Power Management in Embedded Systems Questions and Answers

Answer: A

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct: dynamic power goes as the square of the supply, and leakage rises steeply with temperature, roughly doubling every ten degrees or so. Statement 3 is wrong - a deeper mode saves more current but costs more energy and time to enter and leave, and loses more state, so below a break-even interval a shallower mode wins. The right depth follows from how long the device will stay there.

Answer: B

Clocks and pins are both dealt with before the mode is entered, since a pin left floating or a converter left running dominates whatever the mode. Measuring is the last step and is what proves the other three were done properly.

Answer: C

Polling requires the core awake, or at least waking a hundred times a second whether or not anything has changed; an interrupt lets it sleep until something has. Where the sensor changes rarely the saving is very large.

Answer: D

With the running current dominating the average, the awake fraction is the variable worth attacking. The other three either do nothing or make matters worse, and the last is the commonest unintentional waste in a first design.

Answer: A

The pulse drops the terminal voltage through the cell's series resistance, and a threshold above that trough trips repeatedly while the cell is still largely full. Lowering the threshold, or fitting a bulk capacitor, resolves it.

Answer: B

A low frequency crystal costs under a microampere to run and divides by a power of two to give exactly one second, which is why the odd-looking figure is universal. Running the main oscillator instead would cost milliamperes and defeat the mode.

Answer: C

A small retained block lets the design carry its state across a sleep whose alternative is a full restart, and the leakage of a few kilobytes is well under a microampere. Deciding what must survive and what can be recomputed is the design work.

Answer: D

A controller able to fill a buffer while the core sleeps replaces hundreds of wake-ups with one, and the core is the largest single consumer on the die. Arranging for the peripheral, the DMA controller and the sleep mode to be compatible is what makes this possible.

Answer: A

Counting the wake-ups, the radio transmissions and the sensor readings a day and costing each in microjoules gives a figure that can be compared against the cell and attacked item by item. Average current hides which activity is responsible.

Answer: B

More resolution means a longer conversion and often a higher current, so it costs energy rather than saving it, and it is worth paying only where the extra bits are real. The other three all attack the budget directly.