ST takes flight in this new Feather board. This STM32F405 Feather (video) runs CircuitPython at a blistering 168MHz – our fastest CircuitPython board ever! We put a STEMMA QT / Qwiic port on the end, so you can really easily plug and play I2C sensors.

This Feather has lots of goodies:

  • STM32F405 Cortex M4 with 1MB Flash, 168MHz speed
  • 3.3V logic, but almost all pins are 5V compliant!
  • USB C power and data - our first USB C Feather!
  • LiPo connector and charger
  • SD socket on the bottom, connected to SDIO port
  • 2 MB SPI Flash chip
  • Built in NeoPixel indicator
  • Qwiic/STEMMA-QT connector for fast I2C connectivity
  • We use the built-in USB DFU bootloader to load firmware. It does not come with a UF2 bootloader.

With CircuitPython basics running on this board, it's fast to get all our drivers working, then use the built in plotter in Mu to instantly get sensor data displaying within 3 minutes of unboxing.

You can use MicroPython, CircuitPython or Arduino IDE with this board, with some caveats.

  • CircuitPython support is under development. F4 family boards like this one are considered stable, and support common modules like digital IO, analog IO, I2C, SPI, PWM, and displays. Some less-used modules may be missing compared to the SAMD-type Feathers - you can check the exact list of supported modules on our documentation's Support Matrix.
  • Arduino is supported through STM32duino. There's no auto-reset bootloader support yet so you have to pull the BOOT0 pin high and manually reset before uploading. That said, STM32 support is really good, and we were able to run just about every sketch we tried.
  • MicroPython support is very solid but Adafruit does not provide MicroPython libraries for sensors!

We tested this in Arduino STM32duino with all our FeatherWings and only the RFM69/RFM9x libraries did not work (they are very platform specific). It's an extraordinarily fast Feather, and our first foray into STM32 - very exciting!

The Feather STM32F405 is chock-full of microcontroller goodness. There's also a lot of pins and ports. We'll take you a tour of them now!

Power Pins

  • GND - this is the common ground for all power and logic
  • BAT - this is the positive voltage to/from the JST jack for the optional Lipoly battery
  • USB - this is the positive voltage to/from the USB C jack if connected
  • EN - this is the 3.3V regulator's enable pin. It's pulled up, so connect to ground to disable the 3.3V regulator
  • 3V - this is the output from the 3.3V regulator, it can supply 500mA peak

This is the general purpose I/O pin set for the microcontroller.
All logic is 3.3V, nearly all pins are 5V compliant
Many pins can do PWM output
All pins can be interrupt inputs

  • RX / GPIO 0  / PB11
    Receive (input) pin for Serial3. Hardware USART3
    PWM out on TIM2_CH4
    Alternate uses: I2C2 SDA
  • TX / GPIO 1 / PB10
    Transmit (output) pin for Serial3. Hardware USART3
    PWM out on TIM2_CH3
    Alternate uses: I2C2 SCL
  • SDA / GPIO 14 / PB7
    The I2C (Wire) data pin, this has a 10K pullup to 3.3V. Hardware I2C1
    PWM out on TIM4_CH2
    Alternate uses: USART1 RX
  • SCL / GPIO 15 PB6
    the I2C (Wire) clock pin, this has a 10K pullup to 3.3V. Hardware I2C1
    PWM out on TIM4_CH1
    Alternate uses: USART1 TX, CAN2 TX
  • GPIO 5 / PC7
    PWM out on TIM3_CH2
    Alternate uses: USART6 RX, I2S3 MCK
  • GPIO 6 / PC6
    PWM out on TIM3_CH1
    Alternate uses: USART6 TX, I2S2 MCK
  • GPIO 9 / PB8
    PWM out on TIM4_CH3
    Alternate uses: CAN1 RX, I2C1 SCL
  • GPIO 10 / PB9
    PWM out on TIM4_CH4
    Alternate uses: CAN1 TX, I2C1 SDA
  • GPIO 11 / PC3
    No PWM
    Alternate uses: I2S2 SD, SPI2 MOSI
  • GPIO 12 / PC2
    No PWM
    Alternate uses: I2S2ext SD, SPI2 MISO
  • GPIO 13 / PC1
    Connected to the red LED next to the USB jack
    No PWM or alternate uses
  • SCK / GPIO23 / PB13
    The SPI bus clock pin. Hardware SPI2
    PWM out on TIM1_CH1N (available in Arduino, not CircuitPython)
    Alternate uses: I2S2 Clock, CAN2 TX
  • MISO / GPIO24 / PB14
    The SPI bus clock pin. Hardware SPI2
    PWM out on TIM1_CH2N
    Alternate uses: I2S2ext SD
  • MOSI / GPIO25 / PB15
    The SPI bus clock pin. Hardware SPI2
    PWM out on TIM1_CH3N
    Alternate uses: I2S2 SD

