dw-gdbserver

What is it good for?

This Python script acts as a GDB server for debugWIRE MCUs, such as the ATmega328P. It can communicate with Microchip debuggers such as Atmel-ICE and MPLAB SNAP (in AVR mode), and it provides a pass-through service for the DIY hardware debugger dw-link. For Microchip debuggers, it uses the infrastructure provided by pymcuprog and pyedgblib to implement a full-blown GDB server.

By the way, switching to AVR mode in the SNAP debugger is easily accomplished by using avrdude (>= Version 7.3):

> avrdude -c snap_isp -Pusb -xmode=avr

With PICkit4 it is similar. When you repeat this command, and you get the message again that the debugger is in 'PIC' mode, you need to flash new firmware first using MPLAB X.

Installation

Install the script with pipx like this:

> pipx install dwgdbserver

Usage

If your target board is an Arduino board, you most probably have to first modify it by disconnecting the capacitor that is responsible for the auto-reset feature.

Once you have connected an appropriate hardware debugger to your target board, you can start the gdbserver in a terminal window:

> dw-gdbserver -d atmega328p
[INFO] Connecting to anything possible
[INFO] Connected to Atmel-ICE CMSIS-DAP
[INFO] Starting dw-gdbserver
[INFO] Looking for device attiny1634
[INFO] Listening on port 2000 for gdb connection

In another terminal window, you can now start a GDB session:

> avr-gdb <progname>.elf
GNU gdb (GDB) 15.2
Copyright (C) 2024 Free Software Foundation, Inc.
...
(gdb) target remote :2000
Remote debugging using :2000
0x00000000 in __vectors ()
(gdb) monitor debugwire
debugWIRE mode is disabled
(gdb) monitor debugwire enable
*** Please power-cycle the target system ***
Ignoring packet error, continuing...
debugWIRE mode is enabled
(gdb) load
Loading section .text, size 0x596 lma 0x0
Start address 0x00000000, load size 1430
Transfer rate: 1 KB/sec, 1430 bytes/write.
(gdb) break loop
Breakpoint 1 at 0x470: file /Users/.../varblink0.ino, line 13.
Note: automatically using hardware breakpoints for read-only addresses.
(gdb) continue
...

Command line options

Optionname	Descripion
--device -d	The argument to this option specifies the MCU type of the target chip in lower case. This option is mandatory. If a '?' mark is given, all supported MCUs are listed.
--gede -g	No argument for this option. This option will start the Gede debugger GUI.
--port -p	IP port on the local host to which GDB can connect.
--start -s	Program to start or the string noop, when no program should be started
--tool -t	Specifying the debug tool. Possible values are atmelice, edbg, jtagice3, medbg, nedbg, pickit4, powerdebugger, snap, dwlink. Use of this option is helpful only if more than one debugging tool is connected to the computer.
--usbsn -u	USB serial number of the tool. This is only necessary if one has multiple debugging tools connected to the computer.
--verbose -v	Specify verbosity level. Possible values are debug, info, warning, error, or critical.
--version -V	Print dw-gdbserver version number and exit.

How to get into and out of debugWIRE mode

When the MCU is not already in debugWIRE mode, you must request the switch to debugWIRE mode using the command monitor debugwire enable in GDB. The debugger will then enable the DWEN fuse and either power-cycles the target by itself (if possible) or ask you to power-cycle the target system. Once this is done, the chip will stay in debugWIRE mode, even after terminating the debugging session. In other words, when starting the next debug session, the MCU is already in debugWIRE mode. You can switch back to normal mode using the command monitor debugwire disable before leaving the debugger.

Monitor commands

In addition to the above mentioned command for enabling debugWIRE mode, there are a few other monitor commands.

Command	Action
monitor breakpoints [all|software|hardware]	Restricts the kind of breakpoints the hardware debugger can use. Either all types are permitted, only software breakpoints are allowed, or only hardware breakpoints can be used. Using all kinds is the default.
monitor caching [enable|disable]	The loaded executable is used as a cache in the gdbserver when enabled, which is the default.
monitor debugwire [enable|disable]	DebugWIRE mode will be enabled or disabled. When enabling it, you may be asked to power-cycle the target. After disabling debugWIRE mode, the MCU can be programmed again using ISP programming.
monitor load [readbeforewrite|writeonly]	When loading an executable, either each flash page is compared with the content to be loaded, and flashing is skipped if the content is already there, or each flash page is written without reading the current contents beforehand. The first option is the default option and there is no reason to change it.
monitor onlyloaded [enable|disable]	Execution is only possible when a load command was previously executed, which is the default. If you want to start execution without previously loading an executable, you need to disable this mode.
monitor reset	Resets the MCU.
monitor singlestep [safe|interruptible]	Single-stepping can either be performed in a safe way, where single steps are shielded against interrupts or in the default way, where a single step can lead to a jump into the interrupt dispatch table. The safe option is the default.
monitor timer [freeze|run]	Timers can either be frozen when execution is stopped, or they can run freely. The later option is helpful when PWM output is crucial.
monitor verify [enable|disable]	Verify flash after loading each flash page. The cost for verifying is negligible, and doing so might diagnose flash wear problems. The default is that this option is enabled.
monitor version	Show version of the gdbserver.

