Microcontroller Selection Questions and Answers

Answer: A

Leakage roughly doubles for every ten degrees, so a two microampere figure at room temperature may be twenty at seventy. A battery life calculation done at the room-temperature figure can be wrong by an order of magnitude.

Answer: B

A 220 mAh cell at ten microamperes gives 22 000 hours of load, some two and a half years, and even a much larger cell is limited by a self-discharge of a per cent or two a year. Any predicted life beyond about ten years is governed by chemistry rather than by load.

Answer: C

Correct answer: a few milliamperes to about 20 mA

A typical pin sources or sinks up to about 20 mA, and often less at the guaranteed logic levels. Two further limits apply beyond the per-pin figure: a maximum per port and a maximum total through the supply pins, so a datasheet permitting 20 mA on each of thirty pins does not permit 600 mA altogether. Anything beyond that needs a transistor or a driver.

Answer: D

Eighty milliamperes against a fifty milliampere port total is a clear overload whatever each individual pin can take. Multiplexing so that only a few are lit at any instant is the other legitimate answer, but that has to be designed in rather than hoped for.

Answer: A

Each pin's driver can take its own limit, but every driver returns its current through the same bond wires and package planes, and those set the aggregate. Exceeding the total lifts the internal ground and upsets everything on the die.

Answer: B

Referring to a falling battery makes every reading drift, since the same input maps to a different count as the rail sags. An internal band-gap reference holds its value, at the cost of a smaller full-scale range and some settling time.

Answer: C

Noise, non-linearity and reference error all eat into the nominal figure, so a twelve-bit converter may deliver ten effective bits in a real circuit. Averaging can recover some of it where the noise is random, but not where the error is systematic.

Answer: D

Shipping a product that can be updated over its own communication interface saves recalls and service visits, and it is now expected of anything connected. It costs flash, a protocol and the discipline of never breaking the loader itself.

Answer: A

The requirement comes first, then the memory sizing which may eliminate candidates, then the multiplexing check on those that remain, and price last among parts that all actually work. Pricing first is the sequence that leads to a respin.

Answer: B

Each of the three product requirements lands on a different column of the selection matrix, and each can eliminate an otherwise ideal part. Recognising which requirement drives which criterion is what makes a selection systematic rather than a matter of taste.