Raspberry Pi Pico + ADXL345: Portable resonance measurement

The Raspberry Pi (RPi) Pico offers a convenient and cost efficient way to create a portable resonance measurement device that can be used across different Klipper printers.

ADXL345/343 boards

There are different ADXL345 boards on the market and also the ADXL343 board is a valid alternative.
Depending on the board layout and used components a few things should be considered when connecting the board.


The boards can be mainly distinguished by potentially having a Voltage Regulator or a Level Shifter.

Voltage Regulator

  • The Voltage Regulator allows the board to be connected to either 3.3V or to 5V
  • The ADXL chip itself requires 3.3V. The voltage regulator ensures that only 3.3V actually reach the chip
  • In the following images, a Voltage Regulator is marked with a green rectangle

:warning:WARNING: Boards without such a Voltage Regulator must not be connected to 5V or the board will be destroyed

Level Shifter

  • Digital electronics, like MCUs (aka printer boards) or RPis require a certain voltage on their pins
  • Typical voltages in 3D printing are either 3.3V (most common) or 5V
  • A Level Shifter allows to translate between 3.3V and 5V or vice versa
  • The ADXL family is in the 3.3V domain by default. This means that it outputs only 3.3V even when connected to 5V (see voltage regulator).
  • A Level Shifter changes this: When the ADXL is connected to 5V (see Voltage Regulator) it will also output 5V on the board’s connecting pins
  • In the following images, a Level Shifter is marked with a yellow rectangle

:warning:WARNING: RPIs and many MCU boards are designed for 3.3V only. A board with Level Shifter must not be connected to 5V and to a partner tolerant only to 3.3V. This will destroy the partner.

I2C Boards

  • Most boards use the SPI as communication bus. SPI is fast enough to deliver up to 3200 samples/s, as it is needed for resonance measurement
  • Some boards seem to be forced into I2C mode (another bus type) with a pull-down resistor
  • Such boards do not work for resonance measurement but can potentially be reworked by removing the pull-down resistor
  • Refer to ADXL345 Invalid ID · Issue #3637 · Klipper3d/klipper · GitHub with special thanks to @JanKolibri

:warning:WARNING: Avoid such boards, if you do not feel comfortable with soldering on such a board.

ADXL345 with Voltage Regulator and Level Shifter

ADXL345 with Voltage Regulator

ADXL345 without Voltage Regulator

ADXL343 with Voltage Regulator and Level Shifter

ADXL345 with Voltage Regulator and forced to I2C


ADXL345 pin RPi Pico pin RPi Pico pin name
VIN (or VCC) 36 3V3 Out
CS 2 SPI0 CSn (GP 1)
SDO 1 SPI0 RX (GP 0)
SDA 5 SPI0 TX (GP 3)

Compiling & flashing the firmware for the Pico

Compile the firmware

cd ~/klipper/
make menuconfig



Flash the firmware

  1. Conncect the Pico to the regular Klipper Raspberry Pi (or other SBC) USB port, while holding down the BOOTSEL button
  2. Given no other mass storage devices are existing, the Pico should register as block device /dev/sda
  3. Mount the block device and copy the Klipper firmware file to it
sudo mount /dev/sda1 /mnt
sudo cp out/klipper.uf2 /mnt
sudo umount /mnt
  1. After un-mounting, the Pico should automatically reboot with the new firmware

Configuring Klipper

  1. Get the correct serial path with ls /dev/serial/by-id/*
  2. Add the following to the printer.cfg file:
[mcu pico]
serial: /dev/serial/by-id/usb-Klipper_rp2040_mySerial

spi_bus: spi0a
cs_pin: pico:gpio1

accel_chip: adxl345
    100,100,20 # an example
  1. Restart Klipper with the RESTART command.

:grey_exclamation:Important: The first measurement, after freshly connecting the RPi Pico or restarting Klipper, will fail. This is due to some issues with the SPI initialization. Subsequent measurements will work.

Available SPI buses on the RPi Pico

It is possible to connect multiple ADXL boards to one RPi Pico:

ADXL345 pin RPi Pico pin name spi0a spi0b spi0c spi1a spi1b
VIN (or VCC) 3V3 Out 36 36 36 36 36
GND GND 38 38 38 38 38
CS CSn 2 (GP1) 7 (GP5) 22 (GP17) 12 (GP9) 17 (GP13)
SDO SPI RX 1 (GP0) 6 (GP4) 21 (GP16) 11 (GP8) 16 (GP12)
SDA SPI TX 5 (GP3) 10 (GP7) 25 (GP19) 15 (GP11) 20 (GP15)
SCL SPI SCK 4 (GP2) 9 (GP6) 24 (GP18) 14 (GP10) 19 (GP14)



See my post on Reddit for a nice snap-together (printable) case for the Pico (and an alternate wiring/config for this setup).

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@glabifrons thanks for sharing.
One comment: There are many different ADXL boards out there. Some are only 3.3V tolerant.
Your wiring will potentially kill such boards as you are using a 5V pin


@Sineos Very good to point out!
Yes, mine is a 5V-tolerant version.

Edit: I edited my Reddit post (in bold) to warn people about the wiring. Thanks!


I configured everything as per instructions, however I’m getting an error when querying the ADXL345. ‘got f2 vs e5’

I know that the PICO and ADXL345 are working because I can access it via microPython (via SPI) just fine.

