External Memory Interfaces in Stratix II and Stratix II GX Devices
DLL
The DQS phase-shift circuitry uses a delay-locked loop (DLL) to
dynamically measure the clock period needed by the DQS/DQSn pin (see
Figure 3–10). The DQS phase-shift circuitry then uses the clock period to
generate the correct phase shift. The DLL in the Stratix II or Stratix II GX
DQS phase-shift circuitry can operate between 100 and 400 MHz. The
phase-shift circuitry needs a maximum of 256 clock cycles to calculate the
correct input clock period. Data sent during these clock cycles may not be
properly captured.
1
Although the DLL can run up to 400 MHz, other factors may
prevent you from interfacing with a 400-MHz external memory
device.
1
You can still use the DQS phase-shift circuitry for any memory
interfaces that are less than 100 MHz. The DQS signal will be
shifted by 2.5 ns and you can add more shift by using the phase
offset module. Even if the DQS signal is not shifted exactly to the
middle of the DQ valid window, the IOE should still be able to
capture the data in this low frequency application.
There are four different frequency modes for the Stratix II or Stratix II GX
DLL. Each frequency mode provides different phase shift, as shown in
Table 3–9.
Table 3–9. Stratix II and Stratix II GS DLL Frequency Modes
Frequency
Mode
Available
Phase Shift Delay Chains
Number of
Frequency Range (MHz)
0
1
2
3
100–175
30, 60, 90,
120
12
16
12
10
150–230
200–310
22.5, 45,
67.5, 90
30, 60, 90,
120
240–400 (C3 speed grade)
240–350 (C4 and C5 speed grades)
36, 72, 108,
144
In frequency mode 0, Stratix II devices use a 6-bit setting to implement the
phase-shift delay. In frequency modes 1, 2, and 3, Stratix II devices only
use a 5-bit setting to implement the phase-shift delay.
The DLL can be reset from either the logic array or a user I/O pin. This
signal is not shown in Figure 3–10. Each time the DLL is reset, you must
wait for 256 clock cycles before you can capture the data properly.
Altera Corporation
January 2008
3–25
Stratix II Device Handbook, Volume 2
ALTERA [ ALTERA CORPORATION ]