Analog Pins:

  • A0 / GPIO 16 / PA4
    This pin is analog input A0 (ADC12 IN4)
    Analog output (DAC OUT1) due to having a DAC (digital-to-analog converter). You can set the raw voltage to anything from 0 to 3.3V, unlike PWM outputs this is a true analog output
    No PWM or alternate uses
  • A1 / GPIO 17 / PA5
    This pin is analog input  A1 (ADC12 IN5)
    Analog output (DAC OUT2) due to having a DAC (digital-to-analog converter). This is the second DAC, and is 'independent' of A0. You can set the raw voltage to anything from 0 to 3.3V, unlike PWM outputs this is a true analog output.
    Alternative uses: SPI1 SCK
  • A2 / GPIO18 / PA6
    This pin is analog input  A2 (ADC12 IN6)
    Alternative uses: SPI1 MISO
    PWM out on TIM3_CH1
  • A3 / GPIO19 / PA7
    This pin is analog input  A3 (ADC12 IN7)
    Alternative uses: SPI1 MOSI
    PWM out on TIM3_CH2
  • A4 / GPIO20 / PC4
    This pin is analog input  A4 (ADC12 IN14)
  • A5 / GPIO21 / PC5
    This pin is analog input  A5 (ADC12 IN15)

A6 is also available for reading the battery voltage, see the Power Management page for instructions how

I2S Pins:

  • #1/Tx - I2S2 bit_clock pin.
  • #6 - I2S2 master clock pin
  • #10 - I2S2 word_select pin.
  • #11 - I2S2 data pin.

Note at this time we have not tested I2S in Arduino or MicroPython. There is no support yet in CircuitPython.

CAN Pins:

  • #9 - CAN1 RX
  • #10 - CAN1 TX

CircuitPython has CAN support via the canio module. MicroPython also supports CAN. Arduino has an open issue, no support.

SD Card / SDIO Pins

On the bottom of the PCB is a micro SD card slot. Unlike other Feathers, this is connected to the SDIO port (PC8 thru PC12 plus PD2).

In Arduino, SDIO is well supported via the STM32SD library .CircuitPython and MicroPython support SDIO. In CircuitPython use the sdioio module.

The SD detect pin is on PB12 a.k.a D32.
Rev B of this PCB has an error where SD detect is not usable, we will fix in rev C!

BAT Pins

The bottom has a test point named BAT near the center of the board. You can use it to keep the STM32's real-time clock, backup registers, and backup SRAM running while the rest of the chip is powered down.

DO NOT connect the BAT test point to the BAT pin at the side of the Feather. The voltage from a fully charged LiPo could damage the STM32.

SWD Port

On the bottom there is also a 2x5 connector pad that can be used to connect an SWD debug port for advanced uses. We don't solder the connector in place because it would take up space and make it hard to insert into a breadboard. However, you can pick up a 2x5 connector and solder it yourself! Pinout matches any/all JLink/SWD programmers with 2x5 connectors.

SPI Flash, STEMMA and NeoPixel

As part of the 'Express' series of boards, the Feather STM32F405 Express is designed for use with CircuitPython. To make that easy, we have added two extra parts to this Feather: a mini NeoPixel (RGB LED) and a 2 MB SPI Flash chip.

The NeoPixel is connected to pin #8 in Arduino, so just use our NeoPixel library and set it up as a single-LED strand on pin 8.

CircuitPython, the NeoPixel is board.NEOPIXEL and the library for it is here and in the bundle. The NeoPixel is powered by the 3.3V power supply but that hasn't shown to make a big difference in brightness or color. The NeoPixel is not used by the built in STM32 bootloader! This is different than our M0/M4/nRF52840 boards

The SPI Flash is connected to SPI bus 1 pins that are not brought out on the GPIO pads. This way you don't have to worry about the SPI flash colliding with other devices on the main SPI connection.

