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II CORE BLOCK: BCU (Bus Control Unit)
S1C33L03 FUNCTION PART EPSON B-II-4-31
A-1
B-II
BCU
DRAM Refresh Cycles
The DRAM interface supports a CAS-before-RAS refresh cycle and a self-refresh cycle.
CAS-before-RAS refresh cycle
Before performing a CAS-before-RAS refresh, set RPC2 to "1" while RPC1 = "0" in order to enable the
DRAM refresh function. Once this is done, the BCU executes a CAS-before-RAS refresh by using the
underflow signal that is output by the 8-bit programmable timer 0 as a trigger. Therefore, refresh generation
timing can be programmed using the internal prescaler and 8-bit programmable timer 0.
For details on how to control the prescaler and 8-bit programmable timer 0, refer to "Prescaler and Operating
Clock for Peripheral Circuits", and "8-Bit Programmable Timers".
Example: RPC delay: 1 cycle; Refresh RAS pulse width: 2 cycles; Precharge: 1 cycle
CAS-before-RAS refresh cycle
Refresh
RPC delay
Fixed at
1 cycle
Refresh
RPC pulse width
Precharge
cycle
BCLK
#RAS
#HCAS/
#LCAS
Figure 4.36 CAS-Before-RAS Refresh
When the refresh cycle is terminated, the #HCAS/#LCAS signal boot timing is 0.5 cycles before that of
#RAS. Consequently, the pulse width of #HCAS/#LCAS is determined by the refresh RAS pulse width that
was set using RRA. The number of precharge cycles after the refresh cycle is defined by the value that was
set using RPRC, the same value that is used for both random cycles and page mode accesses.
Self-refresh
To support DRAM chips equipped with a self-refresh function, the BCU has a function to generate a self-
refresh cycle.
To start a self-refresh cycle, set RPC2 to "1" after setting RPC1 to "1". To deactivate a self-refresh cycle,
write "0" to RPC1 or RPC2.
Example: RPC delay: 1 cycle
Self-refresh mode set up
Self-refresh mode
deactivationSelf-refresh mode
Refresh
RPC delay
Fixed at
1 cycle
Precharge cycle
(6 cycles)
BCLK
#RAS
#HCAS/
#LCAS
Figure 4.37 Self-Refresh
For a self-refresh function as well, the RPC delay is determined by setting RPC0 in the same way as for a
CAS-before-RAS refresh.
The refresh RAS pulse width is determined by the timing at which the refresh is deactivated in software and
is unaffected by settings of RRA.
#RAS and #HCAS/#LCAS are booted up simultaneously upon completion of a self-refresh and the precharge
duration that follows is fixed at 6 cycles.