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7721 Datasheet(PDF) 388 Page - Renesas Technology Corp

Part # 7721
Description  16-BIT SINGLE-CHIP MICROCOMPUTER
PDF  577 Pages
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Manufacturer  RENESAS [Renesas Technology Corp]
Direct Link  http://www.renesas.com
Logo RENESAS - Renesas Technology Corp

7721 Datasheet(HTML) 388 Page - Renesas Technology Corp

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14-5
7721 Group User’s Manual
DRAM CONTROLLER
14.2 Block description
14.2.2 Refresh timer
The refresh timer is an 8-bit timer with a reload register and is used to generate refresh requests for DRAM
data. Assuming that the set value of the refresh timer = “n,” the refresh timer counts f16 (n + 1) times. Figure
14.2.4 shows the structure of the refresh timer, and the following formula gives the value to be written to
the refresh timer.
n = {m [
µs] !
} – 1
n: a set value of the refresh timer (n = 0116–FF16)
m: a refresh interval
Examples of “m”: an average of 15.625
µs for 512 refresh cycles at 8-ms intervals
an average of 125
µs for 512 refresh cycles at 64-ms intervals
Fig. 14.2.4 Structure of refresh timer
b7
b0
Refresh timer (Address 6616)
Functions
Bit
At reset
RW
7 to 0
These bits can be set to “0116” to “FF16.”
Assuming that the set value = n, this register divides f16 by
(n + 1).
Undefined
WO
Note: Use the LDM or STA instruction for writing to this register.
Do not set this register to “0016.”
f(XIN)
16



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