Sometimes it makes sense to write everything in assembly, even these days. For example if using a tiny microcontroller. Or just if one just don’t need all the productivity of the C/C++ tools. And it is a good educational experience: getting hands-on on the lower levels.
The Microsoft Visual Studio Code is a great IDE, but does not (yet?) implement features for true embedded usage. Or things are there to some level, but hard to use. One of these things is how to step in the assembly code. This article shows how to do this.
My mantra is *not* to use any floating point data types in embedded applications, or at least to avoid them whenever possible: for most applications they are not necessary and can be replaced by fixed point operations. Not only floating point operations have numerical problems, they can lead to performance problems as in the following (simplified) example:
#define NOF 64Continue reading
static uint32_t samples[NOF];
static float Fsamples[NOF];
float fZeroCurrent = 8.0;
static void ProcessSamples(void) {
int i;
for (i=0; i < NOF; i++) {
Fsamples[i] = samples[i]*3.3/4096.0 - fZeroCurrent;
}
}
In many cases it is very useful to see the generated assembly code produced by the compiler. One obvious way to see the assembly code is to use the Disassembly view in Eclipse:
Disassembly View
But this requires a debug session. An easier way is to use command line options to generate the listing file(s).
By default, the GNU compiler (gcc) optimizes each compilation unit (source file) separately. This is effective, but misses the opportunity to optimize across compilation units. Here is where the Link Time Optimization (LTO, option -flto) can help out: with a global view it can optimize one step further.
The other positive side effect is that the linker can flag possible issues like the one below which are not visible to the compiler alone:
type of '__SP_INIT' does not match original declaration [enabled by default]
Warning by LTO
The ARM mbed USB MSD bootloader which is used on many silicon vendor boards has a big problem: it is vulnerable to operating systems like Windows 10 which can brick your board (see “Bricking and Recovering OpenSDA Boards in Windows 8 and 10“). To recover the board, typically a JTAG/SWD programmer has to be used. I have described in articles (see links section) how to recover from that situation, including using an inofficial new bootloader which (mostly) solves the problem. The good news is that ARM (mbed) has released an official and fixed bootloader. The bad news is that this bootloader does not work on every board because of a timing issue: the bootloader mostly enters bootloader mode instated executing the application.
DAPLink in Bootloader Mode
I’m a fan of all kind of weather stations. When Daniel Eichhorn twittered about his new version using an E-Paper display module, I immediately preordered one. I decided to build a station with a custom enclosure, so here is my version of a 3D printed version, featuring magnets so it can be attached to the fridge:
E-Paper Weather Station
The tools and IDE market is constantly changing. Not only there is every year at least one new major Eclipse IDE release, the commercial tool chain and IDE vendors are constantly changing the environment too. For any ARM Cortex-M development, the combination of Eclipse with the GNU tool chain provided by ARM Inc. is the golden standard. But this does not mean that things can be easily moved from one IDE package to another.
While moving between Eclipse versions and GNU versions is usually not a big deal at all, moving between the Eclipse build tool integration is usually not simple. While the GNU MCU Eclipse plugins are widely used (see Breathing with Oxygen: DIY ARM Cortex-M C/C++ IDE and Toolchain with Eclipse Oxygen), the Eclipse based IDEs from the silicon vendors or commercial Eclipse toolchain vendors are using their own GNU toolchain integration. Which means the project files are not compatible :-(.
NXP FRDM-KW41Z Board
If you are like me – someone who always wants to know what the compiler generates for a piece of source code – then have a look at the Compiler Explorer: A web-based compiler code comparison tool:
Compiler Comparison
Thanks to Matt Godbolt, I can select different compilers and compare their output for a given source code. Very useful to see the impact of a compiler optimization or to compare different GCC compiler versions.
Happy Comparing 🙂
I believe in ‘life-long-learning’. With this I continue to learn and discover new things every day. I’m writing tutorials to give something back to the community from which I have learned so much.
On top of this, I receive emails on a nearly daily basis, asking for help. Many articles have the origin in such requests or questions. I prefer questions or comments in a public forum, because that way I feel all others can benefit from it. Last week Alessandro contacted me with this:
“Hi Erich,
I hope this find you well! I’m starting to using ARM processors, but I find them quite complicated on the configuration side. I started in the past with PIC micro (PIC16) with asm, and I found them quite straightforward to be configured (clock, IO, peripherals, …). Then I moved myself on C language, and on PIC18 without any big issues.
Now I would really like join the ARM community, I see that these processors are what I’ve always looking for, on energy, calc power, peripherals, and FINALLY on IDE (editor, toolchain and utilities)… AMAZING!!!”
The topic is about how to start learning developing for ARM. Alessandro agreed to make this public, so I thought this might be a good topic for an article?
Firmware
MCU on Eclipse 