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14-10
7721 Group User’s Manual
DRAM CONTROLLER
DMAC
CPU
14.4.2 Refresh request
When the DRAM validity bit is set to “1,” the refresh timer starts counting down. The count source is
f16.
When the contents of the refresh timer reach “0016,” a refresh request occurs. The refresh timer reloads
the contents of address 6616 and continues counting.
Refresh requests are sampled as bus requests (DRAMC) by using the bus access controller.
As soon as a refresh request is acknowledged by sampling, the following is performed because DRAM
refresh has the highest priority in using the bus.
However, when the CPU or DMAC uses the bus, no bus request is sampled until the CPU or DMAC
releases the bus.
Therefore, in a period from when a refresh request occurs until DRAM refresh is performed, the delay
listed in Table 14.4.1 occurs depending on the refresh request generating timing.
Figures 14.4.2 and 14.4.3 show refresh delay time examples when CPU is operating and during DMA
transfer. For a bus request, refer to “13.2.1 Bus access control circuit.”
When the refresh request is accepted, the right to use the bus is passed to DRAM refresh (1 cycle of
φ). Both of the output levels of ST1 and ST0 are “L.” (The bus status is indicated as [0, 0].)
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The RAS and the CAS signals are output and the DRAM data is refreshed (refresh cycle: 3 cycles of
φ).
The right to use the bus is passed to the CPU, DRAM or Hold (1 cycle of
φ).
The outputs of ST1 and ST0 change.
Note: In Stop or Wait mode, DRAM refresh is not performed because no refresh request occurs.
Table 14.4.1 Delay time from when refresh request occurs until DRAM refresh is performed
14.4 DRAMC operation
Hold
Maximum (with Wait)
Maximum (no Wait)
Minimum
1.5
4.5
8.5
11.5
6.5
1.5
6.5
12.5
15.5
6.5
1.5
Source of using bus
Array state
Transfer (a unit of 1 transfer)
Transfer (a unit of 1 transfer) + Complete cycle
Delay time (unit:
φ cycle)
Note: The above is applied when Ready is not used. The delay time includes the time for passing the right
to use buses to DRAM refresh (1 cycle).