BRK does not Boot without UART

[Julius Salamon]

Hello,

we have a Problem with the BRK board. We created an image as described in the Cube MX tutorial provided by Octavo.
The image is working fine and also all pin changes made in CubeMx do work.

The problem is this.
If we try to boot the BRK board only with the USB cable to power it up, the board is not booting.
It is only booting when we attach a UART adapter to the UART4 header on the board.

What is the reason for this behaviour and how can I disable it?
I already changed the SD Card detection pin according to the BRK shematic so that is not the problem anymore!

Thanks for your help!

[Aedan Cullen]

This may be due to U-Boot thinking that a key is pressed and interrupting the boot process. If the RX signal is floating, garbage data may be received. Try shorting the RX pin of the BRK’s UART to 3.3V and see if that allows the board to boot.

The current device tree in Octavo’s “MinimalConfig” zip on the CubeMX tutorial page seems not to enable the internal pull-up for RX (check if you see bias-disable near line 189 of stm32mp157c-osd32mp157c-512m-baa_minimalconfig-mx.dts.) To fix this, enable the pull-up by using bias-pull-up instead as in this other BRK device tree on GitHub.

[Julius Salamon]

Hello Aedan,

Thank you for your reply.
Your assumption was correct.
When I short the RX Pin with 3.3V everything is working normal.

Now i have the problem that I enabled the pull up in the device tree as you said.
But the pull up is happening after the critical boot stage.
I made sure that the pull up is also enabled in TF-A and U-Boot.

Any suggestions on how to fix that ?

[Julius Salamon]

Never mind. The trick is, to not delete the files that are already prebuild in the devicetree folder.
Then everything works ok.

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