BeagleBone Black Setup and Recovery

Setup

BeagleBone Black (BBB) comes with an USB Type A to Mini-B cable. So in order to power up BBB, the easiest way is to connect Mini-B head to BBB connector P4 and then Type A head to a computer USB port. The benefit of using this cable instead of a 5V@1A DC adapter is two fold:

1. BBB doesn’t come with the 5V@1A DC adapter
2. We can use the Ethernet-over-USB feature, which is very convenient

Unlike BeagleBone (BB), BBB doesn’t install FT2232H on board, we can confirm this by comparing the BOM/Schematic of BB and BBB. So there is no way for us to do serial/JTAG-over-USB like with BB. In addition to that, JTAG header is also not installed on BBB, therefore unless installing the header by ourselves, there is no way to use JTAG on BBB. Now for serial debug, the option left for BBB is to use an USB-to-TTL cable. My personal favorite is PL2303 based 4-pin cable like this one. The color code of the pins and their functions are listed below:

The default UART configuration is 115200 8N1, no flow control.

Boot Options

AM335x ROM code (or firmware, if you want to call it that way) samples SYSBOOT[15:0] configuration pins during power on reset (i.e. cold reset). A device list is created based on sampled value. ROM code will try devices in the list one by one until it finds a bootable image. BBB latches SYSBOOT[15:0] as 0x403C, but SYSBOOT[2] can be flipped from 1 to 0 by holding button S2. So, when S2 is released, the boot device list will be eMMC -> microSD -> UART0 -> USB0; when S2 is hold, the boot device list will be microSD -> USB0 -> UART0. Here UART0 is the serial debug port and USB0 is the Mini-B USB port which is used as power source in our current setup.

The ROM code implements the RNDIS class driver. When USB0 is selected as boot device and HOST OS has RNDIS device driver installed, an Ethernet-over-USB channel will be established between BBB and the host, which enables the ROM code to talk with HOST OS via BOOTP protocol and download the image to the SRAM using TFTP. Considering that the reset button S1 performs warm reset only, the only way to initiate a cold reset it to re-apply the power. This fact implies that after we holding S2 and re-apply the power, if the microSD card is not installed, then BBB will always try to boot from USB0 first after we pressing the reset button S1. This is very convenient if we are going to do some baremetal programming.

For more detailed information, see BBB Rev C schematic page 6 and AM335x TRM 26.1.5.

One way to setup bootp/tftp under Linux is to use dnsmasq package. Following configuration should work out of the box after replacing 12:34:56:78:9a:bc with the actual MAC of the BBB. But be aware that the MAC used by ROM code is not necessarily the same as what we get from ifconfig under BBB Linux. We might need to use wireshark or tcpdump to get this MAC. The dhcp-host line is essential because BOOTP uses a fixed mapping between MAC and IP.

# disable DNS
port = 0

# tftp server
enable-tftp
tftp-root = /srv/tftp

# dhcp
dhcp-range = 10.0.0.100,10.0.0.200,12h
dhcp-host  = 12:34:56:78:9a:bc,10.0.0.100
dhcp-boot  = smiley

The configuration above is not enough. We should give the host side interface an IP address. Add following snippet to /etc/network/interfaces shall do the trick. Remember to replace enx123456789abc with the real interface name.

allow-hotplug enx123456789abc

iface enx123456789abc inet static
    address 10.0.0.1
    netmask 255.255.0.0

When the host side RNDIS driver detects the new RNDIS device, it creates a new interface. This event triggers some udev rule which calls ifup --allow=hotplug with the new interface name. The configuration above will then take effect and give that interface a static IP address.

Recovery

We can use the microSD card to recover corrupted on board eMMC, or perform the on board Linux system upgrade. The steps are straightforward:

1. Download a new image from BBB official site
2. Write the image to a microSD card with dd
3. Boot from the microSD card
4. Modify /boot/uEnv.txt to un-comment the line #cmdline=init=/opt/scripts/tools/eMMC/init-eMMC-flasher-v3.sh
5. Reboot, and it will start to flash the eMMC. Status message will be dumped to UART0
6. Remove the microSD card after the flashing is finished