Chapter 3
Theory of Operation
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BiPAP Vision Service Manual © Respironics, Inc.
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3.2.4
Watchdog and Power On Reset Circuit
The microcontroller generates pulses that toggle the watchdog timer circuit
when the system is running. If watchdog circuit is not toggled within a 70-ms
period, the microcontroller resets. The microcontroller monitors watchdog
status, and clears the reset when it receives a watchdog timer reset signal. The
system does not shut down in case of a watchdog timeout.
The power on reset circuit monitors the logic supply voltage through a voltage
divider circuit. A separate 1.3-V threshold detector for this power fail warning.
If a low supply voltage level is detected, the microcontroller resets.
3.2.5
Error Line Control Circuit (ELC)
The ELC includes ELC #1, ELC #2, and a 2.38-V reference monitor.
•
ELC #1 is designed to allow normal operation only when the PC and
DC boards are connected. Removing either board or a microcontroller
error signal cause an ELC fault condition.
•
ELC #2 includes pulse generators and pulse detectors on the PC and
DC boards, and an ELC fault condition occurs if a minimum number of
missing pulses occur.
•
The 2.38-V reference monitor includes a window comparator that
monitors the reference signal and alerts the microcontroller if the
signal is out of tolerance.
3.2.6
Microcontroller Interface
The microcontroller interface includes:
•
Microcontroller: the MC board uses a microcontroller with a crystal
oscillator. A phase-locked loop in the microcontroller generates an
internal clock signal that helps to reduce EMI interference. The
microcontroller includes 7 interrupt lines and 7 chip select lines.
•
Memory: a memory decoder PAL decodes the microcontroller chip
selects. Program code is stored in EPROM and data is stored in
nonvolatile, static RAM.
•
ICB interface: a PAL provides the logic that controls the ICB interface
•
Serial interface: +5 V logic voltage is converted into +10-V RS-232
voltages through transmit (TXD) and receive (RXD) lines.