The default setting is always the first one listed, except for debugwire, where it depends on the MCU itself. All commands can, as usual, be abbreviated. For example, mo d e is equivalent to monitor debugwire enable.

List of supported and tested hardware debuggers

Except for dw-link, this list is copied from the readme file of pyedbglib. Boldface means that the debuggers have been tested by me and work with this Python script.

• PKOB nano (nEDBG) - on-board debugger on Curiosity Nano
• MPLAB PICkit 4 In-Circuit Debugger (when in 'AVR mode')
• MPLAB Snap In-Circuit Debugger (when in 'AVR mode')
• Atmel-ICE
• Power Debugger
• EDBG - on-board debugger on Xplained Pro/Ultra
• mEDBG - on-board debugger on Xplained Mini/Nano
• JTAGICE3 (firmware version 3.0 or newer)
• dw-link - DIY debugWIRE debugger running on Arduino UNO R3

List of supported and tested MCUs

This is the list of all debugWIRE MCUs, which should all be compatible with dw-gdbserver. MCUs tested with this Python script are marked bold. MCUs known not to work with the script are struck out. For the list of MCUs compatible with dw-link, you need to consult the dw-link manual.

ATtiny (covered by MicroCore):

• ATtiny13

ATtinys (covered by the ATTinyCore):

• ATtiny43U
• ATtiny2313(A), ATtiny4313
• ATtiny24(A), ATtiny44(A), ATtiny84(A)
• ATtiny441, ATtiny841
• ATtiny25, ATtiny45, ATtiny85
• ATtiny261(A), ATtiny461(A), ATtiny861(A)
• ATtiny87, ATtiny167
• ATtiny828
• ATtiny48, ATtiny88
• ATtiny1634

ATmegas (covered by MiniCore):

• ATmega48, ATmega48A, ATmega48PA, ATmega48PB,
• ATmega88, ATmega88A, ATmega88PA, Atmega88PB,
• ATmega168, ATmega168A, ATmega168PA, ATmega168PB,
• ATmega328, ATmega328P, ATmega328PB

The ATmega48 and ATmega88 (without the A-suffix) sitting on my desk suffer from stuck-at-one bits in the program counter and are, therefore, not debuggable by GDB. I suspect that this applies to all chips labelled this way. In any case, the test for stuck-at-one-bits is made when connecting to the chips.

Other ATmegas:

• ATmega8U2, ATmega16U2, ATmega32U2
• ATmega32C1, ATmega64C1, ATmega16M1, ATmega32M1, ATmega64M1, ATmegaHVE2
• AT90USB82, AT90USB162
• AT90PWM1, AT90PWM2B, AT90PWM3B
• AT90PWM81, AT90PWM161
• AT90PWM216, AT90PWM316
• ATmega8HVA, ATmega16HVA, ATmega16HVB, ATmega32HVA, ATmega32HVB, ATmega64HVE2

Notes for Linux systems

The following text is copied verbatim from the README of pyedbglib.

HIDAPI needs to build using packages: libusb-1.0.0-dev, libudev-dev

USB devices need udev rules to be added to a file in /etc/udev/rules.d

Example of udev rules for supported debuggers:

# HIDAPI/libusb:

# JTAGICE3
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2140", MODE="0666"
# Atmel-ICE
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2141", MODE="0666"
# Power Debugger
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2144", MODE="0666"
# EDBG - debugger on Xplained Pro
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2111", MODE="0666"
# EDBG - debugger on Xplained Pro (MSD mode)
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2169", MODE="0666"
# mEDBG - debugger on Xplained Mini
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2145", MODE="0666"
# PKOB nano (nEDBG) - debugger on Curiosity Nano
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2175", MODE="0666"
# PKOB nano (nEDBG) in DFU mode - bootloader of debugger on Curiosity Nano
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2fc0", MODE="0666"
# MPLAB PICkit 4 In-Circuit Debugger
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2177", MODE="0666"
# MPLAB Snap In-Circuit Debugger
SUBSYSTEM=="usb", ATTRS{idVendor}=="03eb", ATTRS{idProduct}=="2180", MODE="0666"

pyedbglib also provides helper functions for accessing serial ports. The user has to be part of the 'dialout' group to allow this. This can be done by executing:

sudo adduser $USER dialout

It may also be necessary to grant read+write permission to the port, for example:

sudo chmod a+rw /dev/ttyACM0

What the future has in store for us

The script has all the basic functionality but still needs some polishing.

I also plan to provide binaries, which can be used as tools for the Arduino IDE 2. And if it all works, it is only a tiny step to generalize it to the JTAG and UPDI AVR MCUs. So, stay tuned.