Re: BeagleBone Black doesn't boot without serial console (10.0 release)

[Brook Milligan <brook%nmsu.edu@localhost>]

> On Oct 11, 2024, at 6:36 AM, Manuel Bouyer <bouyer%antioche.eu.org@localhost> wrote:
> 
> From the schematic here:
> https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fbeagleboard%2FBeagleBone-Black%2Fblob%2Fmaster%2FBBB_SCH.pdf&data=05%7C02%7Cbrook%40biology.nmsu.edu%7C20c9957b85684c02c51408dce9f1823c%7Ca3ec87a89fb84158ba8ff11bace1ebaa%7C1%7C0%7C638642470732586576%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C60000%7C%7C%7C&sdata=IXknH0lQWQWxeS7YvTwVoVFGRoLOaY84J2uZVzkQygc%3D&reserved=0
> 
> there is a pulldown resitor on UART0_RX input. This means that without
> a (powered up) USB/serial adapter, UART0_RX sees a logical 0 which
> is most probably interpreted as a BREAK by the AM335x UART, and causes
> the kernel to enter DDB.
> 
> The solution is to either rebuild a kernel without DDB, or tie UART0_RX to
> a +3.3V pin.

This agrees with my understanding.  I have had a fair amount of experience with the BBB.  Although I often boot with a serial cable (to be clear: https://ftdichip.com/products/ttl-232r-rpi/), I am sure that I have also done so without and had no problems.  That is, disconnecting the serial cable before powering on the BBB should work fine.

There is, however, a problem with connecting/disconnecting the serial cable on a live BBB; then the signal levels change on the line, which the kernel detects.

I am currently using a 10k pull-up resistor on the Rx line, which allows connection/disconnection of the serial cable on a live BBB without disrupting the kernel.  For example, I can boot the BBB and later connect the serial cable to monitor it without affectiing the kernel.  This works fine for a long-running embedded device.

Cheers,
Brook