ZOLL Base Powercharger 4x4, Руководство по обслуживанию

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FUNCTIONAL DESCRIPTION
IV-7
Clock Test Circuitry
Clock testing is provided by the R123 and C122 R/C network. As part of its start-up
verification sequence the microprocessor drives the port pin low and then releases it
to charge up to VCC. After a delay of approximately 110 uS it reads the voltage to
determine if it has reached the 2/3 charge point or approximately 3.3 volts. If it
perceives the value to be outside acceptable limits it may be because the crystal
oscillator circuit (Y101, C103, C102) is out of specification. In this event the
microprocessor will revert to a fault condition to prevent improper operation.
VCC Test Circuitry
A VCC test is provided by the 3.3V voltage reference circuit which was presented in
the "Common Circuitry" section. The microprocessor reads the reference value
through its A/D port (U104.14). If it perceives the value to be outside acceptable limits
it may be because either the VCC power is out of specification or the reference device
has failed. In either event the microprocessor will revert to a fault condition to prevent
improper operation.
A series resistor (R136) is provided in line with the 3.3V reference to insure that if a
microprocessor fails within one of the charging compartments that it does not prevent
the others from operating.
Watchdog Circuit
The Watchdog device (U101) provides two functions. First, it acts as a power on
reset circuit to insure that the microprocessor is held RESET until the +5 volt VCC
power supply is stable. The watchdog timer continues to monitor the power supply
and drives RESET high if the VCC supply falls below tolerance (4.62 volts nominal)
anytime during normal operation. Secondly, the device will reset the microprocessor if
it does not receive a "refresh strobe" at its input pin (U101.7) at least once every 250
milliseconds. If a reset condition occurs, the microprocessor will be held reset for a
minimum of 250 mS.
Whenever the microprocessor is held reset by the watchdog, the switchmode power
supply circuitry is also disabled via diode D103 and R117. As current is sourced
through this device, the safety current limiting comparator (U103.13 and U103.14)
turns on causing a "shut down" of the power supply output. Refer to Switchmode
Power Supply Section for a detailed description of the switchmode power supply
control circuitry.
Shut Down Control
The Watchdog device (U101) as described above has the ability to both disable the
microprocessor and shut down power supply operation. The microprocessor also has
the ability to shut down power supply operation in the event that a fault condition is
detected. This control function is provided via inverter U106 and diode D102 via
R117. As described in the previous section, current sourced through the diode
causes the safety current limiting comparator (U103.13 and U103.14) to turn on
producing a shut down of the power supply output.
Power supply shut down also occurs when the battery test load is enabled.