Epson PX-8 Technical Manual page 109

REV.-A
2.9 A-O Converter
This is an A-D converter which has an analog input multiplexer and an input/output serial inter-
face built in. A reference voltage of +2V is supplied to the converter, and is used to compare the
various analog input voltages for conversion as shown in Fig. 2-88
-5V
0.1 uF
r
VD
Temperature
--~
Ba'to", voltage
-~~o
. 1
(Analog Input)
~2
13
16
External input signal
~
3
VRF
o---+--~
Barcode input signal /
15
Reference input
2
(2.0V)
I>--:'-c+----<~-----J
14
51 5CK
Fig. 2-88
2.9.1 Operation Control
so
C5
R60
10 K!l
6
7CLO R 12
27K.!l
The +2V reference voltage and a clock signal of approximately 400 kHz are fed from external cir-
cuits when the +5V source is supplied, and the converter is activated. Then, to accomplish an ac-
tual A-D conversion, the following sequence of control operations must be externally provided:
1. Channel Selection
After activating a low CS signal, the first channel selection address bit is supplied via the SI
signal line, together with one shift clock pulse from the SCK signal line. Repeating this opera-
tion eight times, with the adress bit changed each time in sequence, causes the complete de-
sired channel address to be set to the shift register in the converter. This then goes high. This
causes the least significant bits of the shift register to be set to the address latch decoder and
the channel is selected according to the two bits as shown in Table 2-16.
Table 2-16 Channel Selection
Bit 0 Bit 1 Analog channel
. -
0 0
o
(Temperature)
1
0
1 (Battery voltage)
0 1 2 (External input)
1 1 3 (Barcode)
2-85