
3–12
Altera Corporation
Stratix II Device Handbook, Volume 2
January 2008
External Memory Standards
Figure 3–5. Data and Clock Relationship During a QDRII SRAM Read
(1)
This relationship is at the memory device. The timing parameter nomenclature is based on the Cypress QDRII
SRAM data sheet for CY7C1313V18.
(2)
tCO is the data clock-to-out time and tDOH is the data output hold time between burst.
(3)
tCLZ and tCHZ are bus turn-on and turn-off times respectively.
(4)
tCQD is the skew between the rising edge of CQ or CQ# and the data edges.
(5)
tCCQO and tCQOH are skew measurements between the C or C# clocks (or the K or K# clocks in single-clock mode)
and the CQ or CQ# clocks.
When reading from the QDRII SRAM, data is sent edge-aligned with the
rising edge of the echo clocks CQ and CQ#. Both CQ and CQ# are shifted
inside the FPGA using DQS and DQSn logic blocks to capture the data in
the DDR IOE registers in DLL-based implementations. In PLL-based
implementations, CQ feeds a PLL, which generates the clock to capture
the data in the DDR IOE registers.
When writing to QDRII SRAM devices, data is generated by the write
clock while the K clock is 90° shifted from the write clock, creating a
center-aligned arrangement.
Read and write operations occur during the same clock cycle on
independent read and write data paths along with the cycle-shared
address bus. Performing concurrent reads and writes does not change the
functionality of either transaction. If a read request occurs simultaneously
with a write request at the same address, the new data on D is forwarded
to Q. Therefore, latency is not required to access valid data.
f
QA
QA + 1
QA + 2
QA + 3
C/K
C#/K#
CQ
CQ#
Q
tCO (2)
tCLZ (3)
tCCQO (5)
tCQOH (5)
tCQD (4)
tDOH (2)
tCHZ (3)