STM32 Beginner - Getting started

Getting started with STM32 development is a daunting task. The purpose of this page is to provide some basic pointers on how to make this journey easier.

	Arduino Uno	STM32 "Blue Pill"	STM32 "Black Pill"
MCU	ATmega328P	STM32F103	STM32F411
Core	8-bit	32-bit ARM Cortex-M3	32-bit ARM Cortex-M4
Clock	16 MHz	72 MHz	100 MHz
RAM	2 kB	32 kB	128 kB
Flash	32 kB	64 kB/128 kB	512 kB
USB	Yes	Gadget/Device	Gadget/Device or Host
Timers	2	7	11
ADCs	1 (8 channels)	2 (16 channels)	1 (16 channels)
DMA	No	Yes	Yes

As can be seen from the basic technical specs, STM32 are much more complex than the simple AtMega328 used on the Arduino boards and because of that complexity (and flexibility) they represent a much steeper learning curve.

Basics

Understanding the part numbers

When initially looking into STM32 the sheer number of different "versions" appear baffling. I have no idea how many different STM32 MCUs there are but it has got to be in the "hundreds". Fortunately, ST didn't just pick part numbers at random, there is in fact a system to the madness. ST themselves, on their website, provide the following example:

While not specifically mentioned in the above breakdown, the first number in the features field typically indicate the ARM core version. "STM32F1xx" will be based on an ARM Cortex-M3 core, while "STM32F4xx" are based on a M4 core. Also, different models have different built-in features - number of UARTS, number of I2C ports, number of SPI ports and number of timers. Once you have a development board it is absolutely essential that you local the exact datasheet for the relevant MCU and familiarize yourself with the features.

GPIO Voltage

While most Arduinos run on 5 V, the STM32 can only accept up to 3.3 V (a 5 V supply will fry them). However, while the MCU itself require a <= 3.3 V supply voltage, most GPIO pins are so-called "five volt tolerant".

Clock

Manu MCU run at whatever frequency crystal is attached to it. STM32s are slightly different in this aspect. Most STM32 MCUs can use pretty much any crystal oscillator between 8 GHz and 25 GHz. The crystal oscillator is used internally to drive a PLL (Phase-Locked Loop) which is then used to generate various internal clock signals. This clock configuration is rather complex and it is greatly helped by a graphical editor in STM32CubeMX. Here is an example using a STM32F103 with an external 16 MHz crystal:

Starting from the left, we first see that a 16 MHz external (HSE = High Speed External) crystal oscillator is used (as opposed to the less precise internal 8 MHz HSI (High Speed Internal) RC oscillator. This frequency is divided by two, feeding an 8 MHz clock into the PLL. The PLL will multiply this frequency by 9 to end up with a 72 MHz frequency. In this example USB is enabled and that require a stable and precise 48 MHz clock. This is derived from the 72 MHz by dividing with 1.5. 72 MHz is the highest this MCU will run at so no further prescaling is necessary. The 72 MHz clock is also fed to the DMA busses and the timers.

Bootloader

All STM32 MCUs has got a built-in bootloader for "flashing". During startup, the status of the Boot0 pin will be asserted. If "low" (GND) the MCU will start normally. If "high" (= supply voltage - typically 3.3V) the internal bootloader will be executed. This essentially mean that it is very hard to "brick" a STM32 MCU to a point where it can not be recovered.

Prerequisites

ST-Link

A pile of cheap Chinese ST-Link clones

The ST-Link is a device especially designed to flash and debug STM32 MCU's.

ST can provide these at a reasonable price (< $20) but there are also a lot of Chinese "clones" available. The image to the right show a bunch of these clones.

While the original ST-Link devices provide additional features, the Chinese clones do get the job done and they have one benefit over the original that the pins on the device are clearly labelled. For some incomprehensible reason, ST decided not to label the pins on their device and that is really annoying in daily use.

Development Board

As mentioned earlier, there's a lot of different STM32 MCU variants and there's almost as many different development boards.

A very popular model was/is the so-called "Blue Pill" board:

There are different variants of these boards, but they are supposed to include a STM32F103 MCU. The problem is that almost none of them are. There are at least 6-8 Chinese companies making "clones" of the STM32F103 processor. Some of these are probably genuine MCUs with a properly licensed ARM core, that just happens to use the same memory map like the original. Some of these are labelled correctly and identifies themselves as "GD32F103" (from GigaDevice), HK32F103 or something else. Unfortunately some are labelled STM32F103xxx and it can be extremely hard to know which is which:

Fake STM32F103 (possibly a HK32) from some random Blue Pill	Real STM32F103 from my own Green Pill board

In short, while these boards are extremely cheap and readily available, I would suggest avoiding them completely. Working around the various bugs and lack of knowledge about the MCU just make them a gigantic waste of time.

A much better choice are the so-called "Black Pill" boards.

Not only are they generally based on a much more powerful MCU but they are also normally real STM32s.

Arduino

Before diving into using the Arduino IDE with STM32 development boards, I would like to stress that I think it is a horrible approach to learning.

https://github.com/stm32duino/BoardManagerFiles/raw/main/package_stmicroelectronics_index.json

Stm32CubeIde

To be added

Miscellaneous Links

• Serial Debugging

Arduino

To be added

Stm32CubeIde

To be added