source: rtems-docs/user/bsps/arm/beagle.rst @ 94af618

.. SPDX-License-Identifier: CC-BY-SA-4.0

.. Copyright (C) 2019 Vijay Kumar Banerjee

.. _BSP_arm_beagle:
.. _BSP_arm_beagleboardorig:
.. _BSP_arm_beagleboardxm:
.. _BSP_arm_beagleboneblack:
.. _BSP_arm_beaglebonewhite:

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:

.. code-block:: none

    arm-rtems@rtems-ver-major@-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 :ref:`DeviceTree` 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:

.. code-block:: none

     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,

.. code-block:: none

     /dts-v1/;

     / {
        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:

.. code-block:: c

    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:

.. code-block:: none

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

.. figure:: ../../../images/user/bbb-p2-debug-mod.jpg
  :width: 50%
  :align: center
  :alt: BeagleBone Black JTAG Hardware Modification

  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:

.. code-block:: none

    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
            continue

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

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

    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).