Burning Zero bootloader with Beaglebone as SWD programmer

 

OpenOCD has support for bitbanging the jtag or swd protocol over gpio’s. So if you have a board that can run OpenOCD and on which you have access to gpio’s, you can use that as a jtag or swd programmer. The following describes how to use a beaglebone for this purpose but any board with 3V3 gpio’s (e.g. raspberry pie) will do…

I followed these instructions to set up the beaglebone. I opted for the debian file system.

Following list of things to install before building OpenOCD will probably save you from having to iteratively install packages untill OpenOCD builds. Of coarse the exact list will change if the initial content of the debian file system changes:

apt-get install autoconf libtool make pkg-config 

Get the OpenOCD sources:

git clone git://git.code.sf.net/p/openocd/code openocd-code

Build openOCD:

cd openocd-code
./bootstrap
./configure --enable-sysfsgpio
make 
make install

openocd is now installed in /usr/local/bin and the scripts it uses are in /usr/local/share/openocd/scripts.

We need an extra config file to describe the beaglebone programmer. I added it here: /usr/local/share/openocd/scripts/interface/bb.cfg. The choice of the gpio’s is up to you, I picked gpio 38 (i.e. gpio1[6], pin header P8_03) and 39 (i.e. gpio1[7], pin header P8_04) as swd_io and swd_sck respectively, so bb.cfg looks like this:

#
# Config for beaglebone interface
#
# This is best used with a fast enough buffer but also
# is suitable for direct connection if the target voltage
# matches bone's 3.3V
#
# Do not forget the GND connection.
#

interface sysfsgpio

# minimal swd setup
sysfsgpio_swdio_num     38
sysfsgpio_swclk_num     39

# (Did not manage to make srst work:)
# sysfsgpio_srst_num      45

Store dot-cc’s bootloader (samd21_sam_ba.bin) in a directory of your choice on the beaglebone.

In that dir, create a openocd.cfg file. OpenOCD will read this file to know what it has to do, and how. In our case the file instructs OpenOCD to use the beaglebone as interface (programmer) and tells it we’re programming a samd21 chip.

source [find interface/bb.cfg]
transport select swd

set CHIPNAME at91samd21g18
source [find target/at91samdXX.cfg]

# did not yet manage to make a working setup using srst
#reset_config srst_only
reset_config  srst_nogate

adapter_nsrst_delay 100
adapter_nsrst_assert_width 100

init
targets
reset halt

If you invoke openocd in this directory, you should see following output.

# openocd
Open On-Chip Debugger 0.10.0-dev-00040-gd52070c (2015-10-11-18:21)
Licensed under GNU GPL v2
For bug reports, read
        http://openocd.org/doc/doxygen/bugs.html
SysfsGPIO num: swdio = 38
SysfsGPIO num: swclk = 39
SysfsGPIO num: srst = 45
adapter speed: 500 kHz
adapter_nsrst_delay: 100
cortex_m reset_config sysresetreq
none separate
adapter_nsrst_delay: 100
adapter_nsrst_assert_width: 100
Info : SysfsGPIO JTAG/SWD bitbang driver
Info : SWD only mode enabled (specify tck, tms, tdi and tdo gpios to add JTAG mode)
Info : This adapter doesn't support configurable speed
Info : SWD IDCODE 0x0bc11477
Info : at91samd21g18.cpu: hardware has 4 breakpoints, 2 watchpoints
    TargetName         Type       Endian TapName            State       
--  ------------------ ---------- ------ ------------------ ------------
 0* at91samd21g18.cpu  cortex_m   little at91samd21g18.cpu  running
target state: halted
target halted due to debug-request, current mode: Thread 
xPSR: 0x21000000 pc: 0x000028f4 msp: 0x20002c00

OpenOCD is now constantly polling the device and it is ready to accept new instructions from us via telnet or gdb. You could log into openocd using telnet from your pc (or from the bone) and inspect memory, run code, start debugging…

If on the other hand you are just interested in burning the bootloader, you could add following line at the end of openocd.cfg and start openocd again.

at91samd bootloader 0; program samd21_sam_ba.bin verify reset; shutdown

OpenOCD should now produce following extra report about the flashing operation.

** Programming Started **
auto erase enabled
Info : SAMD MCU: SAMD21G18A (256KB Flash, 32KB RAM)
wrote 16384 bytes from file samd21_sam_ba.bin in 4.120137s (3.883 KiB/s)
** Programming Finished **
** Verify Started **
verified 6328 bytes in 0.575388s (10.740 KiB/s)
** Verified OK **
** Resetting Target **
shutdown command invoked

The M0 is now ready to receive sketches from the dot-cc IDE.
Remark: if you previously made the modification to the dot-cc lnker script from my previous post, don’t forget to undo this change.

In case you want to restore the dot-org bootloader, use this line as last line in openocd.cfg instead:

at91samd bootloader 0; program Bootloader_D21_M0_150515.hex verify reset; shutdown