Motorola ColdFire MCF5281 - Manual (Page 275)

Synchronous DRAM Controller Module
Freescale Semiconductor 15-3
15.2 SDRAM Controller Operation
By running synchronously with the system clock, SDRAM can (after an initial latency period) be accessed
on every clock; 5-1-1-1 is a typical burst rate to the SDRAM. Unlike the MCF5272, this processor does
not have an independent SDRAM clock signal. For this processor, the timing of the SDRAM controller is
controlled by the CLKOUT signal.
Note that because the processor cannot have more than one page open at a time, it does not support
interleaving.
SDRAM controllers are more sophisticated than asynchronous DRAM controllers. Not only must they
manage addresses and data, but they must send special commands for such functions as precharge, read,
write, burst, auto-refresh, and various combinations of these functions.
Table 15-1 lists common SDRAM
commands.
SDRAMs operate differently than asynchronous DRAMs, particularly in the use of data pipelines and
commands to initiate special actions. Commands are issued to memory using specific encodings on
address and control pins. Soon after system reset, a command must be sent to the SDRAM mode register
to configure SDRAM operating parameters.
Table 15-1. SDRAM Commands
Command Definition
ACTV Activate. Executed before READ or WRITE executes; SDRAM registers and decodes row address.
MRS Mode register set.
NOP No-op. Does not affect SDRAM state machine; DRAM controller control signals negated;
SDRAM_CS[1:0] asserted.
PALL Precharge all. Precharges all internal banks of an SDRAM component; executed before new page is
opened.
READ Read access. SDRAM registers column address and decodes that a read access is occurring.
REF Refresh. Refreshes internal bank rows of an SDRAM component.
SELF Self refresh. Refreshes internal bank rows of an SDRAM component when it is in low-power mode.
SELFX Exit self refresh. This command is sent to the DRAM controller when DCR[IS] is cleared.
WRITE Write access. SDRAM registers column address and decodes that a write access is occurring.
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3