DDR SDRAM Clocking and Interface Timing
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
12-13
Software should initialize the parameters in the DDR controller registers with the appropriate
values (by boot code, for example) before the controller is enabled. Altering the register values
while the controller is enabled can produce unpredictable controller behavior, can cause data loss,
and can lock the controller or device.
System software is responsible at reset for optimally configuring SDRAM timing parameters.
The programmable timing parameters apply to both read and write timing configuration. The
configuration process must be completed and the DDR SDRAM initialized before attempting any
accesses to SDRAM.
PRETOACT
Precharge-to-Activate Interval
The number of clock cycles from a precharge command until
an activate or a refresh command is allowed. This interval is
listed in the AC specifications of the SDRAM as t
RP
.
DDR SDRAM Timing Configuration
Register 1 (TIMING_CFG_1)
page 12-38
REFINT
Refresh Interval
Represents the number of memory bus clock cycles between
refresh cycles. One row is refreshed in each SDRAM bank
during each refresh cycle. Depending on
DDR_SDRAM_CFG_2[NUM_PR], some number of rows are
refreshed in each SDRAM bank during each refresh cycle.
The value of REFINT depends on the specific SDRAMs used
and the frequency of the interface. This interval is listed in the
AC specifications of the SDRAM as t
REFI
.
DDR SDRAM Interval Configuration
Register
page 12-49
REFREC
Refresh Recovery Time
The number of clock cycles from the refresh command until an
activate command is allowed. This interval is listed in the AC
specifications of the SDRAM as t
RFC
.
DDR SDRAM Timing Configuration
Register 1 (TIMING_CFG_1)
page 12-38
WR_DATA_DELAY Write Data Delay
Provides different options for the timing between a write
command and the write data strobe. This allows write data to
be sent later than the nominal time to meet the SDRAM timing
requirement between the registration of a write command and
the reception of a data strobe associated with the write
command. The specification dictates that the data strobe may
not be received earlier than 75% of a cycle, or later than 125%
of a cycle, from the registration of a write command. This
parameter is not defined in the SDRAM specification. It is
implementation-specific, defined for the DDR memory
controller in TIMING_CFG_2.
DDR SDRAM Timing Configuration
Register 2 (TIMING_CFG_2)
page 12-40
WRREC
Write Recovery
The number of clock cycles from the last beat of a write until a
precharge command is allowed. This interval, write recovery
time, is listed in the AC specifications of the SDRAM as t
WR
.
DDR SDRAM Timing Configuration
Register 1 (TIMING_CFG_1)
page 12-38
WR_DATA_DELAY Last Write Pair to Read Command.
Controls the number of clock cycles from the last write data
pair to the subsequent read command to the same bank. This
interval, write recovery time, is listed in the AC specifications
of the SDRAM as t
WTR
.
Table 12-8. DDR SDRAM Interface Timing Intervals (Continued)
Timing Intervals
Definition
Register/Page
Summary of Contents for MSC8144E
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