AET Digital Electronics and Microprocessors 5 — Questions and Answers
Question 1: In a microcontroller used in aircraft fuel management, what is the purpose of a watchdog timer?
- To measure fuel flow rate electronically
- To reset the processor if software fails to service the timer within a set interval (Correct answer)
- To delay startup until all systems are initialized
- To synchronize the fuel pump motor speed
Correct answer: To reset the processor if software fails to service the timer within a set interval
A watchdog timer automatically resets the microcontroller if the main program becomes stuck and fails to periodically 'kick' (service) the timer.
Question 2: The hexadecimal number 0xAF converted to decimal is:
- 155
- 175 (Correct answer)
- 165
- 185
Correct answer: 175
0xAF = (10 × 16) + 15 = 160 + 15 = 175 in decimal.
Question 3: What type of memory is typically used to store the boot firmware in an avionics Line Replaceable Unit (LRU)?
- DRAM
- SRAM
- Flash (EEPROM) (Correct answer)
- Magnetic core memory
Correct answer: Flash (EEPROM)
Flash (EEPROM) is non-volatile and retains firmware without power, making it standard for avionics LRU boot code storage.
Question 4: In a tri-state bus architecture used in avionics backplanes, what is the purpose of the high-impedance (Hi-Z) state?
- To increase signal voltage for long bus runs
- To allow a device to disconnect electrically from the bus without causing conflicts (Correct answer)
- To provide a fault indication to the bus controller
- To double the data transfer rate of the bus
Correct answer: To allow a device to disconnect electrically from the bus without causing conflicts
The Hi-Z state effectively disconnects a device's output from the shared bus, preventing bus contention when multiple devices share the same lines.
Question 5: A shift register in a serial-to-parallel converter for avionics data acquisition receives bits MSB first. After clocking in the 8-bit word 10110100, what is bit 0 (LSB) of the parallel output?
- 1
- 0 (Correct answer)
- Undefined until latched
- Equal to the clock frequency
Correct answer: 0
The word 10110100 binary has its LSB (bit 0) as 0, which is the last bit shifted in when transmitting MSB first.
Question 6: Which type of analog-to-digital converter (ADC) is preferred in avionics for highest accuracy at lower sample rates, such as pressure sensor digitization?
- Flash ADC
- Successive approximation ADC
- Delta-sigma ADC (Correct answer)
- Ramp (single-slope) ADC
Correct answer: Delta-sigma ADC
Delta-sigma ADCs achieve very high resolution through oversampling and noise shaping, making them ideal for slow but precise measurements like pressure.
Question 7: In ARINC 629 multi-transmitter data bus used on the Boeing 777, how is bus access controlled?
- A central bus controller grants access tokens to each LRU
- Each terminal uses a decentralized CSMA/CA protocol with periodic and aperiodic transmission (Correct answer)
- Priority levels are hard-wired into each LRU at manufacture
- Time-division multiplexing assigns fixed slots to each transmitter
Correct answer: Each terminal uses a decentralized CSMA/CA protocol with periodic and aperiodic transmission
ARINC 629 uses a decentralized protocol where each terminal autonomously manages periodic and aperiodic data transmission without a central bus controller.
AET Digital Electronics and Microprocessors 5
In a microcontroller used in aircraft fuel management, what is the purpose of a watchdog timer?