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9.6.1
CPU
•
System memory — System memory is external to the CPU and consists of
an 8K × 8 static RAM and a 64K × 16 EPROM.
•
Real-Time Clock (RTC) — The RTC keeps track of date and time, which is
printed on each printout. A lithium battery designed to last up to 5 years
before needing replacement powers the RTC.
•
Audio output — A piezoelectric ceramic beeper is used for audio output.
•
Display control — A high-visibility display provides oxygen saturation and
pulse rate values. An ambient light sensor responds to low-light conditions
and turns on the display backlight.
•
User controls — A On/Off button and a battery-check button. The On/Off
button signals the power control circuit to switch on the power supply. Press
and hold the battery-check button to display a percentage of useful life
remaining in the batteries.
•
Power supply/Power control circuitry — The N-20PA receives power
from 4 "C" cell batteries. The power control circuitry discontinues power to
the unit when the batteries are no longer reliable.
•
Thermal printer — Generates a hard copy of oxygen saturation and pulse
rate values. A sensor monitors ambient temperature and adjusts printer
output to ensure consistent print quality.
Measure
N-20/N-20P Control buttons
Check Battery
Power supply
AUX
& control
PCB
CPU
Main PCB
Memory
&
software
To analog section
Real-time
clock
Figure 9-14: Digital Circuitry Block Diagram
The CPU circuit is illustrated in Figure 9-15, at the end of this section.
The Intel 80C196KC CPU is a 16-bit microcontroller with built-in peripherals
including a serial port, watchdog timer, A/D converter with an 8-input analog
multiplexer, three pulse width modulators, two 16-bit counter/timers, up to 48
I/O lines, and a high speed I/O subsystem.
Ambient
Main
light sensor
PCB
Ambient
Main
AUX
PCB
temp. sensor
PCB
Technical Supplement
AUX PCB
Display
drivers
Audio beeper
AUX PCB
(N-20 only)
Printer
Printer Control button
ON
ADV
D/D
Display
9-13
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