What follows is a quick list of potential problems and solutions. If your problem isn't covered here, please post in the Forum.

Do you have a Bluetooth LE device like Puck.js or Pixl.js? Check out the Bluetooth specific troubleshooting page

Getting Started

My board doesn't seem to be recognized by my computer

On the Espruino Pico/WiFi Boards

Hold down the button, and then plug the board in while keeping it held (then release after connecting):

On the Original Espruino Board

Hold down the RST button. Do the blue and red lights glow dimly? If not, there's a problem with your USB cable as power isn't getting to Espruino.

Hold down BTN1, and then press and release RST while keeping BTN1 held. The Blue and Red LEDs should now light brightly for a fraction of a second, and the Blue LED should be pulsing. If not, there is some issue with USB. Try another USB cable (it's surprising how often this is at fault, even if it works for other devices) and if that doesn't work, see the next troubleshooting headings.

On a Bluetooth LE device

Check out the Puck.js troubleshooting page

On other boards

If you've bought a different board, it won't come pre-installed with Espruino. You'll have to go to the Download page and follow the instructions there in order to flash the correct software onto it.

My board doesn't appear as a USB Serial port in Windows

Windows versions before 10 don't come with the correct drivers preinstalled. You'll need to install ST's VCP drivers first.

NOTE: It's not enough to just open the ZIP file and run the installer, you have to then go to the installation directory and run the correct installer as well. See the readme file in the ZIP above for more information.

My board shows with an error in Windows Device Manager

My board doesn't appear in Windows Control Panel's 'Devices and Printers' page.

If you use many COM port devices in Windows, you may find that the COM port numbers quickly get so high that Windows refuses to add more. If this is the case, you'll have to follow the instructions here: []

If not, see the first troubleshooting item above.

In Windows, the COM port appears in the Web IDE, but I can't connect to it

This can sometimes occur if you've reset or unplugged the Espruino board while the Web IDE was connected to it. Chrome can hang onto the serial port and stops Espruino from reconnecting to it.

Try unplugging Espruino and then completely close Chrome (close all windows, not just the Web IDE). However if that doesn't work, try restarting Windows.

If you have never been able to connect, check whether there is an error in 'Device Manager' and see the FAQ item above.

In Windows, Espruino was working and now it won't connect

See above.

I can't get my board into bootloader mode

On official Espruino boards you enter bootloader mode by holding the button held down while resetting or powering them up.

On any of the newer boards (WiFi, Puck.js, etc) holding the button down for longer than a second or two at boot will cause the board to exit the bootloader and enter Espruino without loading any of the code that you might have saved to Flash memory (to allow you to easily recover if you wrote code that you didn't intend to). As a result, to enter the bootloader you need to make sure that you release the button as soon after applying power (or resetting) as you can.

I tried to reflash my Espruino Board, and now it won't work

If you have Windows, check that it's not one of the problems described above.

Pico/WiFi Try reflashing again by holding down BTN1 as the board is plugged in. You should always be able to get the pulsing Green and Red LEDs.
Espruino Board Try reflashing again by holding down BTN1 and pressing and releasing RST. You should always be able to get the pulsing Blue LED.

As Espruino itself won't work, the IDE won't know what type of board it is supposed to flash so you'll have to choose the correct board from the list of available boards when flashing.

My board appears as a mouse or joystick in Windows Control Panel's 'Devices and Printers' page.

This may happen if you are using an ST Discovery board and haven't yet installed the Espuino firmware. Some of these boards are automatically recognized by Windows as a completely different kind of device (because of the 'demo software' that comes installed). Install the firmware as described on the Download page, disconnect the board a reconnect it again.

Using Espruino

I just upgraded Espruino to the latest version, and now I'm getting ReferenceError messages

This is a new feature in Espruino that detects the use of previously undefined variables - it makes it much more likely that your code will work as intended.

In order to fix the error, either create the variable beforehand with var, or just assign a value to it. For instance this code worked before Espruino 1v81, but won't work now:

setInterval(function() {
  on = !on;
}, 500);

To fix it, simply define on:

var  on = false;
setInterval(function() {
  on = !on;
}, 500);

Espruino keeps responding =undefined to my commands

This is actually fine - Espruino writes what your command returned, so if you execute a command that doesn't return a value, =undefined gets returned.

I typed a command and Espruino says Function --- not found!

Have you got the capitalisation correct?

JavaScript is case sensitive, so digitalWrite(LED1,1) will work, but commands like DigitalWrite(LED1,1) or digitalwrite(LED1,1) won't.

I typed save() and it succeeded, but my code isn't loaded at power on

You could try typing dump() to see if your code has actually been saved. If it hasn't, it's possible that BTN1 or the pin it is connected to was held down while Espruino boots (as this stops Espruino from loading saved code).

You could try typing load() to force Espruino to load its saved program.

