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Self-Refreshing
Self-refresh mode is a kind of standby mode in which the refresh timing and refresh addresses
are generated within the synchronous DRAM. Self-refreshing is activated by setting both the
RMODE bit and the RFSH bit to 1. The self-refresh state is maintained while the CKE signal
is low. Synchronous DRAM cannot be accessed while in the self-refresh state. Self-refresh
mode is cleared by clearing the RMODE bit to 0. After self-refresh mode has been cleared,
command issuance is disabled for the number of cycles specified by the TPC bits in MCR.
Self-refresh timing is shown in figure 10.29. Settings must be made so that self-refresh
clearing and data retention are performed correctly, and auto-refreshing is performed at the
correct intervals. When self-refreshing is activated from the state in which auto-refreshing is
set, or when exiting standby mode other than through a power-on reset, auto-refreshing is
restarted if RFSH is set to 1 and RMODE is cleared to 0 when self-refresh mode is cleared. If
the transition from clearing of self-refresh mode to the start of auto-refreshing takes time, this
time should be taken into consideration when setting the initial value of RTCNT. Making the
RTCNT value 1 less than the RTCOR value will enable refreshing to be started immediately.
After self-refreshing has been set, the self-refresh state continues even if the chip standby state
is entered using the SH7709S’s standby function, and is maintained even after recovery from
standby mode other than through a power-on reset. In case of a power-on reset, the bus state
controller’s registers are initialized, and therefore the self-refresh state is cleared.
Self-refreshing is performed in normal operation, in sleep mode, in standby mode, and in case
of a manual reset.
When using synchronous DRAM, use the following procedure to initiate self-refreshing.
1. Clear the refresh control bit to 0.
2. Write H'00 to the RTCNT register.
3. Set the refresh control bit and refresh mode bit to 1.
Summary of Contents for SH7709S
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