iMX6 Hang during boot without UART cable connected

[kal]

Is there a known issue with an iMX6 boot process hanging due to the UART? I have an intermittent problem on some boards where they only boot part-way until a UART cable is physically plugged in.

They will spit out some Rx FIFO overrun errors for a while and then hang, e.g. here the bootup hung until a UART cable was connected 12 minutes later, at which point udevd etc started up:

[ 2.825049] imx-uart 21e8000.serial: Rx FIFO overrun
[ 3.584013] imx-uart 21e8000.serial: Rx FIFO overrun
[ 3.599746] imx-uart 21e8000.serial: Rx FIFO overrun
[ 3.612704] imx-uart 21e8000.serial: Rx FIFO overrun
[ 3.629967] imx-uart 21e8000.serial: Rx FIFO overrun
[ 3.652768] imx-uart 21e8000.serial: Rx FIFO overrun
[ 746.789572] udevd[156]: starting version 182
[ 746.795362] imx-uart 21e8000.serial: Rx FIFO overrun

I've attached a full boot dmesg, where one can see the 12 minute pause before a UART cable was physically connected. After that udevd starts, networking starts etc.

Many thanks

Original Attachment has been moved to: uarts.txt.zip

[erdani80]

Probably you need to place a pull-up resistor on the RX line, if that line has a low level the uart receiver may consider it as a character that needs to be received and this will overflow.

When a cable is connected to another device the the RX will have a high level when no data is available.

[igorpadykov]

Hi kal

one can check if uart rx line is floating or noisy, as this may cause

its overrun errors.

Best regards
igor
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