Few questions:
Where does spi0a come from? Given gpio notation, it seems to me it should be pico:spi0? Could it be that my other mcu SPI is being used?
EDIT: as per reddit linked above CS pin determines mcu being used, so I suppose the correct SPI port is being opened…
How can I bypass device ID detection?
Is there anything else I can try to troubleshoot this?

EDIT2: turns out I just needed to give it some time before spi starts working.

For reference it appears it took about 30 seconds for SPI port to be available for some reason:

00:30:02  !! Lost communication with MCU 'pico'
00:30:34  // Klipper state: Ready
00:30:42  // Invalid adxl345 id (got f2 vs e5).
// This is generally indicative of connection problems
// (e.g. faulty wiring) or a faulty adxl345 chip.
00:30:42  !! Invalid adxl345 id (got f2 vs e5).
00:31:05  // adxl345 values (x, y, z): 2677.215450, -1988.788620, 15910.308960

Heh, I take it back. It appears that first access after reboot results in an error. No matter the time that have passed.

Initial post amended by further available SPI buses. Mind the notes

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Initial post amended by information on various board configurations

@glabifrons: For your guide: There is another potential pitfall with connecting to 5V. Some boards have a Level Shifter which will bring the output to 5V. In this case not the ADXL might get destroyed but the Pico.

Just reporting in that your instructions are still spot on.

To connect 2 ADXLs I use the following pins. For the second ADXL you will need to use GPIO8, GPIO9, GPIO10 and GPIO11 according to the colors. Red and Black are the same.

Klipper Configuration

[mcu pico_adxl345]
serial: /dev/serial/by-id/usb-Klipper_rp2040_use_your_id-xxxxx

[adxl345 head]
cs_pin: pico_adxl345:gpio1
spi_bus: spi0a
axes_map: -z, x, -y #update your axes mapping

[adxl345 bed]
cs_pin: pico_adxl345:gpio9
spi_bus: spi1a

probe_points: 115, 115, 30 #update your position
accel_chip_x: adxl345 head
accel_chip_y: adxl345 bed

For pico case I use this.
For bed mount I use this.


Adxl345 with raspbery chip :slight_smile:

@Sineos , I’d like to take a stab at updating the Klipper Measuring Resonances documentation to reflect the information you provided here as it was really helpful to me. I’d love to use some of the images you posted in this thread.

I believe that means you’d have to release the images either to the public domain or under Klipper’s license, the GPL3. Do I have your consent? You are certainly under no obligation to release your images and even if you don’t, I very sincerely thank you for posting this really helpful information.

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I’m a big fan of the WTFPL - Do What the Fuck You Want To Public License :wink:

But of course. As the official support board of Klipper, all my contributions here are automatically considered as common to the project and as such I would consider them under the same GPL v3.

Appreciated that you are asking :+1:


Removed original response since my problem did not have anything to do with this post.
With the help of this info, I was able to do this with a MPU9250 sensor and PICO as well. I used a short run of twisted pair wire taken from a cat5e cable. It’s important to note that most spooled ethernet wire is solid core wire that is not meant to be flexed repeatedly. It will cause problems in this kind of application. Patch cables (like those you get with new cable modems) typically have stranded wire.

Sineos asked for confirmation on using the other available SPI buses. I managed to mess up spi0a and spi0b by getting the wires wrong and filling adjacent holes with solder. Anyway, I got spi0c working just fine. Config change was obvious -

spi_bus: spi0c
cs_pin: pico:gpio17
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I am adding my notes on the errors returned by the Pico based on what I learned in making so many mistakes in the hope that it may help others -

Recv: !! Invalid adxl345 id (got ff vs e5)
This is the error I got when I did not have any wires connected to the correct GP pins.

Recv: !! Invalid adxl345 id (got 00 vs e5)
This is the error I got when I did not have the correct cs_pin identified in the config file.

Recv: !! Invalid adxl345 id (got f2 vs e5)
Known problem see ADXL345 on Raspberry Pi Pico stopped after specific commit · Issue #4712 · Klipper3d/klipper · GitHub I get this the first time I access the Pico after plugging it in, but thereafter it works

Recv: !! Invalid adxl345 id (got 8f vs e5)
From https://github.com/Klipper3d/klipper/issues/3637 if you get this error it may be a grounding issue.

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Thanks for sharing this. I’m not sure though, if there is a real system behind this. I have seen various error messages all across for various reasons.
Especially nasty are these where basically everything is fine but the sensor is unhappy because the cable is not made of pure gold with a triple screen made of Kryptonite


Very true - correlation is not causation. If nothing else it is a list of almost everything that can be done wrong. The only thing I may have done right is have a relatively short Cat 5e cable between the accelerometer and the Pico ( its about 300 mm long). The USB cable is plenty long to reach the keystone on the back of the printer. I got about half the noise readings compared to the long cable that I originally used to connect directly to the Pi GPIO. Although again I have not done enough research to know why the noise levels changed and it could be something else.

A post was split to a new topic: Use Pico ADXL measurements outside of Klipper

Sorry for the necro, but I thought I’d chime in to confirm that the Adafruit ADXL343 works great with Klipper, if you’d prefer to save some money. I wired an Adafruit ADXL343 up to a Pico and used it to run SHAPER_CALIBRATE on my Trident successfully.

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