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Synchronous DRAM Controller Module
MCF5253 Reference Manual, Rev. 1
7-6
Freescale Semiconductor
15
RE
Refresh enable. Determines when the DRAM controller generates a refresh cycle to the DRAM block.
0
Do not refresh associated DRAM block
1
Refresh associated DRAM block
14
Reserved, should be cleared.
13–12
CASL
CAS latency. Affects the following SDRAM timing specifications. Timing nomenclature varies with manufacturers.
Refer to the SDRAM specification for the appropriate timing nomenclature:
Note: The SDRAM controller only supports CAS latencies of 1 or 2. However very few SDRAM devices are available
that support CASL = 1. So we recommend to only use CASL = 2. Some fast SDRAM are now becoming available
and require a CASL = 3 which is not supported by this SDRAM controller.
11
Reserved, should be cleared.
10–8
CBM
Command (AP) and Bank Select MUX [2:0]. Because different SDRAM configurations cause the command and bank
select lines to correspond to different addresses, these resources are programmable. CBM determines the
addresses onto which these functions are multiplexed.
CBM Command Bit Bank Select Lines
000 17 18 and up
001 18 19 and up
010 19 20 and up
011 20 21 and up
100 21 22 and up
101 22 23 and up
110 23 24 and up
111 24 25 and up
This encoding and the address multiplexing scheme handle common SDRAM organizations. Bank select lines
include a base line and all address lines above for SDRAMs with multiple bank select lines.
7
Reserved, should be cleared.
6
IMRS
Initiate mode register set (
MRS
) command. Setting IMRS generates a
MRS
command to the associated SDRAM. In
initialization, IMRS should be set only after all DRAM controller registers are initialized and
PALL
and
REFRESH
commands have been issued. After IMRS is set, the next access to an SDRAM block programs the SDRAM’s mode
register. Thus, the address of the access should be programmed to place the correct mode information on the
SDRAM address pins. Because the SDRAM does not register this information, it doesn’t matter if the IMRS access
is a read or a write. The DRAM controller clears IMRS after the
MRS
command finishes.
0
Take no action
1
Initiate
MRS
command
Table 7-5. DRAM Address and Control Register (DACR0) Field Descriptions (Synchronous Mode)
(continued)
Field
Description
Parameter
Number of Bus Clocks
CASL = 00 CASL = 01 CASL = 10 CASL = 11
t
RCD
—SRAS assertion to SCAS assertion
N/A
2
3
3
t
CASL
—SCAS assertion to data out
N/A
1
2
2
t
RAS
—
ACTV
command to precharge command
N/A
4
6
6
t
RP
—Precharge command to ACTV command
N/A
2
3
3
t
RWL
,
t
RDL
—Last data input to precharge command
N/A
1
1
1
t
EP
—Last data out to precharge command)
N/A
1
1
1
Содержание MCF5253
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