Ddr2 Read Command - Texas Instruments TMS320C642x DSP User Manual

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2.4.4 READ Command
Figure 8 shows the DDR2 memory controller performing a read burst from DDR2 SDRAM. The READ
command initiates a burst read operation to an active row. During the READ command, DDR_CAS drives
low, DDR_WE and DDR_RAS remain high, the column address is driven on DDR_A[12:0], and the bank
address is driven on DDR_BA[2:0].
The DDR2 memory controller uses a burst length of 8, and has a programmable CAS latency of 2, 3, 4, or
5. The CAS latency is three cycles in
The DDR2 memory controller always configures the memory to have an additive latency of 0, so read
latency equals CAS latency. Since the default burst size is 8, the DDR2 memory controller returns 8
pieces of data for every read command. If additional accesses are not pending to the DDR2 memory
controller, the read burst completes and the unneeded data is disregarded. If additional accesses are
pending, depending on the scheduling result, the DDR2 memory controller can terminate the read burst
and start a new read burst. Furthermore, the DDR2 memory controller does not issue a DAB/DEAC
command until page information becomes invalid.
DDR_CLK
DDR_CLK
DDR_CKE
DDR_CS
DDR_RAS
DDR_CAS
DDR_WE
DDR_A[12:0]
DDR_BA[2:0]
DDR_A[10]
DDR_DQM[3:0]
DDR_D[31:0]
DDR_DQS[3:0]
SPRUEM4A – November 2007
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Figure
Figure 8. DDR2 READ Command
COL
BANK
CAS Latency
8. Read latency is equal to CAS latency plus additive latency.
D0 D1 D2
Peripheral Architecture
D3 D4 D5 D6 D7
DDR2 Memory Controller 17