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Synchronous DRAM Controller Module
MCF5253 Reference Manual, Rev. 1
7-16
Freescale Semiconductor
7.5
Initialization Sequence
Synchronous DRAMs have a prescribed initialization sequence. The DRAM controller supports this
sequence with the following procedure:
1. SDRAM control signals are reset to idle state. Wait the prescribed period after reset before any
action is taken on the SDRAMs. This is normally around 100 µs.
2. Initialize the DCR, DACR, and DMR in their operational configuration. Do not yet enable
PALL
or
REF
commands.
3. Issue a
PALL
command to the SDRAMs by setting DCR[IP] and accessing a SDRAM location.
Wait the time (determined by t
RP
)
before any other execution.
4. Enable refresh (set DACR[RE]) and wait for at least 8 refreshes to occur.
5. Before issuing the
MRS
command, determine if the DMR mask bits need to be modified to allow
the
MRS
to execute properly.
6. Issue the
MRS
command by setting DACR[IMRS] and accessing a location in the SDRAM.
NOTE
Mode register settings are driven on the SDRAM address bus, so care must
be taken to change DMR[BAM] if the mode register configuration does not
fall in the address range determined by the address mask bits. After the
mode register is set, DMR mask bits can be restored to their desired
configuration.
7.5.1
Mode Register Settings
It is possible to configure the operation of SDRAMs, namely their burst operation and CAS latency,
through the SDRAM mode register. CAS latency is a function of the speed of the SDRAM and the bus
clock of the DRAM controller. The DRAM controller operates at a CAS latency of 1 or 2.
Although the MCF5253 DRAM controller supports bursting operations, it does not use the bursting
features of the SDRAMs. Because the MCF5253 can burst operand sizes of 1, 2, 4, or 16 bytes long, the
concept of a fixed burst length in the SDRAMs mode register becomes problematic. Therefore, the
MCF5253 DRAM controller generates the burst cycles rather than the SDRAM device. Because the
MCF5253 generates a new address and a
READ
or
WRITE
command for each transfer within the burst, the
SDRAM mode register should be set either to a burst length of one or to not burst. This allows bursting to
be controlled by the MCF5253 instead.
The SDRAM mode register is written to by setting the associated block’s DACR[IMRS]. First, the base
address and mask registers must be set to the appropriate configuration to allow the mode register to be set.
NOTE
Improperly set DMR mask bits may prevent access to the mode register
address. Thus, the user should determine the mapping of the mode register
address to the MCF5253 address bits to find out if an access is blocked. If
the DMR setting prohibits mode register access, the DMR should be
reconfigured to enable the access and then set to its necessary configuration
after the
MRS
command executes.
Содержание MCF5253
Страница 1: ...Document Number MCF5253RM Rev 1 08 2008 MCF5253 Reference Manual...
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