EP4SE360F35I4 Datasheet, PDF (268/432 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

7–44
Chapter 7: External Memory Interfaces in Stratix IV Devices
Stratix IV External Memory Interface Features
DQS Delay Chain
DQS delay chains consist of a set of variable delay elements to allow the input
DQS/CQ and CQn signals to be shifted by the amount specified by the DQS
phase-shift circuitry or the logic array. There are four delay elements in the DQS delay
chain; the first delay chain closest to the DQS/CQ pin can be shifted either by the
DQS delay settings or by the sum of the DQS delay setting and the phase-offset
setting. The number of delay chains required is transparent because the
ALTMEMPHY megafunction automatically sets it when you choose the operating
frequency. The DQS delay settings can come from the DQS phase-shift circuitry on
either end of the I/O banks or from the logic array.
The delay elements in the DQS logic block have the same characteristics as the delay
elements in the DLL. When the DLL is not used to control the DQS delay chains, you
can input your own Gray-coded 6-bit or 5-bit settings using the
dqs_delayctrlin[5..0] signals available in the ALTMEMPHY megafunction. These
settings control 1, 2, 3, or all 4 delay elements in the DQS delay chains. The
ALTMEMPHY megafunction can also dynamically choose the number of DQS delay
chains needed for the system. The amount of delay is equal to the sum of the delay
element’s intrinsic delay and the product of the number of delay steps and the value
of the delay steps.
You can also bypass the DQS delay chain to achieve a 0° phase shift.
Update Enable Circuitry
Both the DQS delay settings and the phase-offset settings pass through a register
before going into the DQS delay chains. The registers are controlled by the update
enable circuitry to allow enough time for any changes in the DQS delay setting bits to
arrive at all the delay elements. This allows them to be adjusted at the same time. The
update enable circuitry enables the registers to allow enough time for the DQS delay
settings to travel from the DQS phase-shift circuitry or core logic to all the DQS logic
blocks before the next change. It uses the input reference clock or a user clock from the
core to generate the update enable output. The ALTMEMPHY megafunction uses this
circuit by default. Figure 7–25 shows an example waveform of the update enable
circuitry output.
Figure 7–25. DQS Update Enable Waveform
DLL Counter Update
(Every 8 cycles)
DLL Counter Update
(Every 8 cycles)
System Clock
DQS Delay Settings
(Updated every 8 cycles)
6 bit
Update Enable
Circuitry Output
Stratix IV Device Handbook
Volume 1
February 2011 Altera Corporation