Debugging ESP8266 with GDB and VSCode + Platformio

VSCode with the Platformio IDE plugin makes for an excellent platform for developing ESP8266 code, especially using the Arduino Framework for ESP8266 but the Platformio debugging features do not work. It is, however, possible to use GDB together with VSCode and Plaformio to be able to single-step through code, inspecting flow of control and variable values as you go, in an experience that is comparable to VisualGDB but costs nothing.

There are some limitations, “gotchas”, etc, but once these are understood it really does work!

Setting Up Debug Compilation

The program must be compiled differently for debugging to work because additional information must be included, along with the GDBStub (see below). Once debugging is over, however, the firmware uploaded to the ESP8266 should not have this overhead.

Allowing for both options to be compiled and uploaded is easy in Platformio if you use platformio.ini environments. For example:

platform = espressif8266
framework = arduino
upload_port = COM15
upload_speed = 115200
monitor_port = ${env.upload_port}
monitor_speed = ${env.upload_speed}

board = d1_mini_pro

board = d1_mini_pro
build_flags = -Og -ggdb3 -g3 -D WITH_GDB
Note that I have created two targets – “d1_mini_pro” and “debug_d1_mini_pro” – with the latter defining some non-default build_flags. The “-g” flags make the compiler include debug information, while -Og prevents the compiler optimising code (which breaks the nice relationship between lines of source code and compiled MCU code). I have also included “-D WITH_GDB”; the -D directive is equivalent to a “#define” in the source code and means we can use “#ifdef” to selectively include chunks of code depending on whether we’re using the debug_d1_mini_pro target or not.
Remember to recompile and upload before debugging any changes you make.

Adding GDBStub to Your Code

The GDBStub is code which executes on the ESP8266 to communicate with GDB running on your PC via the serial port. Serial.print() will still work so long as you avoid using a rapidly-repeating loop() which contains little more then Serial.print().

The Arduino Framework already contains the GDBStub code and their documentation explains the basics, although in the context of using the Arduino IDE.

Following on from the use of “WITH_GDB” in the previous section, add the following chunks of code.

At the top:

#ifdef WITH_GDB
#include <GDBStub.h>
At the start of setup():
  Serial.begin(115200);  // or whatever baud rate you prefer
  #ifdef WITH_GDB

Command Line GDB

An essential truth is that you must use the correct gdb for the core processor of the ESP8266: xtensa-lx106-elf-gdb.exe. For my Platformio install, this can be found at (“~” denotes my home directory, i.e. C:\Users\Adam): ~/.platformio/packages/toolchain-xtensa/bin/xtensa-lx106-elf-gdb.exe

A second essential truth is that GDB needs to be set up correctly before you can actually debug. This is most easily done with a set of commands stored in a file, which I call “gdbcmds” and which should be created in the home folder of the Platformio project (i.e. in the same folder as platformio.ini). My gdbcmds file contains:

# ESP8266 HW limits the number of breakpoints/watchpoints
set remote hardware-breakpoint-limit 1
set remote hardware-watchpoint-limit 1
# Some GDBstub settings
set remote interrupt-on-connect on
set remote kill-packet off
set remote symbol-lookup-packet off
set remote verbose-resume-packet off
# The memory map, so GDB knows where it can install SW breakpoints
mem 0x20000000 0x3fefffff ro cache
mem 0x3ff00000 0x3fffffff rw
mem 0x40000000 0x400fffff ro cache
mem 0x40100000 0x4013ffff rw cache
mem 0x40140000 0x5fffffff ro cache
mem 0x60000000 0x60001fff rw
# Locate the ELF file
file .pio/build/debug_d1_mini_pro/firmware.elf
# Change the following to your serial port and baud
set serial baud 115200
target remote \\.\COM15

The ELF file location follows the pattern .pio/build/{target}/firmware.elf, where “{target}” matches the relevant entry in platformio.ini.

Obviously: change the serial port and baud rate!

Now open a new terminal in VSCode. It should open with the project home as the working directory (this is necessary). You can open a terminal window outside VSCode too (I like ConEMU) but should make sure the working directory is right.

