docs: Document how to connect ADXL345 to Pico then on to Pi. (#5996)

This is based on documentation from:
https://klipper.discourse.group/t/raspberry-pi-pico-adxl345-portable-resonance-measurement/1757

Signed-off-by: Christopher Thompson <chris@hypocrite.org>
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@ -39,6 +39,7 @@ SCLK+CS
##### ADXL345
###### Direct to Raspberry Pi
**Note: Many MCUs will work with an ADXL345 in SPI mode(eg Pi Pico), wiring and
configuration will vary according to your specific board and available pins.**
@ -60,6 +61,31 @@ Fritzing wiring diagrams for some of the ADXL345 boards:
![ADXL345-Rpi](img/adxl345-fritzing.png)
###### Using Raspberry Pi Pico
You may connect the ADXL345 to your Raspberry Pi Pico and then connect the
Pico to your Raspberry Pi via USB. This makes it easy to reuse the
accelerometer on other Klipper devices, as you can connect via USB instead
of GPIO. The Pico does not have much processing power, so make sure it is
only running the accelerometer and not performing any other duties.
In order to avoid damage to your RPi make sure to connect the ADXL345 to 3.3V
only. Depending on the board's layout, a level shifter may be present, which
makes 5V dangerous for your RPi.
| ADXL345 pin | Pico pin | Pico pin name |
|:--:|:--:|:--:|
| 3V3 (or VCC) | 36 | 3.3V DC power |
| GND | 38 | Ground |
| CS | 2 | GP1 (SPI0_CSn) |
| SDO | 1 | GP0 (SPI0_RX) |
| SDA | 5 | GP3 (SPI0_TX) |
| SCL | 4 | GP2 (SPI0_SCK) |
Wiring diagrams for some of the ADXL345 boards:
![ADXL345-Pico](img/adxl345-pico.png)
#### I2C Accelerometers
Suggested twisted pair order:
@ -164,6 +190,65 @@ probe_points:
It is advised to start with 1 probe point, in the middle of the print bed,
slightly above it.
#### Configure ADXL345 With Pi Pico
##### Flash the Pico Firmware
On your Raspberry Pi, compile the firmware for the Pico.
```
cd ~/klipper
make clean
make menuconfig
```
![Pico menuconfig](img/klipper_pico_menuconfig.png)
Now, while holding down the `BOOTSEL` button on the Pico, connect the Pico to
the Raspberry Pi via USB. Compile and flash the firmware.
```
make flash FLASH_DEVICE=first
```
If that fails, you will be told which `FLASH_DEVICE` to use. In this example,
that's ```make flash FLASH_DEVICE=2e8a:0003```.
![Determine flash device](img/flash_rp2040_FLASH_DEVICE.png)
##### Configure the Connection
The Pico will now reboot with the new firmware and should show up as a serial
device. Find the pico serial device with `ls /dev/serial/by-id/*`. You can
now add an `adxl.cfg` file with the following settings:
```
[mcu adxl]
# Change <mySerial> to whatever you found above. For example,
# usb-Klipper_rp2040_E661640843545B2E-if00
serial: /dev/serial/by-id/usb-Klipper_rp2040_<mySerial>
[adxl345]
cs_pin: adxl:gpio1
spi_bus: spi0a
axes_map: x,z,y
[resonance_tester]
accel_chip: adxl345
probe_points:
# Somewhere slightly above the middle of your print bed
147,154, 20
[output_pin power_mode] # Improve power stability
pin: adxl:gpio23
```
If setting up the ADXL345 configuration in a separate file, as shown above,
you'll also want to modify your `printer.cfg` file to include this:
```
[include adxl.cfg] # Comment this out when you disconnect the accelerometer
```
Restart Klipper via the `RESTART` command.
#### Configure MPU-6000/9000 series With RPi
Make sure the Linux I2C driver is enabled and the baud rate is
@ -228,7 +313,9 @@ Recv: // adxl345 values (x, y, z): 470.719200, 941.438400, 9728.196800
```
If you get an error like `Invalid adxl345 id (got xx vs e5)`, where `xx`
is some other ID, it is indicative of the connection problem with ADXL345,
is some other ID, immediately try again. There's an issue with SPI
initialization. If you still get an error, it is indicative of the connection
problem with ADXL345,
or the faulty sensor. Double-check the power, the wiring (that it matches
the schematics, no wire is broken or loose, etc.), and soldering quality.

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