EP4SE360F35I4 Datasheet, PDF (270/432 Pages) Altera Corporation – This section provides a complete overview of all features relating to the Stratix IV device family, which is the most architecturlly advanced

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Chapter 7: External Memory Interfaces in Stratix IV Devices
Stratix IV External Memory Interface Features
Leveling Circuitry
DDR3 SDRAM unbuffered modules use a fly-by clock distribution topology for better
signal integrity. This means that the CK/CK# signals arrive at each DDR3 SDRAM
device in the module at different times. The difference in arrival time between the first
DDR3 SDRAM device and the last device on the module can be as long as 1.6 ns.
Figure 7–27 shows the clock topology in DDR3 SDRAM unbuffered modules.
Figure 7–27. DDR3 SDRAM Unbuffered Module Clock Topology
DQS/DQ DQS/DQ DQS/DQ DQS/DQ CK/CK# DQS/DQ DQS/DQ DQS/DQ DQS/DQ
Stratix IV Device
Because the data and read strobe signals are still point-to-point, take special care to
ensure that the timing relationship between the CK/CK# and DQS signals (tDQSS,
tDSS, and tDSH) during a write is met at every device on the modules. Furthermore,
read data coming back into the FPGA from the memory is also staggered in a similar
way.
Stratix IV FPGAs have leveling circuitry to address these two situations. There is one
leveling circuitry per I/O sub-bank (for example, I/O sub-bank 1A, 1B, and 1C each
has one leveling circuitry). These delay chains are PVT-compensated by the same DQS
delay settings as the DLL and DQS delay chains.
For frequencies equal to and above 400 MHz, the DLL uses eight delay chains, such
that each delay chain generates a 45° delay. The generated clock phases are
distributed to every DQS logic block that is available in the I/O sub-bank. The delay
chain taps then feeds a multiplexer controlled by the ALTMEMPHY megafunction to
select which clock phases are to be used for that ×4 or × 8 DQS group. Each group can
use a different tap output from the read-leveling and write-leveling delay chains to
compensate for the different CK/CK# delay going into each device on the module.
Stratix IV Device Handbook
Volume 1
February 2011 Altera Corporation