Microcontrollers

Question 1

Data Direction Register (DDR)

Answer 1

 read/write

 specifies for each bit of the corresponding port whether it is an input or an output bit

Question 2

Port Register (PORT)

Answer 2

 read/write
 specifies for the output pins whether the output value is high or low
 ATmega16: also used for controlling pull-up resistors for input pins (see next slides)

Question 3

Port Input Register (PIN)

Answer 3

 read only (writing has no effect or unintuitive semantics)
 contains the current value (high or low) of all pins (input and output)
 usual purpose: reading values of input pins

Question 4

LED: Digital Output

Answer 4

PORT = logic 1 -> VCC 
PORT = logic 0 -> GND

Question 5

Polling

Answer 5

 Periodically check for event
 Disadvantages:
• Waste of CPU time if the event occurs infrequently
•Polling sequence has to fit in the rest of the code (hard to modify or extend)

Question 6

Interrupts (IRs)

Answer 6

 MCU polls the signal and interrupts the main program if a state change is detected.
 MCU calls an interrupt service routine (ISR) which handles the event

Question 7

ISR is triggered by event

Answer 7

 Save return address (PC) to stack
 Clear global interrupt enable bit (I bit)
 Clear interrupt flag bit (usually)
 Jump to corresponding interrupt vector table entry (interrupt vector)

Question 8

(Interrupts vs Polling) Interrupts is better if

Answer 8

 Event occurs infrequently
 Long intervals between two events
 The exact time of the state change is important
 Short impulses, polling might miss them
 Nothing else to do in main, could enter sleep mode

Question 9

(Interrupts vs Polling) Polling is better if

Answer 9

 No precise timing is necessary
 The state is important
 Impulses are long
 The signal is noisy (Interrupts would be triggered very often)

Question 10

Counter

Answer 10

 counts external events

 e.g. number of rising edges at PINB2

Question 11

Timer

Answer 11

 counts clock cycles (with or without prescaler)

 Each timer is basically a counter

Question 12

Input capture

Answer 12

• Used to timestamp (mostly external) events

* Whenever the event occurs, the timer automatically copies its current count value to an input capture register

Question 13

Output compare

Answer 13

• Used to generate signals
• Whenever a certain timer value is reached, the output compare event is triggered (can automatically set or clear an output line).

Question 14

Pulse Width Modulation (PWM)

Answer 14

• Special case of output compare

* Timer generates a periodic digitaloutput signal with configurable high-time and period.

Question 15

DAC

Answer 15

 RC low-pass filter
 Binary weighted resistor circuit
 R-2R ladder

Question 16

ADC

Answer 16

 Simple: Analog Comparator
 Flash Converter
 Tracking Converter
 Successive Approximation Converter