Texas Instruments DM648 DSP - Manual (Page 14)

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2.4.1 Mode Register Set (MRS and EMRS)
COL
MRS/EMRS
BANK
DDR_CLK
DDR_CLK
DDR_CS
DDR_CKE
DDR_RAS
DDR_WE
DDR_CAS
DDR_BA[2:0]
DDR_A[13:0]
2.4.2 Refresh Mode
Peripheral Architecture
DDR2 SDRAM contains mode and extended mode registers that configure the DDR2 memory for
operation. These registers control burst type, burst length, CAS latency, DLL enable/disable, single-ended
strobe, etc.
The DDR2 memory controller programs the mode and extended mode registers of the DDR2 memory by
issuing MRS and EMRS commands. When the MRS or EMRS command is executed, the value on
DDR_BA[1:0] selects the mode register to be written and the data on DDR_A[12:0] is loaded into the
register.
Figure 3 shows the timing for an MRS and EMRS command.
The DDR2 memory controller only issues MRS and EMRS commands during the DDR2 memory controller
initialization sequence. See
Section 2.11 for more information.
Figure 3. DDR2 MRS and EMRS Command
The DDR2 memory controller issues refresh commands to the DDR2 SDRAM device ( Figure 4 ). REFR is
automatically preceded by a DCAB command, ensuring the deactivation of all CE spaces and banks
selected. Following the DCAB command, the DDR2 memory controller begins performing refreshes at a
rate defined by the refresh rate (REFRESH_RATE) bit in the SDRAM refresh control register (SDRFC).
Page information is always invalid before and after a REFR command; thus, a refresh cycle always forces
a page miss. This type of refresh cycle is often called autorefresh. Autorefresh commands may not be
disabled within the DDR2 memory controller. See
Section 2.8 for more details on REFR command
scheduling.
14 DSP DDR2 Memory Controller SPRUEK5A – October 2007
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