Atmel SAM9X25, Руководство пользователя

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429
11054A–ATARM–27-Jul-11
SAM9X25
429
11054A–ATARM–27-Jul-11
SAM9X25
30.5 Functional Description
30.5.1 SDRAM Controller Write Cycle
The DDRSDRC allows burst access or single access in normal mode (mode = 000). Whatever
the access type, the DDRSDRC keeps track of the active row in each bank, thus maximizing
performance.
The SDRAM device is programmed with a burst length equal to 8. This determines the length of
a sequential data input by the write command that is set to 8. The latency from write command to
data input is fixed to 1 in the case of DDR-SDRAM devices. In the case of SDR-SDRAM
devices, there is no latency from write command to data input.
To initiate a single access, the DDRSDRC checks if the page access is already open. If
row/bank addresses match with the previous row/bank addresses, the controller generates a
write command. If the bank addresses are not identical or if bank addresses are identical but the
row addresses are not identical, the controller generates a precharge command, activates the
new row and initiates a write command. To comply with SDRAM timing parameters, additional
clock cycles are inserted between precharge/active (t RP) commands and active/write (t RCD)
command. As the burst length is fixed to 8, in the case of single access, it has to stop the burst,
otherwise seven invalid values may be written. In the case of SDR-SDRAM devices, a Burst
Stop command is generated to interrupt the write operation. In the case of DDR-SDRAM
devices, Burst Stop command is not supported for the burst write operation. In order to then
interrupt the write operation, Dm must be set to 1 to mask invalid data (see
Figure 30-2 on page
430 and Figure 30-5 on page 431 ) and DQS must continue to toggle.
To initiate a burst access, the DDRSDRC uses the transfer type signal provided by the master
requesting the access. If the next access is a sequential write access, writing to the SDRAM
device is carried out. If the next access is a write non-sequential access, then an automatic
access break is inserted, the DDRSDRC generates a precharge command, activates the new
row and initiates a write command. To comply with SDRAM timing parameters, additional clock
cycles are inserted between precharge/active (tRP) commands and active/write (tRCD)
commands.
For a definition of timing parameters, refer to Section 30.7.4 “DDRSDRC Timing Parameter 0
Register” on page 457 .
Write accesses to the SDRAM devices are burst oriented and the burst length is programmed to
8. It determines the maximum number of column locations that can be accessed for a given write
command. When the write command is issued, 8 columns are selected. All accesses for that
burst take place within these eight columns, thus the burst wraps within these 8 columns if a
boundary is reached. These 8 columns are selected by addr[13:3]. addr[2:0] is used to select the
starting location within the block.
In the case of incrementing burst (INCR/INCR4/INCR8/INCR16), the addresses can cross the
16-byte boundary of the SDRAM device. For example, in the case of DDR-SDRAM devices,
when a transfer (INCR4) starts at address 0x0C, the next access is 0x10, but since the burst
length is programmed to 8, the next access is at 0x00. Since the boundary is reached, the burst
is wrapping. The DDRSDRC takes this feature of the SDRAM device into account. In the case of
transfer starting at address 0x04/0x08/0x0C (DDR-SDRAM devices) or starting at address
0x10/0x14/0x18/0x1C, two write commands are issued to avoid to wrap when the boundary is
reached. The last write command is subject to DM input logic level. If DM is registered high, the
corresponding data input is ignored and write access is not done. This avoids additional writing
being done.