Martin Cowen ⁽⁴⁴⁾

After nearly 10 years of continuous operation, my Panasonic DMR-EX77 PVR died. The display went out and it didn't respond to the remote control. Sometimes this type of failure is just a fuse which has been stressed too many times by the inrush current produced when charging large capacitors in the power supply, but not this time. It was immediately obvious that the electrolytic capacitors had failed. I could see their bulging tops which are designed to blow out in the event of failure. All electrolytic capacitors have an expected lifetime which is mainly due to the wet electrolyte drying out at which point the capacitance drops and the equivalent series resistance goes up. It seems to be a common problem with these PVRs. On this product, the capacitors which needed replacing were identified as C1401, C1402, C4056 & C1557. The service manual explains the disassembly procedure and has the schematic and bill of materials which makes finding replacements easy. I ordered spares from RS Components and disassembled the PVR carefully. Since many different types of screws are used, it is a good idea to stick them down to a sheet of paper with a note of which screw was…

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…

Generally, the advice on unit testing in embedded environments is to run your tests on the PC host rather than on the target device. Whilst I agree that this is the most productive arrangement, there are a variety of reasons for needing to test on the target which can be convincing in certain situation. The technique described here allows for both. Mike Long in his GOTO 2015 presentation Continuous Delivery for Embedded Systems says "Test on your host because that's fast - it's a really fast way to develop. But also test on the target because behaviour can change in different ways, different compilers, different hardware..." Niall Cooling in his talk at the EmbeddedOnlineConference 2020 "How agile is changing the face of embedded software development" says (at 46m) on the gap between testing on the host and the target Things like TDD really are based on testing in the host, and really that's fine but of course we are typically using host compilers like host GCC and of course we know that at the moment this is typically going to be an Intel based processor. So we are compiling for the underlying OS. And it is good for finding a…

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…

CRCs are used for error detection in communication systems and storage systems. They have a combination of several advantageous properties whilst only requiring a short FCS, but they do have their limitations. If you require error correction as well as detection then you have to look at a different class of algorithms and will have to add much more redundancy to the codeword. The rest of this post is about message systems, where if an error is detected, the link layer of the protocol will take various actions such as discarding the message, sending a NACK, requesting a retry, waiting for retry until a timeout occurs etc. Another use of CRCs is for protecting data in memory, where similar considerations apply but the corruption possibilities may be different so you have to consider if the single random bit-flips model is still appropriate. If you just need to protect a single 8, 16 or 32 bit value, choose from this list and use a non-zero seed. Unless your protocol design is such that the messages are fixed length, one of the fields in the message will be the message length or an equivalent which points to the position of the FCS.