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Section 9 Bus State Controller
R01UH0134EJ0400 Rev. 4.00 Page 333 of 2108
Sep 24, 2014
SH7262 Group, SH7264 Group
(9) Relationship between Refresh Requests and Bus Cycles
If a refresh request occurs during bus cycle execution, the refresh cycle must wait for the bus cycle
to be completed. If a refresh request occurs while the bus is released by the bus arbitration
function, the refresh will not be executed until the bus mastership is acquired.
If the external bus does not return the bus for a period longer than the specified refresh interval,
refresh cannot be executed and the SDRAM contents may be lost.
If a new refresh request occurs while waiting for the previous refresh request, the previous refresh
request is deleted. To refresh correctly, a bus cycle longer than the refresh interval or the bus
mastership occupation must be prevented from occurring.
If a bus mastership is requested during self-refresh, the bus will not be released until the refresh is
completed.
(10) Low-Frequency Mode
When the SLOW bit in SDCR is set to 1, output of commands, addresses, and write data, and
fetch of read data are performed at a timing suitable for operating SDRAM at a low frequency.
Figure 9.28 shows the access timing in low-frequency mode. In this mode, commands, addresses,
and write data are output in synchronization with the falling edge of CKIO, which is half a cycle
delayed than the normal timing. Read data is fetched at the rising edge of CKIO, which is half a
cycle faster than the normal timing. This timing allows the hold time of commands, addresses,
write data, and read data to be extended.
If SDRAM is operated at a high frequency with the SLOW bit set to 1, the setup time of
commands, addresses, write data, and read data are not guaranteed. Take the operating frequency
and timing design into consideration when making the SLOW bit setting.