What is the difference between volatile and const?
Const = keyword for constant variable and their purpose is to be read-only
Volatile= keyword for variables that can be modified outside normal program flow. Such as in interrupts.
- Compiler doesn’t optimise these variables
Why is volatile important in embedded systems?
Volatile is important to prevent the compiler from optimising away reads which causes incorrect behaviour
- hardware registers can change automatically
- interrupts can modify variables asynchronously
Difference between stack and heap?
Stack = fast and automatically managed
- used for local variables and function calls
Heap = slow and manually allocated/freed
What is segmentation fault?
Illegal access of memory
- occurs when a program tries to access memory it’s not allowed to.
- usually causes the operating system to terminate the program
Explain pointers and pointer arithmetic
Pointers are variables that hold a memory address that points towards another variable
- pointer arithmetic modifies the memory address by the size of the data type
Pass-by-value vs pass-by-reference
Value = pass a direct copy of a variable’s value into a function
Reference = memory address of the variable is passed into a function
What is a watchdog timer?
A timer that resets the MCU if the firmware hangs or doesn’t “feed the watchdog” in time
GPIO input vs output
Input = purpose is to read signals
Output = drive signals
What is an interrupt?
A hardware or software event that halts the program to handle the event in an ISR
Polling vs interrupts
Polling = blocks the cpu until an event occurs
- less efficient
Interrupts = event driven, more efficient and real-time
What is debouncing?
Removing mechanical switch noise
Can be done by:
- State machine
- software filter
- hardware RC filter
- delay
What causes debouncing?
When a button is pressed, the physical contacts vibrate and make/break the electrical connection multiple times
- the microcontroller may read these bounces as multiple presses.
How does a delay fix debouncing?
- you record the initial state change and wait a few milliseconds
- if the switch’s state is the same after the delay the input is valid
How does a state machine prevent debouncing?
- the state machine will have two states idle and debouncing
START: Idle - once the button is pressed. GO TO: debounce state
DEBOUNCE: if the value of the button changes and the timer ends while in debounce stay in debounce
- if the timer ends without a new value from the button finish debouncing and go back to IDLE
How can a software filter prevent debouncing?
When a change in the input signal is detected:
- the software waits for a short period (has to be longer than the bounce duration)
- if the signal is still in the new state it is valid.
How does an RC circuit prevent debouncing
- Specifically use a RC low-pass filter
- purpose is to make the signal into a clean and gradual signal
What is PWM used for?
- controlling motors
- LEDs
- brightness of lights
- servos
- efficient power supplies
Does this by varying duty cycle
How does PWM work?
Constantly turns ON and OFF the power supply at a set frequency
Duty cycle = ratio of the “on” time to the total period of the pulse
How does an ADC work?
ANALOGUE-TO-DIGITAL CONVERTER
- converts an analogue signal into digital values by taking samples at a specific sampling rate.
What does mutable mean?
A data structure that can be changed
What does immutable mean?
A type of data structure that can’t be changed
