RX62N Group and RX621 Group

16-Bit SDRAM Connection and Access Examples

R01AN0585EJ0202 Rev.2.02

Page 7 of 22

Feb 14, 2014

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

Setting Value

Function

Auto refresh required interval setting bits

(RFC[11:0])

02EDh

749 cycles

Auto refresh cycle/auto refresh release cycle

setting bits (REFW[3:0])

0011b

4 cycles