External Memory Controller (EMC)
Symphony DSP56724/DSP56725 Multi-Core Audio Processors, Rev. 0
Freescale Semiconductor
21-65
21.4.3.10 SDRAM MODE-SET Command Timing
The EMC transfers mode register data (CAS latency and burst length) stored in the SDMR register to the
SDRAM device by issuing the MODE-SET command, as shown in
. In this case, the latched
address carries the mode bits for the command.
Figure 21-32. SDRAM MODE-SET Command
21.4.3.11 SDRAM Refresh
The memory controller supplies AUTO-REFRESH commands to any connected SDRAM device
according to the interval specified in SRT (and prescaled by MRTPR[PTP]). This represents the time
period required between refreshes. The values of SRT and MRTPR depend on the specific SDRAM
devices used and the system clock frequency of the EMC. These values should allow for a potential
collision between memory accesses and refresh cycles. The period of the refresh interval must be greater
than the access time to ensure that read and write operations complete successfully.
There are two levels of refresh request priority—low and high. The low priority request is generated as
soon as the refresh timer expires; this request is granted only if no other requests to the memory controller
are pending. If the request is not granted (because the memory controller is busy) and the refresh timer
expires two more times, the request becomes high priority and is served when the current memory
controller operation finishes.
21.4.3.11.1
SDRAM Refresh Timing
The SDRAM memory controller implements bank staggering for the auto refresh function. This reduces
instantaneous current consumption for memory refresh operations.
After a refresh request is granted, the memory controller begins issuing an AUTO-REFRESH command
to each device associated with the refresh timer. After a refresh command is issued to an SDRAM device,
the memory controller waits for the number of bus clock cycles programmed in the SDRAM machine’s
mode register (SDMR[RFCR]) before issuing any subsequent ACTIVATE command to the same device.
To avoid violating SDRAM device timing constraints, you should ensure that the refresh request interval
(defined by SRT and MRTPR) is greater than the refresh recovery interval (defined by SDMR[RFCR]).
1111
ZZZZZZZZ
MODE
ZZZZZZZZ
LCLK
LALE
LCSx
LSDRAS
LSDCAS
LSDWE
LSDDQM
LAD[23:0]
MODE-SET
Command