We give the SPI Flash the 'faster' SPI port 1 because there is no QSPI support, and reading fast from the SPI is important if you want to stream audio clips or GIFs.

In CircuitPython the SPI flash is automatically used as the filesystem exposed over USB.

In Arduino you can access SPI flash with our library and adding this definition to the top of your sketch to instantiate the SPI flash.
Adafruit_FlashTransport_SPI flashTransport(PIN_SPI1_SS, &SPI_FLASH);
Note that our SPI flash library cannot be used at the same time as the SDIO library because they have colliding File definitions.

The Qwiic / STEMMA QT port is a JST SH 1.0mm pitch connector that gives a plug-and-play connection to 3.3V, GND, SDA and SCL. Perfect for attaching a wide variety of sensors. Check out our wide range of cables and devices that can be chained together just like this mini GPS module:

In CircuitPython, you can use the STEMMA connector with board.SCL and board.SDA, or board.STEMMA_I2C().

We ship Feathers fully tested but without headers attached - this gives you the most flexibility on choosing how to use and configure your Feather

Header Options!

Before you go gung-ho on soldering, there's a few options to consider!

The first option is soldering in plain male headers, this lets you plug in the Feather into a solderless breadboard

Another option is to go with socket female headers. This won't let you plug the Feather into a breadboard but it will let you attach featherwings very easily

A few Feather boards require access to top-side components like buttons or connectors, making stacking impractical. Sometimes you can stack in the opposite order—FeatherWing underneath—or, if both Feather and Wing require top-side access, place the boards side-by-side with a FeatherWing Doubler or Tripler.

We also  have 'slim' versions of the female headers, that are a little shorter and give a more compact shape

Finally, there's the "Stacking Header" option. This one is sort of the best-of-both-worlds. You get the ability to plug into a solderless breadboard and plug a featherwing on top. But its a little bulky

Soldering in Plain Headers

Prepare the header strip:

Cut the strip to length if necessary. It will be easier to solder if you insert it into a breadboard - long pins down

Add the breakout board:

Place the breakout board over the pins so that the short pins poke through the breakout pads

And Solder!

Be sure to solder all pins for reliable electrical contact.

(For tips on soldering, be sure to check out our Guide to Excellent Soldering).

Solder the other strip as well.

You're done! Check your solder joints visually and continue onto the next steps

Soldering on Female Header

Tape In Place

For sockets you'll want to tape them in place so when you flip over the board they don't fall out

Flip & Tack Solder

After flipping over, solder one or two points on each strip, to 'tack' the header in place

And Solder!

Be sure to solder all pins for reliable electrical contact.

(For tips on soldering, be sure to check out our Guide to Excellent Soldering).

You're done! Check your solder joints visually and continue onto the next steps

Battery + USB Power

We wanted to make the Feather easy to power both when connected to a computer as well as via battery. There's two ways to power a Feather. You can connect with a USB C cable (just plug into the jack) and the Feather will regulate the 5V USB down to 3.3V. You can also connect a 4.2/3.7V Lithium Polymer (Lipo/Lipoly) or Lithium Ion (LiIon) battery to the JST jack. This will let the Feather run on a rechargable battery. When the USB power is powered, it will automatically switch over to USB for power, as well as start charging the battery (if attached) at 100mA. This happens 'hotswap' style so you can always keep the Lipoly connected as a 'backup' power that will only get used when USB power is lost.

The JST connector polarity is matched to Adafruit LiPoly batteries. Some 3rd party batteries ship with the opposite polarity, and using them can destroy your Feather! Always double check your wires before choosing a battery to use with your project.

The below image shows the USB C jack (left), Lipoly JST jack (above and to the right of the USB), as well as the changeover diode (just below JST jack) and the Lipoly charging circuitry (to the right of the JST jack). There's also a CHG LED, which will light up while the battery is charging. This LED might also flicker if the battery is not connected.

Power supplies

You have a lot of power supply options here! We bring out the BAT pin, which is tied to the lipoly JST connector, as well as USB which is the +5V from USB if connected. We also have the 3V pin which has the output from the 3.3V regulator. We use a 500mA peak regulator. While you can get 500mA from it, you can't do it continuously from 5V as it will overheat the regulator. It's fine for, say, powering an ESP8266 WiFi chip or XBee radio though, since the current draw is 'spikey' & sporadic.

