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I wan't to enable the mini UART on a CM3 for Linux console access in emergency cases. The full UART will be used for other purposes.

With a newly flashed CM3 I can access the Linux console using GPIO14/15.
On the system this is /dev/ttyAMA0.

I have done the following steps:

  1. Disabled the system service: systemctl disable hciuart

  2. Edit /boot/cmdline.txt by removing console=/dev/ttyAMA0,115200
    The plan is to later add console=/dev/ttyS0,115200

  3. Add these lines to /boot/config.txt

    dtoverlay=pi3-miniuart-bt
    enable_uart=1
    core_freq=250

  4. Reboot

After the reboot I connect using ssh.
Inside I can't find /dev/ttyS0

Is the mini UART a viable option for emergency access to the Linux console?

What can I do to enable the mini UART?

What GPIO/pins will each UART appear on? GPIO14/15 vs GPIO32/33

1

At step 3 it was enough to add the following line.

dtoverlay=uart1,txd1_pin=32,rxd1_pin=33

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