Epson PX-8 Technical Manual page 140

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-
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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