Debugging with MRI


If Smoothie crashes, or you manually tell it to, it can go into a special mode ( MRI ), where if you use a serial cable to connect to it's hardware serial port ( UART0 ), you can use GDB to connect to it, and step through the code, add breakpoints, print values, and more generally debug the code.

This is a feature that would otherwise require an expensive JTAG adapter.

If you are using a JTAG debugger with your Smoothie project, you can scroll down to the sections which discuss mini and full dumps since those are applicable to JTAG debugging as well.

Smoothie contains the Monitor for Remote Inspection, MRI, a debug monitor for the NXP LPC1768 device. This monitor allows the GNU Debugger, GDB, to be connected to the device when a hard fault occurs. It can also be configured to break on start and then used to step through any desired Smoothie source code but this isn't the default configuration in the Smoothie project where it is primarily used for crash analysis.

When Smoothie crashes and MRI takes over control of the device, you will see this text dumped to the serial console:


This line is MRI attempting to tell GDB that Smoothie has stopped for some reason. If an unexpected exception was thrown to cause the crash then you will see something like this before the $T packet.

**Hard Fault**
Status Register: 0x40000000
**Bus Fault**
Status Register: 0x04
Imprecise Data Access

Smoothie will go into MRI mode automatically if a problem occurs.

But sometimes you want to -put- Smoothie into MRI mode to poke around in it's brain, run code step by step etc.

If you want to do that, you have two ways to cause Smoothie to enter MRI mode : 

  • Click on the ISP button next to the Reset button
  • Send the break command over any serial interface ( UART, USB/Serial, or Telnet )

GDB should have been installed along with the GCC compiler and other GNU tools used to build the Smoothie source code.

Note that you need to compile Smoothie yourself before you can do any debugging, please see compiling-smoothie

From a Terminal or Command Prompt that is able to build Smoothie, you should be able to set the current directory to the root of the Smoothie project which contains main.bin, main.elf, etc and run one of the following commands to launch GDB. The main difference between the OS's is the type of name used for the serial port. You should use the same serial port name and baud rate when launching GDB as you used for the Terminal application in which you saw the $T packet. Please note: You must first disconnect your terminal application from Smoothie before attempting to connect GDB to MRI.

In these following examples, we will assume that you are in the root directory of the Smoothie source tree and that your .ELF file is therefore located in the LPC1768/ subdirectory.

MRI is over UART, not USB

You need to connect a USB/UART adapter cable to the UART ( Serial ) 6-pin connector on the Smoothieboard ( next to the USB connector, close to the endstops ). And then use that connection to talk to MRI. You can not talk to MRI over the USB cable


arm-none-eabi-gdb LPC1768\main.elf --baud 115200 -ex "set target-charset ASCII" -ex "set remotelogfile mri.log" -ex "target remote com1" -ex "set mem inaccessible-by-default off"

OS X and Linux

arm-none-eabi-gdb LPC1768/main.elf --baud 115200 -ex "set target-charset ASCII" -ex "set remotelogfile mri.log" -ex "target remote /dev/tty.usbmodem412" -ex "set mem inaccessible-by-default off"

If you hit a crash in Smoothie, you should obtain a mini-dump and send it to the Smoothie community for analysis. A mini-dump can be created using this sequence of GDB commands:

set pagination off
set logging on
set var $ptr=$sp
while $ptr < 0x10008000
x/4wa $ptr
set var $ptr+=16
info registers
set logging off
set pagination on

What is all of that doing?

  • Turn pagination off so that you don't have to keep pressing Enter to continue the text as it spews to the debug console.
  • Ask GDB to save the output from the rest of these commands to a file named gdb.txt in the current directory from which GDB was launched. This is one of the files that you will want to send to the Smoothie community for analysis.
  • Get a stack 'back trace'.
  • List the source code around the line where the debug event occurred.
  • Disassemble the function in which the crash occurred.
  • Then a debugger variable is used to enter a loop within the debugger itself to dump the stack 4 words at a time from the current stack location to the end of the stack, which is at 0x10008000 on the LPC1768.
  • Dump the CPU registers.
  • Stop the logging of information to gdb.txt now that we have collected all of the useful information we need.
  • Turn pagination back on which is more friendly for interactive debugging.

