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 Puck.js? Check out the Puck 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):

  • If the Red and Green LEDs lit brightly for a fraction of a second, then they start 'pulsing' everything is fine - you're now in bootloader mode
  • If the Red LED flashes briefly when plugging in, the button wasn't pressed, or isn't working.
  • If nothing lights up at all, there is a bad USB cable/connection, so no power
  • If the Red and Green LEDs stay lit brightly the bootloader has started, but no USB connection could be made. This could be due to a bad cable (try plugging the board in directly), or if that doesn't work it could be drivers - see the next troubleshooting headings.

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 Puck.js

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 XP / Windows 8.1

These versions of Windows 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 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 is probably because you've reset or unplugged the Espruino board while the Web IDE was connected to it. Chrome hangs on to 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.

In order to stop this happening in the future, click 'Disconnect' before resetting or unplugging the board. We're hoping that a new version of Chrome due out soon will help to fix this problem.

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

See above.

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

  • Unplug the board from USB
  • Plug the board in and immediately press the button (but not before you've plugged the board in)
  • Wait 2 seconds
  • Release the button

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

  • Press the RST button
  • Release the RST button and immediately press BTN1
  • Wait 2 seconds
  • Release BTN1

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 the Serial port at startup with function onInit() { Serial1.setConsole(true); }. However 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.

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 '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 won't work.

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.

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) {

That 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 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 the K&R style:

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

This could be for several reasons:

  • The pins and wiring in the examples are designed for the Espruino board. On other boards those pins often conflict with other on-board devices.
  • Many of the other boards don't have enough memory for all the functionality of the Espruino Board, so things (such as Waveform, HTTP, and sometimes even Graphics) have had to be removed.
  • As we only make any money from the Espruino Boards, we can't afford to spend time implementing functionality on other boards - so even if the board you have has enough memory, the functionality the example is using may still not be implemented.

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:

  • There are no pull-up resistors. I2C needs resistors between each of its data lines and 3.3v (or 5v) - these are usually in the 4-10 kOhm range. Many modules already have them in, but if you have a module without one then you need to add them before I2C will work.
  • The device's address is incorrect - sometimes people quote the address as being twice as large as it actually is. If in doubt, try dividing the address by two or multiplying it by two.
  • The device isn't powered or ground isn't shared (see the point above)
  • The SDA and SCL wires are mixed up. SDA should be connected to SDA, and SCL to SCL.

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