If you want to execute certain commands at power-on, add one or more event handlers for the init event on E:

E.on('init', function() {
  // ...

This will then be loaded every time Espruino starts up. If you create a function called onInit, that will be executed too:

function onInit() {
 // ...

But of course there can only be one onInit function, so if you copy and paste two bits of code with two onInit functions then the second function will overwrite the first.

Espruino stopped working after I typed save()

You might have written some code that stops Espruino from working, and Espruino loads it at power on and breaks itself each time. To stop this:

On Espruino Pico/WiFi

If you get pulsing Red/Green LEDs, it's because you actually pressed the button too soon. Try again and leave a bit more of a gap.

On Original Espruino Board

If you get a glowing blue LED, it's because you pressed BTN1 too quickly after pressing RST. Try again and leave a bit more of a gap.


See the instructions on the Puck.js page


This will make Espruino start without loading your saved code. You can then connect with the Web IDE and type save() to overwrite your saved program with the 'empty' state that Espruino is now in.

There's a video of how to do this on the original Espruino board here

When powered on, Espruino just shows a pulsing LED (blue on the Original Espruino board, or red/green LEDs on the Pico/WiFi)

This is the Espruino Bootloader. It starts on the Espruino Board when BTN1 or the pin it is connected to is held down while the reset button is released.

To enter normal mode, just:

Pico/WiFi Unplug from USB and re-plug, without pressing the button.
Espruino Board Press and release RST while BTN1 is not pressed.
Puck.js Remove and re-insert the battery

When I upload code, some characters are being lost

Most likely this is because you're uploading code that is doing calculations that are taking a long time to finish - and so Espruino is unable to process the data that is being received quickly enough.

When you upload code to Espruino, it is executed as it is received (allowing you to upload more code than might otherwise fit into RAM).

In many cases, you will actually want the code to run at power on rather than when you upload (eg. WiFi connection or LCD initialisation). In these cases you could put your code inside a function called onInit:

// variables
// function declarations

function onInit() {
  // initialisation of hardware

// Call onInit right after upload (for testing)
setTimeout(onInit, 1000);

When saving, you may then want to remove the setTimeout line, upload, and then save (unless you're sure that calling onInit twice will not cause problems). See the page on Saving for more information.

I typed save() but my connected device doesn't work at power on

Some devices (such as LCDs and WiFi) require their own initialisation code which Espruino can't remember. To do that initialisation at boot time, write a function called onInit which contains the initialisation code for your device. After typing save(), it will be executed at power on or reset.

See the page on Saving for more information.

I typed save() but Espruino won't work (or stops working quickly) when powered from a computer (it only works from a USB power supply, battery, or the computer when the Web IDE is running)

This is because you're printing information to the console.

When you are not connected to a computer via USB, Espruino automatically moves the console (left-hand side of the IDE) to the Serial port. However when you are connected to a computer, Espruino writes down USB. If no terminal application is running on your computer then your computer won't accept any incoming data down USB, and Espruino can't tell whether that is because the IDE is connected but busy, or because no app is running at all. Espruino won't throw away any of the data that you send down USB, so when Espruino fills up its output buffer, it stops and waits for the computer to accept the data, and this is what causes your program not to work.

To fix this, either remove your console.log and print statements, or explicitly set the console to be on another device at startup with function onInit() { Serial1.setConsole(true); }. The second option will mean that you will no longer be able to program Espruino from USB unless you reset it or you code calls USB.setConsole(); to move the console back.

A second option is to use simply call Serial1.setConsole(); (without true as an argument). This will move the console to Serial1 until USB is connected again. Running this from onInit may not work, since onInit will likely be called at startup before the USB connection is made with your PC. In that case, the console will move to Serial1, but just a fraction of a second later, USB will connect and the console will move to USB. Instead, you could do:

function onInit() {
  setTimeout(function() { Serial1.setConsole(); }, 1000);
  // ...

This will call setConsole 1 second after boot, by which time USB should have initialised. Assuming your Espruino is battery powered, unplugging and replugging USB will then move the console back to USB, where the board can be programmed.

Note: Serial ports are not generally as fast as USB - 9600 baud is only around 1000 characters/second. If your code writes a lot of data to the console then you may find it fills up the output buffer and ends up stalled waiting for characters to send.

Note: We're using Serial1 and USB in the examples here, but depending on the board you have, you may also have access to other devices like Serial2/3/4 or Bluetooth. Check out the Serial class reference for a full list of options.

Espruino works when connected to a computer, but stops when powered from something else

Do you have a Serial device connected to pins B6/B7 on the Pico or WiFi, or A9/A10 on the Original Espruino? When disconnected from USB, Espruino's REPL/'console' (what's on the left-hand side of the Web IDE) gets moved over to Serial1 - which is on those pins. If you've got a Serial device on those pins then it will then stop working.

