5 read command – Texas Instruments TMS320C6452 DSP User Manual

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2.4.5

READ Command

DDR_D[31:0]

DDR_DQS[3:0]

COL

BANK

DDR_A[10]

CAS Latency

DDR_CLK

DDR_CS

DDR_CKE

DDR_RAS

DDR_WE

DDR_DQM[3:0]

DDR_CAS

DDR_BA[2:0]

DDR_A[13:0]

Peripheral Architecture

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

Figure 8

. Read latency is equal to CAS latency plus additive latency.

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 DCAB/DEAC
command until page information becomes invalid.

Figure 8. DDR2 READ Command

SPRUF85 – October 2007

DSP DDR2 Memory Controller

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