The mini-dump discussed in the previous section only contains the contents of the stack. A full dump will provide a dump of all of the lower 32K of RAM on the CPU. It takes longer to capture this full dump but it provides the community with the contents of all globals and even dynamic allocations from the heap. A full dump can be created using this sequence of GDB commands:

set pagination off
set logging on
set var $ptr=0x10000000
while $ptr < 0x10008000
x/4wa $ptr
set var $ptr+=16
info registers
set logging off
set pagination on

The only command which is different from the mini-dump is the line which initializes the $ptr variable. Instead of setting it to the current value of the stack pointer, it is instead initialized to the beginning of RAM.

The above steps will have generated a gdb.txt dump file (mini or full) in the current directory from which you launched GDB. You want to send this file along with your corresponding main.elf to the Smoothie community. You should also let the community know what version of the Smoothie sources you were using when you encountered this crash.

Where to send your dump? You can use any one of these methods to get your crash information to the community.

  • Create a gist and attach to it your gdb.txt and main.elf files. Then create an issue at [] and provide a link to your gist. In the issue, you can also give information about what version of the Smoothie sources you were using and anything else about your setup that you think will be interesting to the developer looking into your issue. This is the preferred method since it provides a convenient method to track progress on your crash, assign responsibility to the appropriate developer, and helps ensure that you item doesn't get lost.
  • If you are already communicating with the Smoothie community on IRC, then you can post a link to your gist or pastebin which contains the text of your gdb.txt. They will probably request that you send them your corresponding main.elf as well if you haven't already attached it to your gist. This approach may give you the quickest response but if it isn't investigated and diagnosed quickly, it could get lost.
  • E-mail your gdb.txt and main.elf to

To get more meaningful use out of GDB the Build should be set to Debug in the src/makefile BUILD_TYPE=Debug, however due to the fact all modules are compiled in this will now cause the image to be bigger than flash, so you need to turn some stuff off, the easiest is to do make NONETWORK=1 which turns off networking and saves a good deal of flash space.

A copy of gdb.pdf should have been installed when you installed the GNU tools used to build Smoothie. “Chapter 1 - A Sample GDB Session” of this manual provides an overview of the most useful GDB commands. Here are some of the commands we have found most useful.

  • bt Dump a stack back trace showing the call graph which generated the debug stop.
  • list Lists the source code around the line on which the program stopped.
  • p expression Prints the result of a C/C++ expression. For example “p *kernel” would attempt to dump the object pointed to by a local variable pointer called kernel. Note: The effectiveness of this command won't be perfect when Smoothie is built with optimizations turned on.
  • up Changes the context for commands like p to be the next function up in the callstack.
  • down Changes the context for command like p to be the next function down in the callstack.

Code optimizations are a way for the compiler to make the compiled machine code more compact and faster to execute. It's enabled by default in Smoothie.

Because Smoothie is now close to filling the fully 512 kB of flash with code and data, if you disable code optimizations, it suddenly becomes too big to fit into the flash.

But when you are debugging, if optimizations are enabled, you will not be able to do some things ( like see some values, use some features ).

This means that when debugging an issue, you might want to disable optimizations, and at the same time, make Smoothie smaller by disabling some things so that they don't take flash space.

This is done by editing the Makefile :

First, change BUILD_TYPE from Checked to Debug : 


Smoothie will still compile, but it's now too big to fit the flash.

One thing you can do for example is to disable networking ( which takes quite a bit of space, and which you don't need while debugging, unless you are debugging a network-specific problem ).

You do this by uncommenting NONETWORK : 

export NONETWORK = 1

If you still need to reduce size further, you can also disable specific modules : 

export EXCLUDE_MODULES = tools/touchprobe tools/laser tools/temperaturecontrol tools/extruder

For the firmware to run correctly, you'll also want to disable the watchdog :

watchdog_timeout      0       # watchdog timeout in seconds, default is 10, set to 0 to disable the watchdog

And reduce the base stepping frequency :

base_stepping_frequency       50000       # Base frequency for stepping

Default excludes

You can add your module excludes in the src/ file. Because it is not saved by git, it is useful for development as you can delete it once you are done, and you do not risk committing a modified makefile with your code.

For example you can add this to it : 

export CNC=1
export NONETWORK=1
export EXCLUDE_MODULES = tools/laser tools/filamentdetector tools/scaracal tools/temperaturecontrol tools/extruder tools/zprobe tools/endstops

Information about using MRI, as well as a more complete debugging session example, can be found here.