To fix this, right at the top of your code that runs on initialisation (eg. the onInit function or E.on('init', ...) event), add the line USB.setConsole(), which will force the console onto USB. After USB.setConsole(), the console can still automatically move back to Serial1 if there is some event that forces it (USB connect and disconnect), so you can force the console to stay on USB regardless with USB.setConsole(true) instead.

Other reasons could be that the power supply you're using isn't supplying enougn current or is too low a voltage.

To debug further, you can take advantage of the console that was mentioned before. Connect a USB-TTL converter to GND and the Serial1 pins (shown above) and connect with the Web IDE at 9600 baud. You can then interact with Espruino as if it was connected to USB - but while it is powered from another source.

When I type dump() the code that is displayed isn't exactly the code that I entered

When you send code to Espruino, it is executed immediately. That means if you say:

var foo = 1+2;

Then foo will forever be set to 3, and not the expression 1+2. In this case it's not a problem, but you might write var foo = analogRead(A0); and then foo will be set to whatever the value of A0 was when you uploaded the program.

When you type dump(), Espruino tries to reconstruct the code that you wrote based on its current state (by adding setWatch, setInterval, digitalWrite, etc) - and sometimes there is not enough information available for it to get it correct.

It's most noticeable for setInterval and setWatch, which return integers - so Espruino has no way of knowing that a given number goes with a given setInterval. In this case:

var x = setInterval("print('Hello')", 10000);

will turn into:

var x = 1;
setInterval("print('Hello')", 10000);

To get around this, it's best to put code that you intend to run every time Espruino starts into a function called onInit().

Note: The problem with setInterval happens because Espruino is trying to turn its internal state back into a human readable form. If you just type save() then the correct state will still be saved.

I've pasted code into the left-hand side of the Web IDE and it doesn't work

There could be several reasons for this, but the likely one is that you have formatted your code in a way that doesn't work well with a command-line interface.

Each time you press enter, Espruino's command-line interface counts brackets to see if the statement you've entered is complete. If it is, it'll try and execute it. For instance:

if (true) {

is a complete statement, so when you hit enter at the end it'll be executed immediately. However if you type:

if (true) {
else {
  console.log("Oh No!");

Then now we have a problem. Halfway through, Espruino sees that the first if statement is complete and executes, and it's now given a line that starts else { that isn't a valid statement.

The easiest way to fix this is to write code in something similar to the K&R style. See the Coding Style page for more information.

if (true) {
} else {
  console.log("Oh No!");

If you're writing code in the right-hand side of the Web IDE, the Web IDE should try and detect the different formatting and insert a special newline character (Alt-Enter) which will fix it for you. If you're using other tools to send data to Espruino then this may not automatically happen for you though.

Note: If you use require(...) on the left-hand side and the module is not already part of Espruino, you may find that you get a 'Module not found' error. In this case, you should add the require(...) command to the right-hand pane of the Web IDE and click Send to Espruino. This will automatically load the module into Espruino, allowing you to use it from the left hand side.

I get Uncaught Error: Module XYZ not found

This could be because the module doesn't exist in Espruino's list of modules - Espruino doesn't directly support inclusion of modules from NPM, because the vast majority have too many dependencies to be used on an embedded target.

However, if you're trying to use example code for Espruino, chances are it's because you typed the command on the left-hand side of the IDE (see the last FAQ item). The majority of modules are loaded on-demand by the IDE (some are built-in) when code is uploaded, but the left-hand side of the IDE is a REPL - a direct connection to the Espruino device. In that case the IDE doesn't have a chance to intercept the command and automatically upload the module for you.

I'm using an unofficial board and some of the examples don't work

This could be for several reasons:

In short, if you want to be sure that all the functionality you want is implemented, support us and buy an Espruino board!


I've connected a Serial device like a GPS, but I'm not getting a signal.

For normal Serial communications, you have two wires - RX and TX. However these are relative to the device - for instance TX on the Espruino transmits data out of Espruino, and TX on a GPS transmits data out of the GPS. That means that for the devices to communicate you must connect RX to TX and vice versa.

Serial ports also have to have a communications speed (baud rate) set up - you'll need to make sure that each device has exactly the same baud rate or they won't be able to communicate. For devices such as a GPS, you should find the baud rate in the datasheet.

If you're still having problems, look at the next item too.

I've connected an external device but I'm not getting a reliable signal

This could be because your devices aren't sharing the same ground. For example if you have Serial communications set up, two wires will get mentioned a lot - RX and TX. However you also need to have ground connected, meaning you actually need 3 wires. Ground is required for pretty much everything: Serial, I2C, SPI, OneWire, and even simple logic signals from something like a Pyroelectric sensor.

I've connected an I2C device, but I get 'timeout' errors

This is usually one of four reasons:

Something else is wrong!

Check out the Espruino Forums

Is your device a Bluetooth device? Check out the Bluetooth Troubleshooting page

This page is auto-generated from GitHub. If you see any mistakes or have suggestions, please let us know.