Boot pins/sequence


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:



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,

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.

From the BBB SRM:
* These pins are also the SYSBOOT pins. DO NOT drive them before the
SYS_RESETN signal goes high. If you do, the board may not boot because you
would be changing the boot order of the processor.

SYSBOOT 14-15 appear to define the crystal frequency. See SPRUH73P (TRM)
table 26-7