interviews ⁽²⁾

A function to reverse the order of bits in a byte makes for a good interview question. The candidate has to think about loops, shifting and masking as well as the mundane but important issues of function and variable naming, data types, spacing and comments. As opposed to those "gotcha" questions apparently beloved by the tech giants where you have to know some trick, this can be worked through methodically. Once there is a solution on the table, there is an opportunity to talk about optimization for speed and/or space. There are lots of ways to approach this problem, but I suspect that many will take this simple idea of testing each bit and setting the LSB of the output in a loop which shifts up the output and mask one place each time. //bit reverse eg 0x80 return x01 //simple but straightforward approach uint8_t bitreverse_simple(uint8_t input) { uint8_t i, output, mask; output = 0; mask = 0x01; for (i = 8;i > 0;i--) { output <<= 1; if (input & mask) { output |= 1; } mask <<= 1; } return output; }; Testing The Simple Approach First we need to test that this produces correct results by running…

I've been using this interview question for the last 20 years to test embedded C engineers: Given the formula for Fahrenheit as 9 * C + 32 = F - 5 Write a function to convert unsigned char tempInC to unsigned char tempInF (ie. All values are bytes, including intermediate results). The processor has no floating point library. Recently I was intrigued to find that a very similar problem was posed by professor James M. Conrad at the University of North Carolina at Charlotte at this point in his lecture on Embedded Systems: Software Testing. The responses I used to get were similar to what the students gave in that video. Whilst some candidates would think this was a maths problem that needed rearranging to solve for tempInC, others thought that I just wanted them to write the formula in C syntax. A large percentage of candidates would ignore the premise of the question and cast the parameters to floats, do the calculation as a floating point one and cast the result back to integers. As well as being not anywhere near what I was asking for, this approach leads to rounding down so that 21°C, which should be converted…