A design needs 6 tasks with 512 B stacks, 2 kB of buffers and 1 kB for the kernel. The minimum RAM is about
Answer: C
Three kilobytes of stacks plus two of buffers plus one for the kernel is six, and a device with eight kilobytes leaves a sensible margin. Choosing the four kilobyte part to save a few pence would leave nothing for growth.
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Data that must survive a power cycle and changes a few times a day is best held in
Answer: D
A few thousand writes over the product's life is well within EEPROM endurance, and where the device has none a flash emulation with wear levelling serves. SRAM loses the data and mask ROM cannot be written after manufacture at all.
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A design must talk to three sensors on one two-wire bus, sharing the wires. The interface is
Answer: A
Two wires with addressing built in is exactly what I2C provides, and three devices on one bus is its ordinary case. SPI would need a select line for each device and CAN needs transceivers and is aimed at long buses between boards.
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A design must reach 20 Mbit/s to a single flash memory over the shortest possible connection. The interface is
Answer: B
Correct answer: SPI
SPI runs at tens of megahertz with a dedicated clock and separate data lines, so 20 Mbit/s is routine for it, and a single device on a short connection is exactly the case it suits - one chip select, no addressing, no negotiation. I2C is limited to 1 or 3.4 Mbit/s in its faster modes, a UART is far slower and needs matched baud rates, and CAN is a differential multi-node bus for metres of noisy cable rather than centimetres of board.
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The peripheral to look for when a design must survive electrical noise on a long bus between two enclosures is
Answer: C
Differential signalling, bit-level arbitration and a well-defined error and retransmission scheme are what make CAN suit that environment. I2C is a board-level bus and is notoriously intolerant of noise and cable capacitance.
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A 12-bit converter is specified where a 10-bit one would do. The likely consequence is
Answer: D
Two extra bits are worth nothing if the sensor, the amplifier and the board noise floor cannot resolve them, and they cost money and often conversion time. Matching the converter to the signal chain rather than to an aspiration is the discipline.
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Counting the PWM channels a design needs before selecting a part matters because
Answer: A
A datasheet's headline count is the total across timers, and two channels may share a timer and therefore a period, or a pin may be needed for something else. The pin multiplexing table rather than the feature list is what has to be checked.
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A design to be hand-assembled in small numbers should avoid
Answer: B
Balls under the body cannot be reached with an iron or inspected visually, so a BGA needs a reflow oven and X-ray inspection. Fine-pitch leaded packages are difficult but possible by hand, and leads can at least be seen.
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Pin count should be tallied against
Answer: C
It is easy to count the signal pins and forget several supply and ground pairs, the reset pin and the debug interface, and then find the package one pin short. The pin multiplexing table also has to allow the chosen functions to coexist.
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Choosing a part from a pin-compatible family is valuable because it allows
Answer: D
Correct answer: flash and RAM to be increased later without a new board
A pin-compatible family shares the same footprint and pinout across parts of different memory sizes, so the design can move up when the firmware outgrows the flash without a board revision, new layout or new qualification. It does not let the vendor be changed, and the package footprint is precisely what is being held constant rather than altered.
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