Note the STM32F405 is a fairly power hungry chip, it will draw up to 80mA when it runs

Measuring Battery

If you're running off of a battery, chances are you wanna know what the voltage is at! That way you can tell when the battery needs recharging. Lipoly batteries are 'maxed out' at 4.2V and stick around 3.7V for much of the battery life, then slowly sink down to 3.2V or so before the protection circuitry cuts it off. By measuring the voltage you can quickly tell when you're heading below 3.7V

To make this easy we stuck a double-100K resistor divider on the BAT pin, and connected it to A6 which is not exposed on the feather breakout

In Arduino, you can read this pin's voltage, then double it, to get the battery voltage.

// Arduino Example Code snippet

#define VBATPIN A6

float measuredvbat = analogRead(VBATPIN);
measuredvbat *= 2;    // we divided by 2, so multiply back
measuredvbat *= 3.3;  // Multiply by 3.3V, our reference voltage
measuredvbat /= 1024; // convert to voltage
Serial.print("VBat: " ); Serial.println(measuredvbat);

For CircuitPython, we've written a get_voltage() helper function to do the math for you. All you have to do is call the function, provide the pin and print the results.

import board
from analogio import AnalogIn

vbat_voltage = AnalogIn(board.VOLTAGE_MONITOR)

def get_voltage(pin):
    return (pin.value * 3.3) / 65536 * 2

battery_voltage = get_voltage(vbat_voltage)
print("VBat voltage: {:.2f}".format(battery_voltage))

ENable pin

If you'd like to turn off the 3.3V regulator, you can do that with the EN(able) pin. Simply tie this pin to Ground and it will disable the 3V regulator. The BAT and USB pins will still be powered

Alternative Power Options

The two primary ways for powering a feather are a 3.7/4.2V LiPo battery plugged into the JST port or a USB power cable.

If you need other ways to power the Feather, here's what we recommend:

Here's what you cannot do:

  • Do not use alkaline or NiMH batteries and connect to the battery port - this will destroy the LiPoly charger and there's no way to disable the charger
  • Do not use 7.4V RC batteries on the battery port - this will destroy the board

The Feather is not designed for external power supplies - this is a design decision to make the board compact and low cost. It is not recommended, but technically possible:

  • Connect an external 3.3V power supply to the 3V and GND pins. Not recommended, this may cause unexpected behavior and the EN pin will no longer enable/work. Also this doesn't provide power on BAT or USB and some Feathers/Wings use those pins for high current usages. You may end up damaging your Feather.
  • Connect an external 5V power supply to the USB and GND pins. Not recommended, this may cause unexpected behavior when plugging in the USB port because you will be back-powering the USB port, which could confuse or damage your computer.

The STM32F405 chip has a built in ROM bootloader that cannot be disabled or erased, this makes it a fool-proof way to always be able to recover your microcontroller code. It's not as easy to use as UF2, but it isnt too difficult either.

The ROM bootloader looks for signal on the serial RX line as well as USB, so make sure no GPS or other serial/uart data device is connected to RX while you are trying to bootload the device!

Enabling DFU bootloader mode

Enabling the DFU bootloader is super easy. Simply connect the BOOT0 (B0) pin to 3.3V logic. Then press the reset button or power cycle while the board is connected to your computer USB port

After you've hit reset, you can remove the BOOT0 jumper - it's only checked on powerup

Check for USB Bootloader device

In Windows, you will see the device show up as STM32 BOOTLOADER under Universal Serial Bus devices

In MacOS X

Visit the AppleMenu->About This Mac->System Report

Select USB and look for the item labeled STM32 BOOTLOADER

Programming Firmware


The easiest way by far to program under windows is to download STM32CubeProg. It's a graphical programmer, does not require Zadig or special command line invocation

You'll need to make an account is the only downside.

When you start it up, it'll look like this.

In the top right, below the Not Connected message, find the dropdown to the left of the Connect button

Select USB

OK if the device is plugged in and the bootloader is running, it will show up under the USB configuration pane. If not, enter bootloader mode by connecting BOOT0 to 3.3V and resetting, and click the refresh button.

Once it appears as a valid Port, click Connect

You should see the Device info pane in the bottom right is updated with info about what chip was found!

Click the 3-lines below the STM32 logo in the top left, to expand the menu.

Then click Erasing & Programming

Click Browse to open the firmware files you want to program

You can program .hex or .bin files, it does not seem to support .dfu

Don't change the Start Address

Make sure Verify Programming and Run after Programmingare clicked, but Skip flash erase is not

Then click Start Programming

It will take a few seconds to erase and reprogram the chip.

