REV.-A
2.14.2
O-RAM
Refresh
by
Sub-CPU
The O-RAM refresh feature is enabled whenever the O-RAM is
not
being accessed
by
the main
CPU.
The feature refreshes the O-RAM using
two
signals: OW and OCAS. Refresh is performed
in one
of
three modes, automatically selected depending on ambient temperature.
1)
Operation modes
The Sub-CPU
7508
monitors
the ambient temperature,
which
is read
as
a resistance variation
of
a
thermistor
connected
to
channel ANO
of
the A-O converter
7001,
and
an
internal control
program determines one
of
three predetermined temperature ranges
into
which
the read valu
falls. The sub-CPU sets the states
of
ports
72
and
73
according
to
the selected temperature
range. The range is indicated
to
the gate array,
GAH400,
which
internally generates
two
re-
--
--
fresh control signals CAS and WE, and supplies them
to
the O-RAM chips. Table
2-24
sum-
marizes the relashionship between the modes and the control signals.
Table
2-24
O-RAM
Refresh
Mode
and
Contrl
Signals
Refresh
Ambient
Control signals
to
O-RAM
Control signals from
7508
to
GAH40D
mode
temperature
range
rC)
CAS
WE
OCAS
OW
TH
45
~
H
H
0
0
TM
25
~
45
H L
0
1
TL
o
~
25
L
-Gh:!
1
1
2.14.3
O-RAM
Refresh
by
Main
CPU
The main
CPU
refreshes the O-RAM
by
issuing the
lower
serven refresh address bits and the
MREQ and
RF
signals after each operation code fetch during the
M1
cycle. The actual waveforms
of
the related signals are
shown
in
the following:
2-116
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