PSA - BTT PAD7 with CM4 'not connected' issue, and how to solve it

General Discussion
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Basic Information:

Printer Model:
MCU / Printerboard:
klippy.log

Fill out above information and in all cases attach your klippy.log file (use zip to compress it, if too big). Pasting your printer.cfg is not needed
Be sure to check our “Knowledge Base” Category first. Most relevant items, e.g. error messages, are covered there

Describe your issue:

I was having an issue and solved it after much headache. No post anywhere had ‘all’ the info I needed, so creating this in case it helps someone else.

The issue: if you add a CM4 to the BTT Pad7 following their instructions you are likely going to get errors like ‘mcu CM4 not connected’ and your klippy.log will show errors for 'no such file or directory: ‘/tmp/klipper_host_mcu’ under the MCU CM4 section. Klipper will also not be able to connect to your printer.

Solution: When using the CB1 out of the box, it comes with BTT’s pre-configured image on SD card which is configured and works. If you switch out to a CM4, they say to use the mainstream PI images. After flashing that image BTTs instructions give you some changes to make to config. txt, and printer.cfg, but don’t provide much in term of explanation. Those instructions are accurate and you must follow them! But this issue they ask you to add the following to your printer.cfg

[mcu CM4]
serial: /tmp/klipper_host_mcu
[adxl345]
cs_pin: CM4:None
spi_bus: spidev0.1
axes_map: z,y,-x #

As some of you who are smarter than me know, what they are doing here is accessing the CM4 as an extra MCU (besides the one on your printer board). What’s not mentioned in the BTT docs is that to use the CM4 MCU you need to compile klipper firmware that uses this extra MCU. This is using your Raspberry Pi as a microcontroller. Follow the instructions here, reboot and you should be fine

https://www.klipper3d.org/RPi_microcontroller.html

I logged the issue with BTT’s GH here: documentation error for CM4 - no such file or directory: '/tmp/klipper_host_mcu · Issue #10 · bigtreetech/Pad7 · GitHub

Hope this post existing here will help some else. I really the Pad7. To get sound effects you have to use their version of Klipperscreen, but I’m sure that will change with a new release in the future. And for some reason the Pad7 cannot have adjustable brightness control with a CM4. These are things we can live with for now.

Enjoy!

1 Like
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Remove the CM4, check the connectors on the CM4 and the main board for unwanted debris.

I had an issue with the USB onboard hub not being recognized, ended up being debris in the connector.

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Also good advice. It’s important for newbies(like me) to remember that (always check the connection) and the fact that out of the box Klipper only has one MCU. If you want to use your pi (CM4) for anything, the firmware must be complied for it.

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