Texas Instruments DM648 DSP - Manual (Page 27)

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2.11 DDR2 SDRAM Memory Initialization
2.11.1 DDR2 SDRAM Device Mode Register Configuration Values
Peripheral Architecture
DDR2 SDRAM devices contain mode and extended mode registers that configure the mode of operation
for the device. These registers control parameters such as burst type, burst length, and CAS latency. The
DDR2 memory controller programs the mode and extended mode registers of the DDR2 memory by
issuing MRS and EMRS commands during the initialization sequence described in
Section 2.11.2 and
Section 2.11.3 . The initialization sequence performed by the DDR2 memory controller is compliant with
the JESDEC79-2A specification.
The DDR2 memory controller performs the initialization sequence under the following conditions:
•
Automatically following a hard or soft reset, see Section 2.11.2 .
•
Following a write to the two least-significant bytes in the SDRAM configuration register (SDCFG); see
Section 2.11.3 .
At the end of the initialization sequence, the DDR2 memory controller performs an auto-refresh cycle,
leaving the DDR2 memory controller in an idle state with all banks deactivated.
When the initialization section is started automatically after a hard or soft reset, commands and data
stored in the DDR2 memory controller FIFOs are lost. However, when the initialization sequence is
initiated by a write to the two least-significant bytes in SDCFG, data and commands stored in the DDR2
memory controller FIFOs are not lost and the DDR2 memory controller ensures read and write commands
are completed before starting the initialization sequence.
The DDR2 memory controller initializes the mode register and extended mode register 1 of the memory
device with the values shown on
Table 9 and Table 10 . The DDR2 SDRAM extended mode registers 2
and 3 are configured with a value of 0h.
Table 9. DDR2 SDRAM Mode Register Configuration
Mode Mode Register
Register Bit Field Init Value Description
12 Power-down Mode 0 Active power-down exit time bit. Configured for Fast exit.
11-9 Write Recovery SDTIM1.T_WR Write recovery bits for auto-precharge. Initialized using the
T_WR bits of the SDRAM timing 1 register (SDTIM1).
8 DLL Reset 0 DLL reset bits. DLL is not in reset.
7 Mode 0 Operating mode bit. Normal operating mode is always
selected.
6-4 CAS Latency SDCFG.CL CAS latency bits. Initialized using the CL bits of the SDRAM
configuration register (SDCFG).
3 Burst Type 0 Burst type bits. Sequential burst mode is always used.
2-0 Burst Length 3h Bust length bits. A burst length of 8 is always used.
Table 10. DDR2 SDRAM Extended Mode Register 1 Configuration
Mode Mode Register
Register Bit Field Init Value Description
12 Output Buffer Enable 0 Output buffer enable bits. Output buffer is always enabled.
11 RDQS Enable 0 RDQS enable bits. Always initialized to 0 (RDQS signals
disabled.)
10 DQS enable 0 DQS enable bit. Always initialized to 0 (DQS signals
enabled.)
9-7 OCD Operation 0h Off-chip driver impedance calibration bits. This bit is always
initialized to 0h.
6 ODT Value (Rtt) 0 On-die termination effective resistance (Rtt) bit. Together
with bit 2, this bit selects the value for Rtt as 75
Ω
.
5-3 Additive Latency 0h Additive latency bits. Always initialized to 0h (no additive
latency).
SPRUEK5A – October 2007 DSP DDR2 Memory Controller 27
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