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Chapter 31 External Bus
2.External Bus Interface Registers
When read by a Read - modify - Write instruction, the SELF, RRLD, and PON bits always return to 0.
(Bit 30) RRLD (Refresh counter ReLoaD): Refresh counter start control
This bit is used to start and reload the fresh counter.
Table 4.2-43 shows the function of refresh counter startup control.
The refresh counter is inactive in the initial state.
If this bit is set to 1 in this state, all the SDRAM areas currently enabled in the CSER are auto - refreshed either
once in distributed refresh mode or the RFC - specified number of times in centralized refresh mode. After that,
the values in the RFINT5 to RFINT0 bits are reloaded.
From then on, the refresh counter starts being decremented. Whenever the counter causes an underflow from
000000
B
, repeatedly, the values in the RFINT5 to RFINT0 bits are reloaded while at the same time auto -
refreshing is performed once.
The bit returns to 0 upon completion of reloading.
To stop auto - refreshing, write 000000
B
to the RFINT5 to RFINT0 bits.
When read by a Read - modify - Write instruction, the bit always returns a zero.
[Bits 29 - 24] RFINT5 to RFINT0 (ReFresh INTerval): Auto - refresh interval
Set these bits to the interval for automatic refreshing.
The auto - refresh interval can be obtained for distributed refresh mode {(REFINT5 - REFINT0 value) x 32 x
(external bus clock cycle)} or for centralized refresh mode {(REFINT5 - REFINT0 value) x 32 x (RFC specified
number of times) x (external bus clock cycle)}
Calculate the design value in consideration of the maximum RAS active time.
The refresh counter keeps on being decremented even while the auto - refresh command is being issued.
[Bit 23] BRST (BuRST refresh select): Burst refresh control
This bit is used to control the operation mode for auto - refreshing.
Table 4.2-44 shows the function of burst refresh control.
When distributed refreshing is set, the auto - refresh command is issued once at every refresh interval.
When burst refreshing is set, the auto - refresh command is issued continuously for the number of times set in the
refresh counter at every refresh interval.
Table 2-27 Function of refresh counter startup control
RRLD Refresh counter startup control
0 Disable (no operation)
1 Execute auto - refreshing once and reload the RFINT value.
Table 2-28 Function of burst refresh control
BRST Burst refresh control
0 Distributed refresh (Auto - refresh is activated at intervals.)
1 Burst refresh (Auto - refresh is activated repeatedly at one time.)