.5
1
1.5
2
2.5
3
3.5
Throughput per Second per Unit Power
PC-100
1X
DDR-266
3X
PC-133
0.8X
DDR-333
2.6X
(est)
What’s next for DDR?
Next: Enhancing DDR from 266 to 333 MHz data rate
• Qualification of DDR333 under way
• Possibly different DDR SDRAM packages for each solution:
– Unbuffered DIMM: FBGA– Registered DIMM: TSOP– SO-DIMM: TSOP– Point to point: TSOP
Next: Small Packages
FBGA•Lower inductance
•Lower capacitance
•Smaller footprint
•Tighter layouts enabled
Details:
Package size = 104 mm2 = 54% smaller
Inductance: 1.7nH lower
Inductance variation, pin to pin: 3X less
Capacitance: 0.5pF lower
Performance gain: 300ps of data valid time
Next: DDR FET Switched DIMM
• Quadruples density of each module ordoubles number of DIMM slots
• Address bus latched before going to DDR SDRAMs• Data bus sees a single load per slot• Additional bus turnaround latency
Data Data Data DataAddress
RegisterFET FETFETFET
DDRSDRAM
DDRSDRAM
DDRSDRAM
DDRSDRAM
Next: DDR MicroDIMM
•Half the size of the DDR SO-DIMM
•Half the capacity if using TSOP – or –
•Same capacity if using FBGA
•Target markets:–PDAs–Internet appliances–Subnotebook computers
Next: DDR II
•Work well under way on DDR II
•Double the speed
•Lower power
•Migration path from DDR I–Same controller can use DDR I and DDR II–Compatible process technologies
Conclusions
• DDR is a result of collaboration between many companies
• Cost drives incremental evolutionary steps
• DDR is a simple evolution of SDRAM technology
• Configuration options available for different applications
• Use tricks and techniques to exploit DDR’s features
• The future of DDR is in evolutionary steps