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SDRAM Controller (SDRAMC)
21-20
Freescale Semiconductor
21.5.1.6
Load Mode/Extended Mode Register Command (
LMR
,
LEMR
)
All SDRAM devices contain mode registers that configure the timing and burst mode for the SDRAM.
These commands access the mode registers that physically reside within the SDRAM devices. During the
LMR
or
LEMR
command, SDRAM latches the address and bank buses to load the values into the selected
mode register.
NOTE
The
LMR
and
LEMR
commands are only used during SDRAM initialization.
Use the following steps to write the mode register and extended mode register:
1. Set the SDCR[MODE_EN] bit.
2. Write the SDMR[BA] bits to select the mode register.
3. Write the desired mode register value to the SDMR[ADDR]. Do not overwrite the SDMR[BA]
values. This step can be performed in the same register write in step 2.
4. Set the SDMR[CMD] bit.
5. For DDR, perform steps 2–4 more than once to write the extended-mode register and the mode
register.
For DDR2, perform step 2–4 four times. The first is for the third extended mode register, second
is for the second extended mode register; third is for the first extended mode register; the last is for
the mode register.
6. Clear the SDCR[MODE_EN] bit.
21.5.1.6.1
Mode Register Definition
shows a typical mode register definition. This is the SDRAM’s mode register, not the
SDRAMC’s mode/extended mode register (SDMR) defined in
Section 21.4.1, “SDRAM Mode/Extended
.” Refer to the SDRAM manufacturer’s device data sheet to confirm correct
settings.
BA1
BA0
A11
A10
A9
A8
A7
A6
A5
A4
A3
A2
A1
A0
Field
0
0
OP_MODE
CL
BT
BLEN
Figure 21-9. Typical Mode Register
Table 21-11. Mode Register Field Descriptions
Field
Description
BA1–BA0
Bank address. These must be zero to select the mode register.
A11–A7
OP_MODE
Operating mode.
00000 Normal Operation
00010 Reset DLL
Else
Reserved
A6–A4
CL
CAS latency. Delay in clocks from issuing a
READ
to valid data out. Check the SDRAM manufacturer’s spec because
the CL settings supported can vary from memory to memory.
Содержание MCF54455
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