You need to run xtensa-lx106-elf-gdb.exe, passing the commands file by adding “-x gdbcmds”. I tend to use a bash shell, so the command is:

~/.platformio/packages/toolchain-xtensa/bin/xtensa-lx106-elf-gdb.exe -x gdbcmds

If you prefer a normal Windows command prompt just replace the “~” with C:\Users\You.

If you get an error like “\\.\COM15: No such file or directory.” then you will have the Serial port open somewhere such as the “Monitor”. Close it.

I suggest you read the Arduino Framework notes for “what next”. The GDB documentation is available online but is rather long and has much which is not applicable. Generally useful commands are: thb (see below), next, step, finish, print, and info locals. Interesting to explore are: disassemble and frame.

When single-stepping (step or next), the execution will sometimes step over several lines of code.

Avoid resetting the device while a GDB session is connected; it gets rather confused!

ESP8266 Limitation – One Breakpoint

This causes things to get ugly if you don’t work with care. The first thing to do is to use temporary breakpoints. Rather than the normal “break” GDB command, use “thb”. This means that once the breakpoint has been reached it is deleted. Otherwise you need to use the “delete” command to remove the breakpoint.

Typical uses are to break on entry to a function. So long as the function name is unique, something like “thb loop” works. Since the compiled code contains source from many files, line numbers must identify the file they relate to, so the command should be something like “thb src/main.cpp:25”.

“Visual” Debugging

This is how to get the debugging experience more like what the Platformio and VisualGDB screenshots suggest.

The launch.json file (within the .vscode folder for the Platformio project) should be edited. Ignore the warning about this being an automatically-generated file and replace the platformio debug entries with the following:

    "version": "0.2.0",
    "configurations": [
            "name": "GDB",
            "type": "cppdbg",
            "request": "launch",
            "program": "${workspaceFolder}/.pio/build/debug_d1_mini_pro/firmware.elf",
            "args": [],
            "stopAtEntry": false,
            "hardwareBreakpoints": {"require": true, "limit": 1},
            "cwd": "${workspaceFolder}",
            "environment": [],
            "externalConsole": true,
            "MIMode": "gdb",
            "miDebuggerPath": "${userHome}/.platformio/packages/toolchain-xtensa/bin/xtensa-lx106-elf-gdb.exe",
            "miDebuggerServerAddress": "\\\\.\\COM15",
            "setupCommands": [
                {"text": "set remote hardware-breakpoint-limit 1"},
                {"text": "set remote hardware-watchpoint-limit 1"},
                {"text": "set remote interrupt-on-connect on"},
                {"text": "set remote kill-packet off"},
                {"text": "set remote symbol-lookup-packet off"},
                {"text": "set remote verbose-resume-packet off"},
                {"text": "mem 0x20000000 0x3fefffff ro cache"},
                {"text": "mem 0x3ff00000 0x3fffffff rw"},
                {"text": "mem 0x40000000 0x400fffff ro cache"},
                {"text": "mem 0x40100000 0x4013ffff rw cache"},
                {"text": "mem 0x40140000 0x5fffffff ro cache"},
                {"text": "mem 0x60000000 0x60001fff rw"},
                {"text": "set serial baud 115200"}

Note that: the entry for “program” will need changing to match your debug compilation target; the serial port appears as “miDebuggerServerAddress” and has additional back-slashes compared to the previous configuration.

This uses the “machine interface” to GDB, which you can see by adding “–interface=mi” after the “-x gdbcmds”. There is a Python package which can interpret the gdb/mi responses: pygdbmi.

You should now be able to use the “Run and Debug” feature of VSCode; there should be an entry called “GDB” (using the “Quick Access” Plaformio Debug > Start Debugging also seems to work but I avoid that route as I don’t know what else it might do). I won’t repeat information on using the VSCode debugging interface here; there is plenty on the web! You should be able to see variables and to inspect the call stack while stepping through the code. What I will provide are some caveats and gotcha-avoidance measures.

