Mallinckrodt Nellcor N-20 Service Manual page 51

9.10.3 Real-Time Clock (RTC)
The RTC keeps track of date and time, which is printed on each printout. The RTC is powered by a
lithium battery designed to last up to 5 years before needing replacement.
9.10.4 Audio Output
A piezoelectric ceramic beeper is used for audio output.
9.10.5 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.
9.10.6 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.
9.10.7 Power Supply/Power Control Circuitry
The N-20/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.
9.10.8 Thermal Printer (N-20P only)
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.
9.10.9 CPU
The CPU circuit is illustrated in Figure 9-14.
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.
When the power supply is first switched on by the power control circuit, the reset generation circuit
holds the CPU RESET pin low for at least 20 ms, then allows the internal pull-up resistor to bring it
high; this assures a good CPU reset.
An internal watchdog timer is enabled and runs continuously. The watchdog timer provides a means
of recovering from a software upset caused by ESD, EMI, etc. If the software does not clear the timer
at least every 64K state-times (13.1 ms), the CPU will drive RESET low, resetting the entire unit. The
reset output by the CPU is only 16 state-times long (3.2 µs). Q22 provides isolation from C65 so the
CPU can drive a good reset to the display control circuit.
Technical Supplement
9-13