It's normal to get a Warning Connection is lost alert

Click away until you get the File download complete alert

That's it! You should close STM32 CubeProg now - leaving the program open may conflict with other connections to the board.

Mac (and Linux)

For Mac users, install dfu-util with brew

dfu-util can only program .bin and .dfu files. It cannot program .hex files (but there are tools to convert .hex's to .bin's)

Then upload the firmware with the command

dfu-util -a 0 --dfuse-address 0x08000000 -D firmware.bin

Don't change the address value, only the firmware filename!

Or, if you have a dfu file - use

dfu-util -a 0 -D firmware.dfu

Upon success, reset the board without the BOOT0 jumper and you will see after a few seconds the CIRCUITPY disk drive appear

The first thing you will need to do is to download the latest release of the Arduino IDE. You will need to be using version 1.8 or higher for this guide

Thankfully the Adafruit board support is now supported directly from STM32duino so you can simply install it:

From the File menu select Preferences

Find the Additional Board Manager URLs text box.

If it's empty add the text

or, if its not empty, add a comma at the end of the current text, and then add the line above

From the Tools menu, go down to Board submenu and select Board Manager...

Search for STM32 and click Install - make sure you have the latest version, at least 1.8.0 selected and installed!

Quit and restart the Arduino IDE

From the Tools menu, select Generic STM32F4

Then select Board part number -> Adafruit Feather STM32F405

Under USB Support select CDC supercedes USART so that Serial points to the USB port not the hardware serial

Finally select STM32CubeProgrammer (DFU) as the upload method

These are your Tool menu selections to verify!

Note that if you tried the STM32 Cube Programmer on the "DFU Bootloader Details" page, you need to close it before using this Arduino version! Our users report the application and the Arduino upload method can conflict with each other, so make sure you are only using one at a time or you may find your uploads failing to connect.

Activate the Bootloader

At this time, you must manually put the board into bootloader mode every time you want to upload.

Do that by connecting the B0 pin to 3.3V and clicking reset

STM32CubeProgrammer will run the code immediately after DFU, so you can connect a wire on a breadboard between B0 and 3.3V and keep it connected. When you are about to upload, click the reset button. After upload, your code will be running automatically.


Once you are bootloader mode, click Upload to compile and upload your sketch

STM32duino Notes

We don't really support MicroPython explictly at Adafruit - our drivers are for CircuitPython. However, for people who like MicroPython, we submitted a build definition!

You can build the latest version from the github or load this MicroPython 1.9.4 build we crafted for you.

Load it by following the DFU Bootloader tutorial in this guide.  Follow the instructions for when you have a .dfu file.

Upon success, reset the board without the BOOT0 jumper and you will see after a few seconds the PYBFLASH disk drive appear

MicroPython Notes

The Feather uses the same chip as the PyBoard 1.1 so technically anything available on the PyBoard should work on the Feather, given the pin differences

We use Dx and Ax pin names, to match the Feather markings. You can see the pin names here

The SD card slot can be used for file and code storage

SPI flash is not used by MicroPython (it's something specific to CircuitPython)

To load CircuitPython, follow the DFU Bootloader instructions to get the board into bootloader mode

Visit To get the latest firmware available

Download the bin file, and then program it using dfu-util or STM32CubeProgrammer

Upon success, reset the board without the BOOT0 jumper and you will see after a few seconds the CIRCUITPY disk drive appear

CircuitPython Notes

If you are intending to start a project that is very RAM intensive, note you cannot access the full 196KB of RAM that listed on the F405 datasheet and website - only 128KB is available to Circuitpython programs for system reasons. You'll find the same limitation on Micropython and most other F405 devices.

STM32F4 support is new compared to the SAMD and nRF boards, but is now considered stable. Working modules on this board include:

  • Digital IO (LEDs/buttons)
  • analog input
  • analog output (DAC)
  • PWM output on timer pins
  • I2C
  • SPI
  • NeoPixel Support
  • UART Support
  • DisplayIO
  • PulseIO

To come:

  • I2S
  • Audio
  • TouchIO
  • many others!

If you find something missing or flawed, please open an issue in circuitpython

Schematic & Fabrication Print

This guide was first published on Nov 05, 2019. It was last updated on Nov 05, 2019.