Editing Synchronous dynamic random-access memory (section)

== Command interactions ==
The no operation command is always permitted, while the load mode register command requires that all banks be idle, and a delay afterward for the changes to take effect. The auto refresh command also requires that all banks be idle, and takes a refresh cycle time t<sub>RFC</sub> to return the chip to the idle state. (This time is usually equal to t<sub>RCD</sub>+t<sub>RP</sub>.) The only other command that is permitted on an idle bank is the active command. This takes, as mentioned above, t<sub>RCD</sub> before the row is fully open and can accept read and write commands.

When a bank is open, there are four commands permitted: read, write, burst terminate, and precharge. Read and write commands begin bursts, which can be interrupted by following commands.

=== Interrupting a read burst ===
A read, burst terminate, or precharge command may be issued at any time after a read command, and will interrupt the read burst after the configured CAS latency. So if a read command is issued on cycle 0, another read command is issued on cycle 2, and the CAS latency is 3, then the first read command will begin bursting data out during cycles 3 and 4, then the results from the second read command will appear beginning with cycle 5.

If the command issued on cycle 2 were burst terminate, or a precharge of the active bank, then no output would be generated during cycle 5.

Although the interrupting read may be to any active bank, a precharge command will only interrupt the read burst if it is to the same bank or all banks; a precharge command to a different bank will not interrupt a read burst.

Interrupting a read burst by a write command is possible, but more difficult. It can be done if the DQM signal is used to suppress output from the SDRAM so that the memory controller may drive data over the DQ lines to the SDRAM in time for the write operation. Because the effects of DQM on read data are delayed by two cycles, but the effects of DQM on write data are immediate, DQM must be raised (to mask the read data) beginning at least two cycles before write command but must be lowered for the cycle of the write command (assuming the write command is intended to have an effect).

Doing this in only two clock cycles requires careful coordination between the time the SDRAM takes to turn off its output on a clock edge and the time the data must be supplied as input to the SDRAM for the write on the following clock edge. If the clock frequency is too high to allow sufficient time, three cycles may be required.

If the read command includes auto-precharge, the precharge begins the same cycle as the interrupting command.