Remember there is only one breakpoint

Unfortunately, VSCode does not let you set temporary breakpoints. So you must adopt a working practice of setting a breakpoint and then deleting it as soon as you get there (the config line ‘”hardwareBreakpoints”: {“require”: true, “limit”: 1}’  prevents you from adding more than one, but it isn’t that simple! It appears that the VSCode gdb/mi implementation uses breakpoints for “step over” operations, so you really must delete the breakpoint before stepping. If you get horrible red boxes popping up reporting exceptions etc, the cause is likely to be because you have left a breakpoint.

Where is my Serial.print() output?

In spite of the debugger, I still find Serial.print() very useful for reporting sensor readings etc.

These appear in the Debug Console.

Not all variables are locals

Although the VARIABLES panel shows local variables (and changes these as you select different levels in the call stack), global variables will need you to issue the GDB print command. Locate the Debug Console and note that it has a command entry line at the bottom. GDB commands may be entered here, but must be prefixed by “-exec” (e.g. -exec print foo). You can also add these as WATCH entries.

Adding a breakpoint while things are running

Adding a breakpoint while the ESP8266 is running causes it to stop execution where it is, rather than where the breakpoint has been added. Simply use Continue (F5) to make it continue until the breakpoint is met.

Some things don’t work

Although VSCode suggests otherwise, some things are not supported: PERIPHERALS, MEMORY, DISASSEMBLY.

The Debug Console command line entry comes to the rescue, e.g. try “-exec disassemble”.

Debugging Inside Setup

The problem with attempting to debug inside setup() is that the ESP8266 will normally have exited setup() before you connect GDB.

The neatest way of handling this is to add the following code into setup() just before you want to start debugging:

  #ifdef WITH_GDB

When you reset the ESP8266, this triggers entry into the debugger on the ESP8266, which looks like a breakpoint hit, and means that when you connect GDB you find you are stopped inside gdb_do_break(). Send the “finish” command to GDB twice and you’ll be inside setup().

This works nicely with the command line use of GDB but I haven’t been able to find how to make the VSCode debugger play nicely with it. In this case, the quickest option is to use something like delay(3000) instead of gdb_do_break(). This gives you enough time to start the debugger after resetting the device.

Make sure you quit any GDB session before resetting; things get all messed up otherwise.

Setting Variables

GDB permits variable setting through commands such as “set var my_var = 5”.

Unfortunately, this does not always work. Various comments on the web suggest this comes down to where the variable data is to be found when the program is running, because GDBStub cannot access all possible places.

A work-around is to declare variables with the “volatile” keyword.

Sonoff Basic R3 – DIY Switch via HTTP POST of JSON

The latest firmware of the Sonoff BasicR3 Wifi Switch now allows the device to be controlled through the DIY mode built into its firmware (v3.6.0) without any re-flashing with Tasmota firmware or fooling about with jumpers. The only obvious limitation for some people is that the “eWeLink” app and DIY access cannot both be used together; the switch is either in DIY mode or app-control mode. This is not an issue for me as I have no requirement for remote control from outside my LAN and don’t intend to use the app at all. In any case it would not be hard to either use a general purpose HTTP “shortcuts” app (e.g. HTTP Shortcuts!) for control from a phone from my LAN, or to write/run my own web app of a Raspberry Pi. I begin to digress… the point of this post is to make some notes for myself in a way which someone else might find useful (and adding some extra helpful comments). Much of what follows can be found on the web, but there are some holes, and I didn’t find the documentation quick and easy in parts.

Getting into DIY Mode

The key is to get the device into what the Sonoff User Manual calls “Compatible Pairing Mode”. The push button is used in two stages, with long (5s) holds first to get the LED signalling “..-” and then a continuous rapid flashing.

The switch is now acting as a WiFi access point with SSID (name) of “ITEAD-{device_id}”, where device_id will be something like “100136da83”. Connect to this network and then visit to access a web page which allows you to enter your own WiFi SSID and password. After a short while, the Sonoff device will have closed its access point and connected to your local network. It is now in DIY mode, and will stay this way when its power supply is disconnected and reconnected. The LED will signal “..”. You can still use the eWeLink app to add the switch to its device list, but this cancels the DIY mode. Also: once in DIY mode, you cannot access, no matter how many times you force the device into making “ITEAD-{device_id}” available. The exception to this is if you power the switch up so it cannot connect to your local network. In which case, the device will eventually (20s) give up trying to connect and become eligible to be forced into Compatible Pairing Mode again.

