EP2AGX95EF29C6N Datasheet, PDF (237/380 Pages) Altera Corporation – Device Interfaces and Integration

Chapter 7: External Memory Interfaces in Arria II Devices
Arria II External Memory Interface Features
7–35
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 with the
dqs_delayctrlin[5..0] signals available in the ALTMEMPHY megafunction and
UniPHY IP core. These settings control 1, 2, 3, or all 4 delay elements in the DQS delay
chains. The ALTMEMPHY megafunction and UniPHY IP core can also dynamically
choose the number of DQS delay chains required 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 and
UniPHY IP core use this circuit by default. Figure 7–22 shows an example waveform
of the update enable circuitry output.
Figure 7–22. 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)
Update Enable
Circuitry Output
6 bit
DQS Postamble Circuitry
For external memory interfaces that use a bidirectional read strobe such as in DDR3,
DDR2, and DDR SDRAM, the DQS signal is low before going to or coming from a
high-impedance state. The state in which DQS is low, just after a high-impedance
state, is called the preamble; the state in which DQS is low, just before it returns to a
high-impedance state, is called the postamble. There are preamble and postamble
specifications for both read and write operations in DDR3, DDR2, and DDR SDRAM.
The DQS postamble circuitry ensures that data is not lost if there is noise on the DQS
line at the end of a read postamble time.
Arria II devices have dedicated postamble registers that you can control to ground
the shifted DQS signal used to clock the DQ input registers at the end of a read
operation. This ensures that any glitches on the DQS input signals at the end of the
read postamble time do not affect the DQ IOE registers.
June 2011 Altera Corporation
Arria II Device Handbook Volume 1: Device Interfaces and Integration