Epson PX-8 Technical Manual page 72

2.3.4 Operations of the 7508 Sub-CPU
2.3.4.1 Operation During Power Off
REV.-A
During power off, the 7508 sub-CPU performs two important functions; it monitors pin 23 at port
00, checking for a change of state indicating the power switch has been turned on; it also moni-
tors battery voltage every 10 seconds to assure that there is adequate power supply to maintain
data in RAM and to provide backup power for the main battery.
• Battery voltage monitoring
The sub-CPU 7508 enables port 23 (pin 5) every 10 seconds for a period of approximately 7.5
ms in order to supply the Vcc source to the A-D converter. If then turns on port 21, pin 3, (low)
to put the A-D converter in the interface mode. One-byte serial data is then output with a series
of eight shift clock pulses over the SI and SO signal lines respectively to select the A-D con-
verter channel for battery voltage check (AN 1). After channel selection data is issued, the 7508
returns port 21 from low to high to change the A-D converter mode from interface to A-D con-
version. This causes the converter to initiate an A-D conversion through the specified cnannel.
The converted result is stored in the shift register in the converter. After returning port 21 low
(the interface mode), the 7508 issues shift clock pulses to read in the digital data, bit by bit.
This data is examined to determine the main battery voltage. If the voltage is found to be below
a certain level, the battery voltage line is backed up from the auxiliary battery by using a port 42.
2.3.4.2 Operation While Power Is On
While power is on, the 7508 sub-CPU performs the following functions:
• Keyboard Control- Controls the keyboard matrix using key scan signals (KSCO-8) and key re-
turn signals (KRTNO-7)
• DIP Switch (SW1) Monitoring - Checks DIP switch 1 during initialization using signals KSC9
and KRTN1-7.
• Battery Voltage Monitoring Support Functions - Includes such operations as main CPU inter-
rupt via gate array GAH40M (Low voltage in-
terrupt) and back-up power provision via the
auxially battery.
• Analog Trigger Signal Detection - Detects a triggering signal in the analog input interface to
control the A-D converter and read analog input data.
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