Finding the Device on the Network

This is not as simple as it might be; it would have been neater if the web page reported its network name before disappearing. This is a bit messy.

Option 1 is to look at the client list in your router admin web interface. This is likely to require the “advanced” option. In my case, my modem-router does provide a client list, but rather confusingly the client name which is reported, and the MAC address have no obvious connection with the device_id revealed above. The client name is like “ESP_80DD76”, which is the standard way an ESP8266 (etc) chip reports its name, which takes the last 3 bytes of the MAC address prefixed by “ESP_”. If you can see something like this, you’ve found the IP address and can progress to the “Control the Switch” section, below, although you will have to trust that the default port is used (I see no reason why it would not be).

Option 2 is to use an mDNS browser. The essence of mDNS is a service which links device names to IP addresses and other info. MS Windows doesn’t seem to have an easy way to do this, but if you also run a Raspberry Pi it is not hard. Mac users can use Bonjour, there are Android Apps, and linux users (of course) can do as follows anyway. The service/tool for the Pi is Avahi. This should already be up and running, which can be confirmed using “service avahi-daemon status”. The mDNS browser requires the avahi-utils package, which was not on my Pi, but installed with “sudo apt-get install avahi-utils”. The simple command is not “avahi-browse –all –resolve”, which will reveal that the mDNS name is of the form “eWeLink_{device_id}.local”. The port entry will report which TCP port should be used to connect to the device, which should be the default of 8081. The txt entry reveals the device state if in DIY mode, or a base-64 string of gobbledy-gook if in eWeLink app mode.

Option 2a is an Android App. I tried one called Service Browser, but there are other mDNS browsers

Option 3 is to take advantage of the observation that the mDNS name is of the form “eWeLink_{device_id}.local”. This is fine so long as your method of controlling the switch can work with mDNS names, which Postman cannot (at present).

Caveat: there is no guarantee that your router will assign the same IP address. In my case, I operate a router which allows “address reservation”, so that several IoT devices I have get known IP addresses. Unless your router has this feature, beyond tinkering, you will have to resort to using mDNS-aware software. If writing your own in Python, it looks like the “zeroconf” package is suitable for dealing with mDNS names and not worrying about reliable IP addresses.

Control the Switch

I use Postman for manual testing, and Python for anything automated.

The documentation for interacting with the device is fairly good, and can be found at This includes info on how to perform an OTA (“over the air”) firmware update, control how the device behaves when its power is restored (allowing you to choose whether it starts up off, on, or in its previous state), and to change the WiFI SSID and password which the device connects to, but I’m content with the info, on, off, and pulse options.

Note that all messages to the device are HTTP (not HTTPS) POST operations, with a JSON body (payload). Although the documentation includes “deviceid” in the JSON, this is completely redundant and can be omitted.


Operation URL JSON Body
Info http://{{ip_addr}}:{{port}}/zeroconf/info
 "data": {}
On http://{{ip_addr}}:{{port}}/zeroconf/switch
  "data": { 
    "switch": "on" 
Off http://{{ip_addr}}:{{port}}/zeroconf/switch
  "data": { 
    "switch": "off" 
Pulse http://{{ip_addr}}:{{port}}/zeroconf/pulse
  "data": { 
    "pulse": "on", 
    "pulseWidth": 2000

Pulse changes the behaviour of “On” operations; if Pulse is on then an “On” operation will only last for pulseWidth milliseconds, after which the device will turn off again. The maximum duration is 1 hour. The JSON payload for Pulse can omit the “pulseWidth” to enable/disable the feature, but it is not possible to change the pulse duration by only specifying “pulseWidth”.

Using MQTT in Open Energy Monitor to Capture External Device Data

I struggled to find information about this using web searches, so here is a condensed “how to”. The scenario I have is using a home-brew ESP8266 based device attached to my solar PV inverter which I want to relay definitive power output to my Open Energy Monitor via MQTT. This seems quite simple in principle, and is simple in practice, but seemingly not well documented.

