Datascope Passport 2, Руководство по обслуживанию

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Passport 2®/Passport 2 LT™ Service Manual 0070-10-0441 2 - 51
Theory of Operation Passport 2 NIBP Module (P/N 0670-00-0730 or 0670-00-0746-01)
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.
2.15.12 Secondary Microcontroller, U4
The PCB incorporates an Atmel AVR microcontroller to perform over-pressure sensing. The
Atmel AT90S4433-8AC is an 8-bit RISC architecture AVR microcontroller with 4Kbytes of in-
system programmable FLASH, 128 bytes of RAM and 256 bytes of EEPROM. The device
contains 32 byte-wide general-purpose registers, two timer/counters, a 10-bit ADC with 6
input channels, programmable Watchdog, SPI Serial interface and 20 individually
programmable I/O lines. The device comes in a 32-pin TQFP package and operates
between 4 to 6 volts at speeds up to 8 MHz.
A 2.45MHz ceramic resonator (Y2) with built-in capacitors is used for the AVR’s oscillator.
The AT90S4433 is informed of the desired measurement mode by reading the MODE_IN3:0
signals fed from the HC16Z1. It in turn sends back four signals, STATE_OUT3:0, to indicate
that the AT90S4433 has not encountered any faults, has set its over-pressure and
measurement duration limits to match the type of measurement indicated, has completed the
offset reading (zero) and is ready for the measurement of the type encoded on
STATE_OUT3:0.
The over-pressure signal (PVO) is sampled by an ADC internal to the AT90S4433 at a rate of
153.125kHz. If it detects an over-pressure condition, the AT90S4433 goes into an
emergency shutdown state. In this state, the AT90S4433 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 AT90S4433 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
AT90S4433 from the shutdown state.
The ADC uses an external reference voltage (T5V). Since T5V is the same voltage source that
is powering the ratiometric transducer, direct ADC readings represent the pressure.
In addition to the input signals MODE_IN3:0 and PVO, the AT90S4433 also has the 2.5V
reference voltage (25VR) applied to its A/D inputs. This signal is used as a ‘back-door’
verification of the T5V reference voltage. Since 25VR is generated from a separate supply
than T5V, and since T5V is also used as the ADC reference voltage, if the T5V reference
voltage were to fail, the ADC would read full scale when comparing 25VR to the ADC
reference (T5V).
The AT90S4433 device includes a brown-out reset circuit which guarantees the device is
reset when VCC is below 4.0V +/-0.2V. This brown-out reset is required since the
AT90S4433 is not connected to the HC16Z1 RST*.