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SDRAM Controller (SDRAMC)
Freescale Semiconductor
21-13
The read and write latency fields govern the relative timing of commands and data and must be exact
values. All other fields govern the relative timing from one command to another; they have minimum
values, but any larger value is also legal (but with decreased performance).
For DDR2, burst reads and writes are the normal operation. The DDR2 specification does not use the
BURST
TERMINATE
command; therefore, the entire burst occurs. If the burst length is eight and a 32-bit
write is occurring, the first two cycles have data, while the last six cycles have the data mask disabled.
The minimum values of certain fields can be different for DDR, and DDR2 SDRAM, even if the data sheet
timing is the same, because:
•
In DDR mode, the memory controller counts the delay in 2 x SD_CLK (also referred to as
SD_CLK2)
•
In DDR2 mode, the memory controller counts the delay in SD_CLK (this is not SD_CLK2).
•
SD_CLK—memory controller clock—is the speed of the SDRAM interface and is equal to the
internal bus clock.
•
SD_CLK2—double frequency of SD_CLK—DDR uses both edges of the bus-frequency clock
(SD_CLK) to read/write data
NOTE
In all calculations for setting the fields of this register, convert time units to
clock units and round up to the nearest integer.
Address: 0xFC0B_8008 (SDCFG1)
Access: User read/write
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
R
SRD2RWP
0
SWT2RWP
RD_LAT
0
ACT2RW
W
Reset
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
R
0
PRE2ACT
REF2ACT
0
WT_LAT
0
0
0
0
W
Reset
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Figure 21-6. SDRAM Configuration Register 1 (SDCFG1)
Table 21-7. SDCFG1 Field Descriptions
Field
Description
31–28
SRD2RWP
Single read to read/write/precharge delay. Limiting case is read to write.
DDR: SRD2RWP = CL + 1
DDR2: SRD2RWP =
BurstLength/2 + AL; If AL (additive latency) is less than 2, then use 2.
t
HZ
is the time the data bus uses to return to hi-impedance after a read and is found in the SDRAM device
specifications.
Note:
Count value is in SD_CLK periods for DDR mode.
27
Reserved, must be cleared.
Содержание MCF54455
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