Right...
I'm catching up on this CPU, I've done a bit more of this on the iMX6,
but it appears
to be quite similar...
Referring to the schematic here:
https://github.com/CircuitCo/BeagleBone-Black/blob/master/BBB_SCH.pdf?raw=true
When the CPU first starts out of a cold reset, it will execute code that
is internal
to the Sitara chip, totally independent of what memory devices are tied
to it.
It will initially look at some of its GPIO pins (configured via pullups
on pg 6) to
determine how it is supposed to attempt to access external devices to
pull in the initial
image you're referring to.
Note that the boot-mode button (pg 6) simply allows the user to select
either eMMC
or uSD; however there are other modes by which the CPU can boot. Based
on the
table on pg 6, apparently this device (as is typical) can boot from
UART, USB or SPI flash
as well as the eMMC/uSD it is currently configured for.
For example, a few years ago I worked on a Blackfin based board that
only had SPI
flash and DRAM. We had a jumper on the board (similar to the button on
pg 6) that
we could set so that upon reset, the boot mode was Boot-from-UART.
Then I was able to simply use a terminal emulator to push down an
initial set of images
(in an expected format documented for the Blackfin) that eventually
loaded a real boot
image into the SPI flash. Its very likely that this will be the same
case, just using eMMC
and uSD as the boot sources instead.
The important thing to realize here is that the CPU is booting up
initially with built-in
code, and then it looks at some pins to figure out how the actual
hardware design
is configured to boot the next stage.
Good stuff!
Ed
It looks like the internal ROM-based bootloader looks for a secondary
program loader (SPL) that initializes the necessary devices to
continue the boot process and pass control to a third-stage
bootloader. So now I believe it's a matter of finding whether there
are existing code implementations of this SPL, or last-case scenario
this would have to be implemented. Time for more investigating. :)
On Thu, Mar 26, 2015 at 9:27 AM, Jarielle Catbagan
<jcatbaga...@gmail.com> wrote:
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
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Ed Sutter
Alcatel-Lucent Technologies -- Bell Laboratories
Phone: 908-582-2351
Email: ed.sut...@alcatel-lucent.com
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