32.5.2 DDR-SDRAM Controller Read Cycle
The MPDDRC provides burst access or single access in Normal mode (MPDDRC_MR.MODE = 0). Whatever the access type, the MPDDRC keeps track of the active row in each bank, thus maximizing performance of the MPDDRC.
The DDR-SDRAM devices are programmed with a burst length equal to 8 which determines the length of a sequential data output by the read command that is set to 8. The latency from read command to data output depends on the memory type, as shown in the following table. This value is programmed during the initialization phase (see Product Dependencies, Initialization Sequence).
| Memory Devices | CAS Read Latency |
|---|---|
| Low-power DDR1-SDRAM | 2/3 |
| DDR2-SDRAM | 3 |
To initiate a single access, the MPDDRC checks if the page access is already open. If row/bank addresses match with the previous row/bank addresses, the controller generates a read 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 read command. To comply with DDR-SDRAM timing parameters, additional clock cycles are inserted between precharge/active (tRP) commands and active/read (tRCD) commands. After a read command, additional wait states are generated to comply with CAS latency. The MPDDRC supports a CAS latency delay of 2 or 3 clock cycles. As the burst length is set to 8, in case of a single access or a burst access lower than 8 data requests, it has to stop the burst, otherwise an additional seven or X values could be read. The Burst Stop command (BST) is used to stop output during a burst read. If the DDR2-SDRAM Burst Stop command is not supported by the JEDEC standard, in a single read access, an additional seven unwanted data will be read.
To initiate a burst access, the MPDDRC checks the transfer type signal. If the next accesses are sequential read accesses, reading to the SDRAM device is carried out. If the next access is a read non-sequential access, then an automatic page break can be inserted. 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 read command. If page access is already open, a read command is generated.
To comply with DDR-SDRAM timing parameters, additional clock cycles are inserted between precharge/active (tRP) commands and active/read (tRCD) commands. The MPDDRC supports a CAS latency delay of 2 or 3 clock cycles. During this delay, the controller uses internal signals to anticipate the next access and improve the performance of the controller. Depending on the latency, the MPDDRC anticipates 2 or 3 read accesses. In case of burst of specified length, accesses are not anticipated, but if the burst is broken (border, Busy mode, etc.), the next access is treated as an incrementing burst of unspecified length, and depending on the latency, the MPDDRC anticipates 2 or 3 read accesses.
For the definition of timing parameters, see MPDDRC Configuration Register.
Read accesses to the DDR-SDRAM are burst oriented and the burst length is programmed to 8. The burst length determines the maximum number of column locations that can be accessed for a given read command. When the read command is issued, eight columns are selected. All accesses for that burst take place within these eight columns, meaning that the burst wraps within these eight columns if the boundary is reached. These eight columns are selected by addr[13:3]; addr[2:0] is used to select the starting location within the block.
In case of incrementing burst (INCR/INCR4/INCR8/INCR16), the addresses can cross the 16-byte boundary of the DDR-SDRAM device. For example, 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 0x00. Since the boundary is reached, the burst wraps. The MPDDRC takes into account this feature of the SDRAM device. In case of the DDR-SDRAM device, transfers start at address 0x04/0x08/0x0C. Two read commands are issued to avoid wrapping when the boundary is reached. The last read command may generate additional reading (1 read cmd = 4 DDR words).
To avoid additional reading, it is possible to use the Burst Stop command to truncate the read burst and to decrease power consumption. The DDR2-SDRAM devices do not support the Burst Stop command.