Altera DDR SDRAM High-Performance Controllers and ALTMEMPHY IP - Manual (Page 67)

Chapter 5: Functional Description—ALTMEMPHY 5–21
Block Description
June 2011 Altera Corporation External Memory Interface Handbook Volume 3
Section I. DDR and DDR2 SDRAM Controllers with ALTMEMPHY IP User Guide
Timing constraints ensure that the data resynchronization registers are placed close to
the DQ pins to achieve maximum performance. Timing constraints also further limit
skew across the DQ pins. The captured data (rdata_2x_p and rdata_2x_n) is
synchronized to the resynchronization clock (resync_clk_2x), refer to
Figure 5–5 .
Data Demultiplexing
Data demultiplexing is the process of SDR data into HDR data. Data demultiplexing
is required to bring the frequency of the resynchronized data down to the frequency
of the system clock, so that data from the external memory device can ultimately be
brought into the FPGA DDR or DDR2 SDRAM controller clock domain. Before data
capture, the data is DDR and n-bit wide. After data capture, the data is SDR and 2n-bit
wide. After data demuxing, the data is HDR of width 4n-bits wide. The system clock
frequency is half the frequency of the memory clock.
Demultiplexing is achieved using a dual-port memory with a 2n-bit wide write-port
operating on the resynchronization clock (SDR) and a 4n-bit wide read-port operating
on the PHY clock (HDR). The basic principle of operation is that data is written to the
memory at the SDR rate and read from the memory at the HDR rate while
incrementing the read- and write-address pointers. As the SDR and HDR clocks are
generated, the read and write pointers are continuously incremented by the same
PLL, and the 4n-bit wide read data follows the 2n-bit wide write data with a constant
latency.
Read Data Alignment
Data alignment is the process controlled by the sequencer to ensure the correct
captured read data is present in the same half-rate clock cycle at the output of the read
data DPRAM. Data alignment is implemented using either M4K or M512K memory
blocks. The bottom of
Figure 5–6 shows the concatenation of the read data into valid
HDR data.