Summary

This is a collection of (NetBeans 8.2) baremetal projects for various Raspberry Pi models.

The JtagBoot projects builds an image that, when copied onto the SD card, will cause the RPI to configure some of the GPIO for JTAG usage. That in turn enables another RPI to act as a JTAG server and can, using OpenOCD, connect to the booted target RPI and load new images without removing the SD card. It also enables the same server RPI to use GDB to debug the Target RPI. See the README.md in the JtagBoot folders for details.

The LebBlink project illustrates the use of C++ classes and standard libraries (newlib); it does NOT use -nostdlib, -nodefaultfiles nor -nostartfiles.

Terminology

Three computers are required:

• The Debian based development PC. Not a Raspberry Pi.

• The Server RPI runs Debian and will have OpenOCD installed on it. It is connected through five P1 header wires to...

• The Target RPI. This is the baremetal RPI. It will boot of an SD card with just enough to run the JtagBoot program once.

Setup

Toolchain

The code is compiled using the crosstools-ng cross compiler toolchain on the Development PC. To set it up on Debian Stretch with gdb support:

sudo apt-get install gperf bison flex gawk libtool libtool-bin texinfo libncurses5-dev help2man automake gcc g++ subversion python-dev
wget http://crosstool-ng.org/download/crosstool-ng/crosstool-ng-1.23.0.tar.bz2
tar -xjf crosstool-ng-1.23.0.tar.bz2
cd crosstool-ng-1.23.0/
sudo mkdir ~/cross
./configure --prefix=~/cross
make
sudo make install
sudo cp ct-ng.comp /etc/bash_completion.d/

Add to ~/.bashrc and restart the terminal to get the new path:

export PATH=~/cross/bin:$PATH

Build a toolchain for arm-unknown-eabihf with gdb support:

cd
mkdir staging
cd staging
ct-ng arm-unknown-eabihf
ct-ng menuconfig

and enable GDB in "Debug facilities" and "hardware (FPU)" in "Target options", "Floating point". Then:

ct-ng build

The last step will take a while. When done, the new compilers can be found in ~/x-tools/arm-unknown-eabi/bin.

OpenOCD

Install OpenOCD on the Server RPI:

wget http://sourceforge.net/projects/openocd/files/openocd/0.10.0/openocd-0.10.0.tar.bz2
tar -xf openocd-0.10.0.tar.bz2
cd openocd-0.10.0
sudo apt-get install texinfo texlive git autoconf libtool make pkg-config libusb-1.0-0 libusb-1.0-0-dev telnet
./configure --enable-bcm2835gpio --enable-sysfsgpio
make
sudo make install

Connect the Server and Target RPIs. The numbers below are PI header numbers (not GPIO numbers).

Signal	Server	Target	Alt	Notes
GND	6	6
TCK	23	33	5
TMS	22	32	5
TDI	19	37	4
TDO	21	29	5
TRST	(26)	(15)	4	Not required. Internal pull-up de-asserts. But note that GPIO 22 cannot be used for anything else!

The JtagBoot program sets up the Alt-mode for there pins on the Target RPI.

Add a file /usr/local/share/openocd/scripts/target/rpi.cfg:

adapter_khz 1000
adapter_nsrst_delay 400
reset_config trst_only

if { [info exists CHIPNAME] } {
 set _CHIPNAME $CHIPNAME
} else {
 set _CHIPNAME rspi
}

if { [info exists CPU_TAPID] } {
 set _CPU_TAPID $CPU_TAPID
} else {
 set _CPU_TAPID 0x07b7617f
}

jtag newtap $_CHIPNAME arm -irlen 5 -expected-id $_CPU_TAPID
set _TARGETNAME $_CHIPNAME.arm
target create $_TARGETNAME arm11 -chain-position $_TARGETNAME
rspi.arm configure -event gdb-attach { halt }

Different PIs have different configuration needs:

# Raspi2 and Raspi3 peripheral_base address
bcm2835gpio_peripheral_base 0x3F000000

# Raspi1 peripheral_base address
bcm2835gpio_peripheral_base 0x20000000

# Raspi3 BCM2837 (1200Mhz): 
bcm2835gpio_speed_coeffs 194938 48

# Raspi2 BCM2836 (900Mhz):
bcm2835gpio_speed_coeffs 146203 36

# Raspi1 BCM2835: (700Mhz)
bcm2835gpio_speed_coeffs 113714 28

Jtag Boot

• Build the JtagBoot project for the correct RPI.
• Copy the kernel.img, bootcode.bin and start.elf to an SD card.
• Boot the Target RPI of that SD card.

The Target RPI is now ready to act as a JTAG device. To connect to it, on the Server RPI:

cd openocd-0.10.0/
sudo openocd -f interface/sysfsgpio-raspberrypi.cfg -f target/rpi.cfg

GDB

Create a .gdbinit in ~ on the Development PC:

set auto-load safe-path /

Create a .gdbinit in the project working directory if it is not already there on the Development PC:

target remote 192.168.0.11:3333
set remotetimeout 10

Running

• Copy the just-built kernel.img from the Development PC to the Target RPI:

  scp kernel.img pi@192.168.0.11:/home/pi

Connect to the OpenOCD from the Development PC and transfer it to the Target RPI:

telnet 192.168.0.11 3333
halt
load_image ../kernel.img 0x8000 bin
resume

TO exit telnet:

Ctrl-]
quit

Printf Support

For printf support, connect pin 8 (TX) on the Target RPI to pin 10 (RX) on the Server RPI and make sure the program executes uart_init(). On the Server RPI, run

minicom -b 9600 -o -D /dev/ttyAMA0

To add CR to LF: Ctrl-A U.

Debugging

Run GDB on the Development PC. Provide the .elf file that was built for the project:

~/x-tools/armv6-rpi-linux-gnueabi/bin/armv6-rpi-linux-gnueabi-gdb -tui blah.elf

Commands:

step        (s)
next        (n)
continue    (c)
finish      (fin)  runs until the end of the current function
load
print       (p)
display

break main
break main.c:34

info br
delete <br>