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13.5 PSEUDO STATIC RAM REFRESH FUNCTION
13.5.1 Function
The µ PD78214 provides the pseudo static RAM refresh function to enable pseudo static RAM to be connected
directly.
The pseudo static RAM refresh function outputs refresh pulses at arbitrary intervals. The refresh pulse output cycle
period is specified in the refresh mode register (RFM), and the external access cycle is changed to the refresh bus
cycle that matches the pseudo static RAM bus cycle. (The write pulse width becomes shorter by half a clock pulse
than that observed when pseudo static RAM is not connected.)
The µ PD78214 provides a function to support self-refresh and thus reduce the power dissipation of pseudo static
RAM application systems.
13.5.2 Refresh Mode Register (RFM)
The RFM register is an 8-bit register that controls the refresh cycle of pseudo static RAM and switching to self-
refresh.
The register can be read and written with 8-bit manipulation instructions and bit manipulation instructions. Fig.
13-18 shows the format of the register.
When the RESET signal is applied, the register is set to 00H. The REFRQ pin is set to port mode such that it functions
as pin P67.
7
RFM
RFLV
Fig. 13-18 Format of Refresh Mode Register (RFM)
6
5
4
3
2
0
0
RFEN
0
0
RFT1
0
0
1
1
RFLV
×
0
1
Chapter 13 Local Bus Interface Function
1
0
RFT1
RFT0
RFT0
Specifies refresh pulse output cycle
µ
Note
0
16/f
(2.6 s)
CLK
µ
1
32/f
(5.3 s)
CLK
µ
0
64/f
(10.7 s)
CLK
µ
1
128/f
(21.3 s)
CLK
RFEN
Controls REFRQ pin output
0
Port mode
Self-refresh operation (REFRQ low level)
1
Refresh pulse output enabled
Remark ×: 0 or 1
Note f
: System clock frequency (f
CLK
f
= 6 MHz
CLK
/2)
XX
367
13
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