optimisation ⁽⁴⁾

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…

While learning about RTOSes, I came across the idea of Timer Coalescing which improves performance by eliminating unnecessary context switches. But there is no reason that this technique cannot be applied to bare metal firmware (without RTOS). To test out how well it works, I implemented it using a STM32F4 discovery board. I wanted to see the improvement factor in a typical use case, so I decided on running 8 software timers on a 1ms timer peripheral. The software timers count down to 0 then reload themselves. From previous experience, typical periods were chosen for these timers of 1000, 200, 125, 50, 18, 27, 40 and 600ms. (If they had a common factor of say 5ms, then it would make more sense to use that as the peripheral timer rate, but it restricts the flexibility for future changes.) TIM6 was chosen because we only need a Basic 16 bit timer in the STM32F4 architecture. It runs off APB1 at 42 MHz (HCLK at 168 MHz). Prescaler was set to 41, Counter Period ARR set to 1999 for 1ms and auto-reload preload set to Disable. TIM6 interrupts were enabled. Once all this is set up in the Device Configuration Tool, all…

This Youtube video by Controllers Tech shows how to use an STM32 to measure the frequency of an incoming rectangular wave using the input capture functionality of the timers. The code shown the video works but there are a few areas which could be improved, as I will now discuss. Then I make a performance improvement by using DMA instead of interrupts. The code is written using STM's HAL library which means that it does not have to handle the specific STM32's registers, which makes it more portable. However, HAL is still specific to the ARM and STM architecture, so you need to know what features are available in your internal peripherals to be able to write HAL function calls with the appropriate parameters. Initialising Global variables Controllers Tech declares a set of global variables at the top of main.c and initialises them to zero. Since we can rely on the C runtime to clear the RAM, this is unnecessary for globals and can use up code space and boot time. I would remove the initialisations unless it is thought likely that these lines would be moved into a function, making them local automatic variables, in which case they would need to be…

Everyone expects software to be soft. To be easily changeable. At least more easily changeable than hardware. But that is not always the case, sometimes it is easier to redesign the hardware than the software, but why and what do we mean when we say it is difficult to change software? Before the product has shipped, it should be easy to change both the hardware and the software as necessary to achieve the design. After the product has shipped, the hardware is fixed and you can only change the software. But there are often difficulties in changing software that mean it isn't as flexible as it might appear. What do we mean when we say it is difficult to change software? Let's start with what we don't mean. We don't mean that it's too much typing, or too many files, or we can't find all the source code. We don't mean that we have no ideas about how to produce the functionality required. We don't even necessarily mean that it will take a long time. Ok then, what do we mean? Often, the difficulty comes from the intertwined nature of software. Each part has some dependency on another part. In…