9.2.3. beagle

This BSP supports four variants, beagleboardorig, beagleboardxm, beaglebonewhite and beagleboneblack. The basic hardware initialization is not performed by the BSP. A boot loader with device tree support must be used to start the BSP, e.g., U-Boot.

TODO(These drivers are present but not documented yet):

  • Clock driver.

  • Network Interface Driver.

  • SDcard driver.

  • GPIO Driver.

  • Console driver.

  • PWM Driver.

  • RTC driver. Boot via U-Boot

To boot via uboot, the ELF must be converted to a U-Boot image like below:

arm-rtems6-objcopy hello.exe -O binary app.bin
gzip -9 app.bin
mkimage -A arm -O linux -T kernel -a 0x80000000 -e 0x80000000 -n RTEMS -d app.bin.gz rtems-app.img Getting the Device Tree Blob

The Device Tree Blob (DTB) is needed to load the device tree while starting up the kernel. We build the dtb from the FreeBSD source matching the commit hash from the libbsd HEAD of freebsd-org. For example if the HEAD is at “19a6ceb89dbacf74697d493e48c388767126d418” Then the right Device Tree Source (DTS) file is: https://github.com/freebsd/freebsd/blob/19a6ceb89dbacf74697d493e48c388767126d418/sys/gnu/dts/arm/am335x-boneblack.dts

Please refer to the Device Tree to know more about building and applying the Device Trees. Writing the uEnv.txt file

The uEnv.txt file is needed to set any environment variable before the kernel is loaded. Each line is a u-boot command that the uboot will execute during start up.

Add the following to a file named uEnv.txt:

setenv bootdelay 5
uenvcmd=run boot
boot=fatload mmc 0 0x80800000 rtems-app.img ; fatload mmc 0 0x88000000 am335x-boneblack.dtb ; bootm 0x80800000 - 0x88000000 I2C Driver

The Beagle i2c initialization is based on the device tree. To initialize a i2c device, the user has to enable the respective node in the device tree using overlays.

For registering an I2C device with a custom path (say /dev/i2c-eeprom) an overlay has to be provided. The overlay must add an additional attribute rtems,path with the custom path as value to the respective i2c node.

For example,


/ {
   compatible = "ti,am335x-bone-black", "ti,am335x-bone", "ti,am33xx";

   fragment@0 {
      target = <0xffffffff>;

      __overlay__ {
         compatible = "rtems,bsp-i2c", "ti,omap4-i2c";
         status = "okay";
         rtems,path = "/dev/i2c-eeprom";

   __fixups__ {
      i2c0 = "/fragment@0:target:0";

The above example registers a custom path /dev/i2c-eeprom for i2c0. SPI Driver

The SPI device /dev/spi-0 can be registered with bbb_register_spi_0()

For registering with a custom path, the bsp_register_spi() can be used.

The function prototype is given below:

rtems_status_code bsp_register_spi(
   const char         *bus_path,
   uintptr_t           register_base,
   rtems_vector_number irq
); Debugging using libdebugger

RTEMS’s libdebugger requires the ARM debug resources be enabled for it to work. The TI SOC used on the beagleboneblack board provides no access for software to the ARM defined debug enable signal DBGEN. The signal is negated on power up locking software out of the ARM debug hardware. The signal can only be accessed via the JTAG interface.

The beagleboneblack BSP provides a low level solution to enable the DBGEN signal via the JTAG interface if the board has the following hardware modification installed. The modification requires the addition of two small wire links soldered to the pads of the JTAG connect on the underside of the board. A small length of fine wire, a fine tip soldering iron, some good quality solder and a pair of fine tip pliers are required. If you are new to soldering I suggest you find something to practice on first.

The modification details and software driver can be found in the BSP in the file bsps/arm/beagle/start/bspdebug.c. The driver is automatically run and the DBGEN is asserted via JTAG when libdebugger is started.

The modification is:

  1. Locate P2 on the bottom side of the board. It is the JTAG connector pads. If you look at the underside of the board with the SD card holder to the right the pads are top center left. There are 20 pads in two columns. The pads are numbered 1 at the top left then 2 top right, 3 is second top on the left, 4 is second top to the right, then the pin number increments as you move left then right down the pads.

  2. Connect P2 to P5.

  3. Connect P7 to P13.

The resulting wiring is:

 1 ===  /--=== 2
 3 ===  |  === 4
 5 ===--/  === 6
 7 ===--\  === 8
 9 ===  |  === 10
11 ===  |  === 12
13 ===--/  === 14
15 ===     === 16
17 ===     === 18
19 ===     === 20
BeagleBone Black JTAG Hardware Modification

Fig. 9.1 BeagleBone Black JTAG Hardware Modification

If libdebugger fails to detect the registers open the bspdebug.c source and change has_tdo to 1, save then rebuild and install the BSP. This will turn on an internal feeback to check the JTAG logic. Discard the edit once the hardware is working. Debugging Beagle Bone Black using a JTAG debugger and gdb

Debugging a Beagle Bone Black (or variants) is also possible using a hardware JTAG debugger. The JTAG is available via P2. The footprint is for an ARM 20 pin cTI connector. That connector should be used, if it is necessary to have access to commercially available adapters.

For hand-made cables and adapters a standard 1.27mm pitch header and a 0.635mm ribbon cable can be much cheaper. But note that even if it looks compatible, it’s not the same pin out as a ARM Cortex 20 pin connector!

A lot of JTAG adapters that are working together with OpenOCD will work. There are also commercially available systems (like Segger J-Link) that work well with the Beagle. Note that the JTAG debugger has to be compatible with ARM Cortex A8. Cortex M only debuggers (like the Segger J-Link Edu Mini) won’t work.

If the debugger offers a gdb server (like OpenOCD or Segger J-Link) the following gdb start script can be used:

define reset
        echo -- Reset target and wait for U-Boot to start kernel.\n
        monitor reset
        # RTEMS U-Boot starts at this address.
        tbreak *0x80000000
        # Linux starts here.
        tbreak *0x82000000

        echo -- Disable watchdog.\n
        set *(uint32_t*)0x44e35048=0xAAAA
        while (*(uint32_t*)0x44e35034 != 0)
        set *(uint32_t*)0x44e35048=0x5555
        while (*(uint32_t*)0x44e35034 != 0)

        echo -- Overwrite kernel with application to debug.\n

target remote :2331

Note that you might have to replace the monitor reset by some other command that resets the target using your specific debugger. You also have to replace the target remote :2331 to match the port of your gdb server.

The script expects that the Beagle Bone Black starts some application from an SD card or from eMMC. It defines a reset command that does the following:

  • reset the target

  • let U-Boot run, initialize the base system, load an FDT and an application

  • break at the application entry point

  • disable the watchdog

  • overwrite the application that has been loaded by U-Boot with the application provided as an command line argument to gdb

This method has the advantage that the application is executed in nearly the same environment like it would be executed if loaded by U-Boot directly (except for the watchdog).