Passport 2®/Passport 2 LT™ Service Manual 0070-10-0441 2 - 25
Theory of Operation NIBP Module (P/N 0997-00-0501)
The IRF7303 MOSFET part (Q6), was chosen due to its lead-frame design which offers
superior power dissipation capabilities. As such, the maximum junction rise when driving the
0.526W, 274 ohm linear valve coil under the worst case conditions, does not warrant any
heat-sinking to achieve reasonable reliability.
2.4.8 Primary Microcontroller, U10
The NIBP PCB utilizes a Motorola MC68HC16Z1CPV16 microcontroller. This specifies a 5V,
16.78MHz, commercial temperature range part in a 144 pin TQFP package. This processor
is responsible for controlling a pump and two valves in the generation of the patient pressure
signal. This data is processed by the HC16Z1 and fed to the host via an RS-485 interface
through the processor’s SCI port.
A 32.768kHz crystal (Y1) is used for the HC16Z1 reference oscillator circuit. The HC16Z1
SYNCR (clock synthesizer control) register will be set with X=1, W=0 and Y=60. This
produces a nominal system clock frequency of 15.99MHz. The 32.768kHz crystal is
specified with a load capacitance of 20pF. To present this load, C20 and C21 are set to
33pF. The load capacitance is calculated as the series combination of C21 and C22 plus the
stray capacitance which is estimated to be less than 5pF.
At Power-On Reset, the state of the HC16Z1 pins controlling the pump and valves is such that
they are disabled (pump off / valves open).
The HC16Z1 has an internal ADC which will be used to monitor both pressure transducer
outputs, a 2.5V reference source, A5V, 12V and the switched 12V voltage, 12VSW.
Dividers are used where needed to insure the sampled voltage is within the usable ADC input
range. A filtered version of T5V is used as the ADC reference.
2.4.9 Secondary Microcontroller, U4
The PCB incorporates a Microchip Technologies PIC16LC710 to perform over-pressure
sensing.
A 2.45MHz ceramic resonator (Y2) with built-in capacitors is used for the PIC16 oscillator.
The PIC16 is informed of the desired measurement mode by reading the MODE2:0 signals
fed from the HC16Z1. It in turn sends back three signals, STATE2:0, to indicate that the
PIC16 has not encountered any faults, has set its over-pressure and measurement duration
limits to match the type of measurement indicated, and is ready for the measurement of the
type encoded on STATE2:0.
The over-pressure signal (PVO) is sampled by an ADC internal to the PIC16 at a rate of
1.196kHz. If it detects an abnormal condition, the PIC16 goes into an emergency shutdown
state. In this state, the PIC16 turns off the 12VSW power feeding the pump and valves by
toggling the 12VEN* signal line high. Since the valves are normally open, turning off their
power will vent any cuff pressure to atmosphere. The PIC16 then remains in the emergency
shutdown state until the system’s power is cycled. Note, a software reset issued from the
HC16Z1 and asserting RST* will have no effect on releasing the PIC16 from the shutdown
state.
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