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Background
AN 436: Using DDR3 SDRAM in Stratix III and Stratix IV Devices
© November 2008
Altera Corporation
All DDR3 SDRAM interfaces use the following two classes of signal type:
■
Unidirectional class I terminated signals, which include clocks, and address and
command signals
■
Bidirectional class II terminated signals, which include
DQS
,
DQ
, and
DM
signals
Unidirectional Class I Terminated Signals
All class I signals are multiload signals—they either go to a DIMM that has multiple
memory devices, or they go to all memory devices that make up the interface. Altera
recommends the ideal topology is a daisy-chained serial structure. Altera gives the
following recommendations for the class I termination to V
TT
:
■
Do not use for interfaces using DIMMs, as it is implemented directly on the DIMM
■
Place directly after the last device in the chain for discrete devices
Memory clocks are typically chosen to ensure an even and matched number of loads
on each clock pair, so that the timing to each memory device is consistent assuming
equal trace delays. Each clock pair should be loaded to ensure that significant slew
rate distortion does not occur. Memory clocks are typically differentially terminated
with an effective 100-
Ω
resistance. You can achieve 100-
Ω
differential termination in
one of the following ways:
■
100-
Ω
single resistor directly between the positive and negative signal
■
50-
Ω
single-ended resistor to V
TT
on each positive and negative pin
■
100-
Ω
up to V
CC
and 100-
Ω
down to ground on each positive and negative pin
Electrically all these solutions look the same to differential AC signals
f
For information about the electrical I/O termination, refer to the
chapter of the
Stratix III Device Handbook
.
FPGA drive strength and series termination setting should maximize edge rate while
ensuring that over or undershoot are not encountered.
The combined use of drive strength and slew rate, or output series termination
options mean Stratix III is ideally configurable for any Class I termination schemes.
f
For further information, refer to
Micron Technical Note TN4720: Point-to-Point Package
Sizes and Layout Basics
.
Bidirectional Class II Terminated Signals
Class II signals are typically point-to-point, unless you are using either:
■
Multiple DIMMs
■
Stacked or dual rank DIMMs or topologies
Stratix III devices include on-chip series and parallel termination. So generally,
discrete termination at the FPGA end of the line is not required.
DDR3 SDRAM devices support dynamic parallel ODT at the memory end of the line.
So typically, discrete termination is not required.