12-13
•
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 measure button and a battery-check button. The measure 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-20P 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 hardcopy of oxygen saturation and pulse rate values. A sensor monitors
ambient temperature and adjusts printer output to ensure consistent print quality.
Measure
Check Battery
Power Supply
& Control
CPU
Main PCB
Memory
&
Software
Ambient
Light Sensor
Real-time
Clock
Ambient
Temp. Sensor
ON ADV D/D
Printer
Audio Beeper
Display
Drivers
Display
N-20/N-20P Control Buttons
AUX
PCB
Main
PCB
To Analog Section
AUX
PCB
Main
PCB
AUX PCB
AUX PCB
(N-20P only)
Printer Control Buttons
Figure 12.4 Digital Circuitry Block Diagram
CPU
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.
The CPU is capable of running up to 16 MHz; but, it is run at 10 MHz for decreased power consumption. All
unused inputs are tied to either Vcc or ground through resistors – this prevents unused inputs floating to any
voltage and causing excess power drain. The READY input pin is tied high, thereby disabling wait-state
generation; all bus accesses are zero-wait state. The EA pin is tied low to enable addressing of the external
EPROM.