First thing is to publish the data to an MQTT broker on the emonpi with a topic which starts “emon/{source}/{key}”, replacing {source} with a suitable name for the data source and {key} with the attribute name for the data being sent. In my case, I used “emon/solis/power” as the data source is the output power for my Solis PV inverter. The message payload is simply the data value. This immediately makes the published data appear on the “Inputs” screen of EmonCMS.

Two refinements/possibilities:


Rather than just a single value, send several in the same message. There are two ways to do this:

a) use a topic of form “emon/{source}”

b) use a topic of form “emon/{source}/{key}”

If the message payload sent to the MQTT topic “emon/solis/power” looks like {“ac”: 90, “dc”: 120}, option (a) creates EmonCMS inputs “ac” and “dc” under “solis”, while option (b) creates inputs called “power_ac” and “power_dc”.

Include a timestamp

To do this, simply include an extra field in the JSON called “time”, with a value which is the Unix time. If you are testing, the Unix time needs to be fairly close to the actual time (ignoring summer time) otherwise EmonCMS will indicate “inactive”, but it still captures the data.

Aside: I used the VSMqtt plugin for VSCode as the MQTT client, as I’m using the PlatformIO plugin to develop my ESP8266 code (using the Arduino libraries). Update: these days I’ve changed to using MQTT Explorer as my go-to software for viewing MQTT messages.

Safe Use of GPIO on ESP8266 NodeMCU Boards

It is well known that, unlike a Raspberry Pi or old-school Arduino, the ESP8266 has some quirks which make using the broken-out pins D0-D8 non-trivial. Google/Bing/etc will easily confirm this. I have, however found lots of long winded, unclear, and conflicting information. This post is my aide memoir, shared in case anyone else might find it useful. The hardware platform is “HW-628 V1.1”, from one of the many cheap Chinese sellers on Alibaba. I’m using the Arduino libraries with PlatformIO (within VSCode), but would expect the same results using the Arduino IDE.

My baseline assumption is that inputs and outputs will normally be used in “active low” mode, with INPUT_PULLUP used as the pinMode for inputs. There are two problems which will easily be found without care: 1) [as outputs] some pins are driven low on boot, usually with quite a few pulses, which will cause an active low relay to stutter; 2) [as inputs] holding some pins low will block the booting.

Tests for the effect of booting (and flashing) on output states were undertaken using a logic analyser (an Open Bench Logic Sniffer with OLS software) with a weak pull-up on the pin under test. I am using the D0-D8 notation as marked on the NodeMCU board. The Arduino library pin numbers differ, but the mapping is widely published.

The following pins were found to be OK to use without any restrictions, as inputs or outputs: D0, D1, D2, D5, D6, D7. The only caveat for my hardware is that D0 is connected to one of the on-board LEDs (active low).

The following CANNOT be used as outputs (with the exception of LED signalling*) since they are affected on boot (and when flashing): D3, D4, D8. D8 was found to hold a low value during flashing and during and slightly after the reset pulse. D3 and D4 showed a burst of pulses. You might get away with using D8 in its “TX2” UART alter ego since the flash/boot glitch wont make a valid data frame. [* – the NodeMCU Devkit board I have does have an LED on D4 = GPIO2 and this can be used for signalling from programmes].

D3, D4, and D8 can be used as inputs but only with some care:

  • D3 and D4 can be used as active low inputs only AFTER the boot sequence is completed. Push-switch inputs would be safe. Anything which would pull these pins low on boot will block it. D4 is connected to the on-board LED which is marked “COM” (and is active low).
  • D8 can be used as an active high input only AFTER the boot sequence. Push-switch inputs would be safe. I would use this last as I don’t like circuits to contain a mix of active low and active high inputs; logic should normally be consistent to reduce bug risk.
  • Since these GPIOs have output functions during boot and flashing, if there is a chance that a push switch input would be operated during those states, the switch should connect to ground or supply via a current limiting resistor.

An aside: when setting pins as outputs with pinMode, it is worth setting the output value to HIGH with digitalWrite() BEFORE changing the mode from its default as an input. Otherwise, assuming active low working again, you will be likely to see an output glitch due to a default low state pertaining when the pinMode() is applied.