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Technical Supplement
9.7.3
Content Addressable Memory (CAM)
The CPU controls the HSO lines with the CAM. CAM is software controlled and programmed with
events scheduled relative to one of two internal timers.
9.7.4
High Speed Outputs (HSO)
The 6 HSO lines control most of the timing of the LED signal pulse and the demodulation of the
received signal.
9.7.5
Input and Output (I/O)
Input and Output (I/O) are digital lines that are used by the CPU to read in data and output data.
9.7.6
Light-Emitting Diodes (LEDs)
Two LEDs are used in Nellcor oximetry sensors. Light is transmitted through body tissue and received
by a photodetector circuit that converts it to photocurrent. The two wavelengths, which are used for
calculation of pulse rate and oxygen saturation in blood, are transmitted at the following frequencies:
•
infrared (IR) light at approximately 915 microns
•
red light at approximately 660 microns
9.7.7
Pulse Width Modulation (PWM)
The three 8-bit PWM outputs can be software controlled; their duty cycle can be changed from
0-255/256 of the total pulse duration. PWM frequency is the crystal frequency of the CPU, which is
10 MHz divided by 1024. The PWMs control the gains within the analog circuit.
9.7.8
RCal
Sensor RCal value is a resistance value specific to an individual sensor. This value is used by the
software during oxygen saturation computations to maximize accuracy.
9.7.9
Real-Time Clock (RTC)
The RTC is used with the optional printer to track time and date for printouts.
9.8
Overall Block Diagram
Exclusive of covers, buttons, and external connectors, the N-20/N-20P consists of three main
components: the main PCB, the auxiliary PCB, and the display assembly and analog shield.
9.8.1
Main PCB
Contains the SpO 2 analog circuitry; the CPU; support memory circuits; sensor circuits for ambient
light, temperature, and battery voltage; the check battery circuit; a serial data port; and some display
control circuits.
9.8.2
Auxiliary PCB
Contains the power supply circuitry; the display driver circuits; the real-time clock; the interface
circuitry for the printer flex circuit board (which is not used unless a printer is present); and audio
output hardware.
9-6
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