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Short and General Question

I'm running Debian 9 (Stretch) on a Beaglebone Black with kernel 4.9. I have tried following the instructions that I've found online, but have thus far been seemingly unable to configure its UART devices. How do I configure my device? How do I debug/trouble-shoot my configuration? How can I verify whether my configuration is correct?

Long and Detailed Question

I'm very new to embedded Linux in general and to Beaglebone in particular. I followed the instructions given here:

• BeagleBoard.org: Getting Started

I downloaded the latest "Stretch IoT (without graphical desktop) for BeagleBone and PocketBeagle via microSD card" image (specifically bone-debian-9.3-iot-armhf-2018-03-05-4gb.img) from the latest images page and flashed the Beagleboard from an SD card. After installation I was able to access the Beaglebone from my Linux desktop via a serial-to-USB cable - in a manner similar to what's described in the following article:

• How to Connect the Beaglebone Black via Serial over USB

I was able to use the following screen command successfully:

screen /dev/ttyUSB0 115200

I was also able to a Micro-USB to USB cable to connect via SSH:

ssh debian@192.168.7.2

Once connected to the Beaglebone I verified that my kernel version was Linux beaglebone 4.9.82-ti-r102. I then read through several articles on Beaglebone UART configuration, including the following:

• Trevor's Electronics Blog: Enable UART’s on Element 14 Beaglebone Black rev C




• StackOverflow: Enabling UART on Beaglebone Black




• Thing Printer: Cape Manager is Back, Baby!




• Flaming Lotus Girls: Beaglebone Black - enable UART1




• Billwaa's Blog: Enable All UART Ports at Boot




• Tender Love Making(?): Enabling ttyO1 on BeagleBone




• ELinux: BeagleBoneBlack Debian

My take-away was that (generally speaking) there are several different configuration files to check for, depending on the operating system and kernel version. These included the following:

• /boot/uEnv.txt


• /boot/uboot/uEnv.txt


• /etc/default/capemgr


• /media/BEAGLEBONE/uEnv.txt

There were also several device files mentioned, such as:

• /sys/devices/bone_capemgr.*/slots

I tried adding each of the following lines to each of the various uEnv.txt files:

• cape_enable=capemgr.enable_partno=BB-UART1,BB-UART2,BB-UART4,BB-UART5




• optargs=capemgr.enable_partno=BB-UART1,BB-UART2,BB-UART3,BB-UART5

I also tried adding the following line to /etc/default/capemgr:

CAPE=BB-SPI-01,BB-UART1,BB-UART2,BB-UART4

After rebooting I used the following command to check to see if the UART devices had appeared:

ls -l /dev/ttyO*

Unfortunately I got the following (unexpected results):

debian@beaglebone:~$ ls -l /dev/ttyO*
lrwxrwxrwx 1 root root 5 Jan 1 00:00 /dev/ttyO0 -> ttyS0

NOTE: I'm not concerned with ttyO0, but rather with ttyO1, ..., ttyO5.

I also tried a few other things but (in the interest of saving time and space) I won't describe every attempt unless it's requested.

What am I doing wrong here? What's the correct way to enable the UART devices?

I had this problem, and resolved it by simply dropping Debian 9 and using this Debian 8.6 image instead, under the "Older Debian images" category on this page.

Upon booting into it you'll see the /dev/ttyO1,O2,O4 files, but they won't work until you set the pins to the right pinmux (I was thrown for a loop by this, as it all used to be handled by device tree overlays):

#ttyO1:
config-pin P9_24 uart
config-pin P9_26 uart
#ttyO2:
config-pin P9_21 uart
config-pin P9_22 uart
#ttyO4:
config-pin P9_11 uart
config-pin P9_13 uart

You have to do that step on each re-boot. It belongs in some sort of startup script.

You can confirm the UARTs are working with a simple loopback test, e.g. for UART4:

1. Place a wire between P9_11 and P9_13




2. screen /dev/ttyO4




3. Text you type into the window should show up, disconnect the wire and further typing will not show up. The UART works.

This doesn't fix your problem if you need to use the latest Kernel/Debian, but if you just need UART to work (like me) then you should be all set.