STM32 stop mode & EXTI wakeup – example without HAL

Low-power modes come in handy (apart from obvious power saving) when the electromagnetic emissions of an MCU have to be reduced. For example when an MCU is placed close to antenna switching relays – you definitely do not want to receive the noise from the MCU in your HF transceiver (even if the clock frequency does not fall in an amateur band).

The trouble with low-power modes is that they are notoriously hard to debug. Often the clocks and voltage regulators have to be (re)configured. The debugger can prevent from entering into some of the modes or change their behavior. The debugger may also not be available because various clocks are stopped and supply rail lose power. Instead you have to rely on an ammeter or a simple LED to indicate which state the MCU is in.
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Matching STM32 hardware CRC with standard CRC-32

Hardware CRC generators come handy in embedded systems when data or code have to be validated. For example: storing settings in flash or EEPROM, receiving new firmware via a bootloader. Software implementations are either slow (when calculating the CRC directly) or need quite some memory for lookup tables. There are many CRC standards. The checksum does not necessarily have to be standards-compliant when used for internal data storage because the data will never leave the device in raw binary. However, when the data has to be exchaned with an external system it helps greatly when off the shelf libraries can be used.

I tried using the CRC peripheral available in STM32L011 and failed miserably so I had to come up with a more systematic approach to get a proper CRC-32 that is compatible with zlib, python etc.
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Fast integer scaling on Cortex-M0

Integer scaling is very common in embedded systems. For example to make human-readable values of ADC readings (correcting for gain or voltage dividers). Two operations are needed to do scaling without using floating-point numbers – multiplication and division. Cortex-M0 is a nice little beast with a single cycle integer multiplier. ARM provides a smaller area 32-cycle multiplier but I have not yet come across a chip that would have it. All chip makers seem to choose the faster option. Cortex-M0 however totally lacks integer division. Division is handled by library functions. A simple code like y = x * 123 / 120; gets compiled to a function call. Software division of course takes time (and energy). There is a nice hack though 🙂
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Reducing FFT code size of CMSIS DSP

CMSIS DSP is a fantastic library develeoped by ARM that provides various math primitives (like matrices, filters and FFT). It is usually my first pick when implementing signal processing on microcontrollers bacause it is highly optimized for ARM Cortex-M cores and is free. I was therefore quite surprised when my project suddenly stopped fitting into a 128 KB flash MCU after adding a simple FFT call.
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Debouncing buttons on EFM32 Happy Gecko

Button bounce is always a problem for microcontrollers. There are many ways to deal with the issue. The pins can be sampled at a low frequency, so that the bounce will settle between consecutive samplings. They can be low-pass filtered in software. Some approaches require the pin to be stable for some amount of time to register a press.

Interrupts are usually avoided because the MCU could register almost every edge of the button bounce. With a small piece of code however interrupts can be used for one-shot operation and “rearmed” later from a timer interrupt (after the bounce period). This is the approach the driver uses. A major benefit is that the system reacts to the press immediately.
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STM32L4 I2C driver for FreeRTOS without HAL

I2C remains a popular communication interface between MCUs and all kinds of auxiliary chips like ADCs, digipots and GPIO expanders. I had to make a simple and universal driver for an upcoming STM32L432 project to control Microchip digipots. STM32 I2C peripheral is simple enough to use without the burden of HAL libraries, additionally I needed a custom driver because my application uses FreeRTOS.
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STM32L4 UART driver for FreeRTOS without HAL

This is a driver for STM32L432 LPUART. It should also work with the “full” UART. The LPUART is a simple peripheral (compared to the clock tree or ADC). In this case it is easier to master the usage of a couple of registers, than use full-size HAL drivers, as they are very generic to cover every possible flavor of a peripheral across the whole STM32 line, which in turn makes them big in terms of code size and actually harder to follow than the register layout.

The driver can be safely used within FreeRTOS, It can even be used by multiple tasks, but it probably would make little sense anyway, unless there can be different devices connected at runtime to the same UART or the application has separate operating modes implemented in different tasks.
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