GSOC 2015: Porting MicroMonitor to the Beaglebone Black

[Jarielle Catbagan]

To put things into context in regards to the conversation that I was
having with Ed, Dr. Joel, and Gedare:

I am currently in the process of looking into porting MicroMonitor to
the Beaglebone Black.  As indicated by Ed, "[t]he difficulty of the
port will depend on how much existing CPU-initialization
(clocks, cache, etc..) code we can reuse."

Ed has also indicated to me that there might be an internal bootloader
stored in a ROM-based memory that might look for an image in a
specific format.  I will definitely be investigating more into this.
I did manage to briefly browse through the Beaglebone Black System
Reference Manual Rev C.1 [1], and I have found that the boot
configuration/process is briefly elaborated in section 6.7.  For
convenience, since it's a short section I will post it here:

"The design supports two groups of boot options on the board. The user
can switch between these modes via the Boot button. The primary boot
source is the onboard eMMC device. By holding the Boot button, the
user can force the board to boot from the microSD slot. This enables
the eMMC to be overwritten when needed or to just boot an alternate
image...

[T]the processor-external boot code is composed of two stages. After
the primary boot code in the processor ROM passes control, a secondary
stage (secondary program loader -- "SPL" or "MLO") takes over. The SPL
stage initializes only the required devices to continue the boot
process, and then control is transferred to the third stage "U-boot".
Based on the settings of the boot pins, the ROM knows where to go and
get the SPL and UBoot code. In the case of the BeagleBone Black, that
is either eMMC or microSD based on the position of the boot switch."

I was kindly guided to look into programming a uSD card as it might be
more efficient to run MicroMonitor off of the uSD for quick testing
after every build.  If all goes well, either an application image will
be located and booted off of the same SD card or via a network boot.
For serial debugging I have an FTDI 3.3V USB-to-Serial cable that I
have been previously using to access the U-boot monitor on the
Beaglebone Black.


[1] https://github.com/CircuitCo/BeagleBone-Black/blob/master/BBB_SRM.pdf?raw=true