File IO and SD cards

To access files from Espruino (if you're not getting them from the Internet) you'll need an SD card.

On the original Espruino board there's a Micro SD card slot built-in, or on the Espruino Pico you'll have to wire a card up externally and then tell Espruino about it with E.connectSDCard:

// Wire up up MOSI, MISO, SCK and CS pins (along with 3.3v and GND)
SPI1.setup({mosi:B5, miso:B4, sck:B3});
E.connectSDCard(SPI1, B6 /*CS*/);
// see what's on the device

Espruino has two main forms of File IO available:

FS Library - Simple Node.js-compatible File IO

The fs library allows you to read directories, read, write, stat and delete files. See here for more details about the specific functions.

We've tried to keep it a lot like the Node FS Library however note that while Asynchronous functions exist (like fs.readFile) they are not currently asynchronous and behave the same way as their ...Sync equivalents.

Read directory contents

var files = require("fs").readdirSync();
for (var i in files)
  console.log("Found file "+files[i]);

Read and write files

require("fs").writeFileSync("hello.txt", "Hello World");
console.log(require("fs").readFileSync("hello.txt")); // prints "Hello World"
require("fs").appendFileSync("hello.txt", "!!!");
console.log(require("fs").readFileSync("hello.txt")); // prints "Hello World!!!"

However, with the exception of appendFileSync, these functions only allow you to act on whole files. This means that you will quickly exhaust Espruino's RAM when loading larger files.

E.openFile - Stream File IO

While we'd have liked to keep a node.js-compatible Filesystem stream API, Node's implementation just didn't make sense on a Microcontroller.

Instead we have developed our own simple API. Just use E.openFile to open a file. This returns a File Object, which has functions such as read, write and seek on it.

For example the following would open a file, get one byte out of it, and then write to an analog (PWM) output 10 times a second.

var f = E.openFile("data.raw","r");

setInterval(function() {
  var val =;
  analogWrite(A0, val/256);
}, 100);

Or the following would take a series of readings and write them as a comma-separated file:

var f = E.openFile("data.csv","w");

setInterval(function() {
  var readings = [
  f.write(readings.join(", ")+"\n");
}, 100);

Finally you'll need to close the file with f.close() or you may get corruption.

Note that you could do also this with appendFileSync. However appendFileSync will close the file after each write, which is safer (but also slower!).


You can also 'pipe' to and from files, which will perform the data transfer in the background.

For example the following will create an HTTP server that serves up the file rick.webm from the SD card.

This is hugely useful, as sometimes you will want to serve up a file that is too big to fit into Espruino's RAM.

function onPageRequest(req, res) {
  res.writeHead(200, {'Content-Type': 'video/webm'});
  new File("rick.webm").pipe(res);


Just like on a PC or camera, if you unplug an SD card while a device is writing to it, you can corrupt (and in some cases physically damage) it.

To avoid this, Espruino has the E.unmountSD() function. Just close your open files, and call it. The SD card can then be removed without problems.

Just be sure not to perform any File IO after calling it, as they will 're-mount' the SD card.

For example you might want to create a logger that writed to a log file when the Red LED is lit, but that allows you to press a button to toggle between recording and not recording:

var logFile;
var logInterval;
function doLog() {
  digitalPulse(LED2,1,50); // pulse green led as indicator
  logFile.write(getTime()+","+E.getTemperature()+"\r\n"); // write the time and temperature for example
setWatch(function() {
  if (logFile===undefined) {
   logFile = E.openFile("log.txt", "a");
   digitalWrite(LED1,1); // red indicator on
   logInterval = setInterval(doLog, 1000);
  } else {
    logInterval = undefined;
    logFile = undefined;
    E.unmountSD(); // card can now be pulled out
    digitalWrite(LED1,0); // red indicator off
}, BTN,  { repeat:true, edge:'rising', debounce:50 });

Wiring up an SD card adapter

For the Espruino Pico, there is an adaptor shim available that makes connecting an SD card a lot easier!

If you don't have an SD breakout board at your disposal, you can make your own by soldering directly to a microSD/SDHC to SD card adapter (often provided with microSDs).

SD Card Wiring

Example of wiring it up to the Espruino Pico:

SD Card Wiring

Example code for the above wiring:

  R: the following function is auto-called when the Espruino is booting up
     to flash the Espruino & save the following to its flash, just call 'save()' in the IDE
function onInit() {
  // initial SPI1 for SDCard module
  SPI1.setup({sck:A5, miso:A6, mosi:A7 });
  // ...

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