NEC PD78214 - Manual (Page 396)

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Chapter 13 Local Bus Interface Function
13
13.5 PSEUDO STATIC RAM REFRESH FUNCTION
13.5.1 Function
The
µPD78214 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
µPD78214 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.
Fig. 13-18 Format of Refresh Mode Register (RFM)
Remark ×: 0 or 1
Note f
CLK
: System clock frequency (f
XX
/2)
RFLV
7
0
6
0
5
RFEN
4
0
3
0
2
RFT1
1
RFT0
0
RFM
RFT1
32/f
CLK
16/f CLK
Note
Specifies refresh pulse output cycle
0
0
RFT0
0
1
64/f CLK
1 0
128/f
CLK
1 1
(2.6 s)
(5.3 s)
(10.7 s)
(21.3 s)
µ
µ
µ
µ
RFLV
Self-refresh operation (REFRQ low level)
Port mode
Controls REFRQ pin output
×
0
RFEN
0
1
Refresh pulse output enabled 1
f CLK = 6 MHz