That pin is used to tell the BBB about its boot mode. SYS_BOOT14
If you require to use that pin in external circuitry you need to use a
buffer to isolate
it until the board boots up.
On 3/31/2021 2:46 AM, 'Adam' via BeagleBoard wrote:
Hi,
I'm using a BBB with Debian on an SD card, and am having issues
booting up whilst Pin 31 on P8 is held low.
When P8.31 is held high, i.e. held at 3V3, or not connected to
anything, the normal boot-up sequence occurs. I can see LEDs D2 to D5
flashing, and can see the standard boot-up output from the debug
serial connection on my terminal emulator.
When P8.31 is held low, i.e. 0V, all 4 LEDs light up, but remain lit
and don't flash. It appears the BB doesn't finish it's boot-up
sequence, and am unable to communicate with it via ethernet/USB.
Viewing the Debug output doesn't help either as no matter what speed I
select on my PC's terminal emulator, it outputs garbage characters.
After a few seconds all 4 LEDs turn off and appears as though it tries
to boot again as all 4 LEDs light up.
Holding the boot switch (S2) on the BBB and then applying power has no
affect on the observations described above.
P8.31 is to be used as UART5 CTS and is connected to R2OUT of a
MAX13234EEUP+ RS232 converter on a custom board. When P8.31 is
connected to the RS232 Converter, this pin is held low.
In the /boot/uEnv.txt file I have the following lines enabled to allow
P8.31 to be used as UART5's CTS:
uboot_overlay_addr3=/lib/firmware/BB-UART5-00A0.dtbo
uboot_overlay_addr4=/lib/firmware/BB-UART5-RTSCTS-00A0.dtbo
disable_uboot_overlay_video=1
disable_uboot_overlay_audio=1
However enabling/disabling these lines seem to make no difference to
the boot-up issue.
Any help with this would be appreciated.
Thanks in advance,
Adam.
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