ACE - Stuff happens

25 Apr 2021

This was not a planned blog post, but I bricked the Arduino Nano and may be my experience could be useful to anyone else who may have the same kind of problem in the future

What happened

While testing low-power modes, I created an application that put to sleep the Arduino Nano's Cortex M0 CPU every second... which means I had no enough time to download a different application from the Arduino IDE...

What I did

The only option you have when nothing else works is a low-level interface like JTAG or the even simpler SWD. The latter is the default in-circuit programming interface for ARM devices so it is supported by all the ARM processors.

I had in my lab an STLink/V2 programmer, which is supposed to support both SWD and JTAG interfaces but for some reason I was unable to connect to the board

Luckily I found one amazing open source project called OpenOCD which can work on different debuggers and microcontrollers, having configuration files of almost all available ARM Cortex SoC and the best part is that it has support for Raspberry PI GPIO bit-bang programmer profile. This means we can use Raspberry Pi GPIOs to behave as a programmer pins and connect it to our microcontroller to flash bootloaders.

Prepare the hardware

You need to connect only two wires: SWD (pin 2) and SWCLK (pin3)

I connected SWCLK to GPIO 25 of the Raspberry Pi and SWD to GPIO 24 (see picture below). Blu wire is connected to SWD, the yellow wire to SWCLK. The red wire is the reset, but looks like it is not strictly required

I powered up the Arduino board using a USB cable connected to Raspberry Pi so that extra ground wiring is not required

Install openOCD

sudo apt-get install autoconf libtool libusb-dev
git clone --recursive git://git.code.sf.net/p/openocd/code openocd-code
cd openocd-code

Build openOCD

./bootstrap
./configure --enable-bcm2835gpio
make
sudo make install

Prepare a config file

cd ~
mkdir openocd-config
cd openocd-config
nano openocd.cfg
Paste the following content

interface bcm2835gpio

# Raspi1 peripheral_base address
# bcm2835gpio_peripheral_base 0x20000000
# Raspi2 and Raspi3 peripheral_base address
bcm2835gpio_peripheral_base 0x3F000000

# Raspi1 BCM2835: (700Mhz)
# bcm2835gpio_speed_coeffs 113714 28
# Raspi2 BCM2836 (900Mhz):
# bcm2835gpio_speed_coeffs 146203 36
# Raspi3 BCM2837 (1200Mhz): 
bcm2835gpio_speed_coeffs 194938 48

# SWD GPIO set: swclk swdio
bcm2835gpio_swd_nums 25 24

transport select swd

set CHIPNAME at91samd21
source [find target/at91samdXX.cfg]

# Uncomment & lower speed to address errors
# adapter_khz 1000

init
targets
reset halt