SPRUIE9D May 2017 – May 2024 DRA74P , DRA75P , DRA76P , DRA77P
The EMIF looks at all the commands stored in the command FIFO to schedule commands to the external memory. All commands with the same MTagID on a particular local interface complete in order. The EMIF does not ensure ordering between commands with different MTagIDs or between commands from two local interfaces.
However, the EMIF does maintain data coherency. Therefore, the EMIF blocks a command, regardless of priority or the local interface, if that command is to the same block address (256 bytes) as an older command that is not complete. Thus, the EMIF may have one pending read or write for each MTagID. For information about MTagID, see Section 16.1.1, Terminology in Chapter 16, Interconnect. Among all pending reads, the EMIF selects all reads that have their corresponding SDRAM banks already open. Similarly, among all pending writes, the EMIF selects all writes that have their corresponding SDRAM banks already open. Accesses to memory mapped registers are treated as accesses that have open banks.
As a result of this reordering, the EMIF may now have several pending reads and writes that have their corresponding banks open. The EMIF then selects the highest priority read from pending reads, and the highest priority write from pending writes. If two or more commands have the highest priority, the EMIF selects the oldest command. As a result, the EMIF may now have the next read and a write command. If the return command FIFO and the read data FIFO have space and the external bus conflict is resolved, the EMIF performs the final read command before the final write command. If the return command FIFO has space but the read data FIFO is full, the EMIF performs the final write command before the final read command. Resolution of external bus conflict means all the SDRAM command-to-command counters are satisfied and the read-to-write or write-to-read turnaround time is met.
The EMIF does not support tag interleaving. In other words, for an local interface, the EMIF completes executing an local command before it switches to another command. The EMIF can, however, interleave execution between commands from two local interfaces.
The data coherency inside the EMIF is ensured only in a single level of local infrastructure. For example, if a write from a secondary local bus segment is blocked by a bridge element, the read from a tertiary bus can still beat the write to the EMIF. In such a case, to confirm that a write from master A has landed before a read from master B is performed, master A must wait for the write status from the EMIF before indicating to master B that the data is ready to be read. If master A does not use the local wait status, it must do the following:
Apart from reads and writes, the EMIF must also open and close SDRAM banks and maintain the refresh counts for an SDRAM. The priority of SDRAM commands with respect to refresh levels are:
To avoid continuous blocking effect which can be caused by a continuous stream of high-priority commands which thus block the lower priority commands, the EMIF momentarily raises the priority of the oldest command over all other commands when the time for the oldest command configured through the EMIF_COS_CONFIG[7:0] PR_OLD_COUNT bit field expires.
It should be taken into account that while performing the scheduling algorithm described, the EMIF may also encounter a condition in which continuous stream of SDRAM commands to a row in an open bank can block commands to another row in the same bank.
In addition to this scheduling, the highest priority condition is a reset command. If this condition occurs, the EMIF abandons what it is currently doing and begins its start-up sequence. In this case, commands and data stored in the FIFOs are lost. The EMIF also starts its start-up sequence whenever the EMIF_SDRAM_CONFIG register is written and the EMIF_SDRAM_REFRESH_CONTROL[31] INITREF_DIS bit is set to 0. In this case, commands and data stored in the FIFOs are not lost. The EMIF ensures that in-flight read or write transactions to the SDRAM are complete before starting the initialization sequence.
All the accesses to an SDRAM are pipelined to maximize use of the external bus. All of these are done while fulfilling the access timing requirements of an SDRAM.