Bare Metal Application on the BeagleBone
We are using BOOTP (Bootstrap Protocol) and TFTP to transfer a Boot image (binary) and run it. The boot image is downloaded directly into internal RAM at the location 0x402F0400 on GP devices. The maximum size of downloaded image is 109 KB.

Setup

StarterWare

• Download Linaro GCC Compiler: https://launchpad.net/gcc-arm-embedded/4.7/4.7-2012-q4-major
  (Version 9 works too: https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads)
• Download StarterWare installer for AM335x and BeagleBone Black patch from http://www.ti.com/tool/starterware-sitara
  
• Build the libraries. The easiest way is to build all examples:
  

# path/to/AM335X_StarterWare/build/armv7a/gcc/makedefs:
LIB_PATH=${HOME}/src/gcc-arm-none-eabi-9-2020-q2-update/
LIB_GCC=${LIB_PATH}/lib/gcc/arm-none-eabi/9.3.1/
LIB_C=${LIB_PATH}/arm-none-eabi/lib/
GCCR=${HOME}/src/gcc-arm-none-eabi-9-2020-q2-update/bin
PREFIX=arm-none-eabi-

/path/to/AM335X_StarterWare/build/armv7a/gcc/am335x/beaglebone$ make all

Network Manager

Assign the static IP 192.168.8.1/24 to the enp0s4f0u3 interface (BBB - RNDIS Ethernet over USB)
c.f. 01-bbb.rules

firewall

# In case you are using iptables:
# iptables.conf:
-A INPUT -p udp --dport 67:68 --sport 67:68 -j ACCEPT
-A INPUT -p udp -s 192.168.8.2 -d 192.168.8.1 -j ACCEPT
-A OUTPUT -p udp -s 192.168.8.1 -d 192.168.8.2 -j ACCEPT
-A OUTPUT -p udp --sport 67:68 --dport 67:68 -j ACCEPT

Hardware

Connect a 10K pulldown resistor between P8_43 and GND; this will make the boot order: SPI0, MMC0 (SD card), USB0, UART0. Without an SD card (and no SPI device present), USB0, will be the boot device chosen. The ROM code implements the RNDIS class driver, and from a user's perspective, USB boot is indistinguishable from Ethernet boot. (Note: for EMAC boot, pullup P8_45)

UART Console:

BBB (J1)       FTDI
───────┐      ┌─────
DGND  1├──────┤ GND
NC    2│      │ CTS
NC    3│      │ 5V
RX    4├──────┤ TX
TX    5├──────┤ RX
NC    6│      │ DTR

Connect USB0 (P4) to a host computer.

dnsmasq.conf

dhcp-vendorclass=set:bbrom,AM335x ROM BOOTP Identifier string, from EMAC Boot Procedure, AM335x TRM
dhcp-range=192.168.8.0,static must match the subnet assigned to the device
dhcp-host=a0:f6:fd:8b:6a:c9,192.168.8.2,barebone this MAC can show up on LOGs from iptables. Different than the one from ifconfig

Compilation & Run

c.f. Makefile test.c

make all #make test.c
stty -F /dev/ttyUSB0 115200 cs8 -cstopb -parenb  #-parenb means no parity bit
minicom -D /dev/ttyUSB0 #or screen /dev/ttyUSB0, in one terminal
                        # CCCC... on /dev/ttyUSB0 means boot sequence cycling?
                        #                                xmodem does that too
sudo dnsmasq -d -C dnsmasq.conf #on another terminal
# debug: tshark -i <usb-rndis>
# "Hello World" should appear on /dev/ttyUSB0

References

https://github.com/beagleboard/beaglebone-black/wiki/System-Reference-Manual
https://octavosystems.com/app_notes/bare-metal-on-osd335x-using-u-boot/
https://www.ti.com/lit/ug/spruh73q/spruh73q.pdf