The C standard library (libc) is a component that gets little attention. It is just there. However for embedded systems it brings some challenges and overhead in terms of code size. As firmware size is often critical, it sometimes makes sense to use a trimmed version of the standard library or to remove it entirely. I will focus on reducing the code size that may be beneficial for a small application like a bootloader.
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Category: ARM
Fixing Cortex-M startup code for link-time optimization
Link-time optimization is a powerful output size reducing feature. Even though (as of 2018) still regarded as somewhat experimental, LTO is worth trying, if the binary size is very important and the application can be reliably tested afterwards, as link-time optimized code is hard to debug. A bootloader can be an ideal example. LTO is very easy to enable, but there are some small quirks that have to be taken care of. I will use GCC 7.2.1 from GNU Arm Embedded as an example.
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Practical FFT on microcontrollers using CMSIS DSP
Fourier transform is a vast domain of knowledge with many practical applications within signal processing. Virtually all communications protocols use Fourier transform at one step or another (including LTE, GPS and WiFi). Another popular example are the “jumping bars” in music players showing levels of low and high tones in real time. In this post I show the basics of obtaining spectrum of an audio signal on an EFM32 Cortex-M3 microcontroller. No scary math!
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EFM32 Cortex-M firmware project from scratch – step by step
Most popular microcontrollers come with IDEs and tools provided by the manufacturer, like NXP, STM, TI or Silicon Labs. IDEs are commonly based on Eclipse and creating a project for almost any chip in those IDEs is usually just a click away, so why would you ever want to make such a project from scratch, gather all header files, libraries and scripts? Read to find out why and how 🙂
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Using the internal EEPROM of STM32L
Most STM32 microcontrollers feature an internal EEPROM. It is useful for storing settings or calibration data. Regular flash (that stores code) can also be used, but the EEPROM can be updated byte-by-byte and is independent from regular flash. This may come handy during application updating, as whole flash can be simply erased without affecting the EEPROM.
I wrote a generic driver for keeping settings in the EEPROM based on the standard peripherals library for STM32L, that is easier to understand than the official demos from ST. It was tested on an STM32L151RC.
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Open OBD datalogger
This is a fully open-source car datalogger that reads engine data using the OBD2 interface in real time and stores it on an SD card. It also stores GPS data. All communication is done directly by the MCU without a translator chip like ELM327 or STN1110. The datalogger supports all CAN and K-Line OBD2 protocols. All hardware fits into an off-the-shelf OBD2 connector enclosure.
This project is available on Github.
Kinetis E – writing to internal flash
While working on a bootloader for a Kinetis KE06 I obviously ran in to the task of having to write the internal flash memory of the microcontroller. Freescale’s driver is over 1500 lines of code, exposes every single bit of the flash controller so the simple operation of erasing a sector and writing to it requires many function calls. I wrote my own driver that is less than 200 lines of code and has everything needed to make a bootloader.
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Hard fault in __libc_init_array
When starting my STM32 makefile project for the first time I encountered a very early hard fault in the startup code. It happened exactly when calling libc function at
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/* Call static constructors */ bl __libc_init_array |
The whole startup code came straight from ST, so I did not suspect it to be faulty. Here is what I have found out to fix the problem.
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STM32L151 makefile project
I have published a basic Makefile project for STM32L151 on Github. I plan to develop it into a low-power wireless sensor network using SP1ML modules. The modules contains STM32L151RB MCU with 128K of flash, SPIRIT1 transceiver chip, antenna and passive components. Basically everything needed to make an 868MHz radio network.
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Reading OBD2 data without ELM327, part 2 – K-Line
K-Line is another popular OBD2 interfacing standard, that has been used in European cars before CAN bus became common. There are a couple of physical variations (K-line, K+L, KKL) and slightly different protocols (KWP2000 or Keyword Protocol, and ISO 9141) running on those lines. Basically all you need to talk to an older car is an MCU with a UART and a single transistor. 🙂
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