RX62N Datasheet, PDF (7/25 Pages) Renesas Technology Corp – 100 MHz 32-bit RX MCU with FPU, 165 DMIPS, up to 512-Kbyte Flash, Ethernet, USB 2.0

RX62N Group and RX621 Group
16-Bit SDRAM Connection and Access Examples
4.3 Auto Refresh Settings
In this application note, auto refresh is performed for the SDRAM used. Auto refresh must be performed while
observing the auto refresh required interval and auto refresh release cycles as stipulated in the SDRAM data sheet. The
methods for determining the setting values are shown below. Table 5 lists the setting values used.
(1) Auto refresh required interval
The auto refresh required interval can be determined with the following formula.
RFC (auto refresh required interval setting) = (auto refresh required interval/SDCLK period) – 1
Since the SDRAM used in this application note requires that 4096 auto refresh operations be performed every 64 ms,
the auto refresh interval can be determined from the following formula.
Auto refresh required interval = 64 ms/4096
= 15.62 µs
Also, since the SDRAM clock (SDCLK) frequency used in the application note is 48 MHz, the SDCLK period is
1/48 MHz.
Therefore,
RFC (auto refresh required interval setting) = (15.62 µs/(1/48 MHz)) – 1
= 749
= 2EDh
Therefore the auto refresh required interval setting bits (RFC[11:0]) are set to 2EDh.
(2) Auto refresh release cycle
Since the auto refresh period (tRFC) for the SDRAM used in this application note is 66 ns (minimum), the auto
refresh release cycle must meet the following condition.
66 ns (min) ≤ auto refresh release cycle
Since this means that this cycle will be
66 ns/(1/48 MHz) = 3.17 cycles
an auto refresh release cycle count of at least 4 cycles is required.
Therefore the auto refresh release cycle setting bits (REFW[3:0]) are set to 0011b.
Table 5 SDRAM Auto Refresh Control Register (SDRFCR)
Bit Name
Auto refresh required interval setting bits
(RFC[11:0])
Auto refresh cycle/auto refresh release cycle
setting bits (REFW[3:0])
Setting Value
02EDh
0011b
Function
749 cycles
4 cycles
R01AN0585EJ0202 Rev.2.02
Feb 14, 2014
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