Bangle.js 2 Technical Information

For general Bangle.js information, try the Bangle.js 1 or the Bangle.js 2 pages

This page contains detailed information on Bangle.js's 2's internals. If you're just writing JS then you should have no need for this information, but if you plan on modifying the Bangle.js firmware it might be of interest.

This file is based on reverse engineering of publically available firmware and information, buy Gordon Williams, fanoush, and atc1441. It was originally published as a project page



  • IP67 Water Resistant (everyday use, not swimming or baths)
  • Nordic 64MHz nRF52840 ARM Cortex-M4 processor with Bluetooth LE
  • 256kB RAM 1024kB on-chip flash, 8MB external flash (GD25Q64C/GD25Q64E)
  • 1.3 inch 176x176 always-on 3 bit colour LCD display (LPM013M126)
  • Full touchscreen (Hynitron CST816S)
  • Single button on side of watch
  • GPS/Glonass receiver (AT6558 / AT6558R)
  • Heart rate monitor (Vcare VC31 / VC31B)
  • 3 Axis Accelerometer (Kionix KX023)
  • 3 Axis Magnetometer
  • Air Pressure/Temperature sensor (Bosch BMP280 / Goertek SPL06)
  • Vibration motor
  • SWD program/debug pins on rear of watch
  • 175mAh battery (XK302627, 3 x 26 x 27mm), 4 weeks standby time
  • 36mm x 43mm x 12mm watch body, with standard 20mm watch straps


Where mentioned in the source code or filenames, Q3 references Bangle.js 2, and F18 references Bangle.js 1.

The Espruino version for Bangle.js has a few specific modifications:

  • jswrap_bangle.c contains all Bangle.* functions and pretty much all Bangle.js functionality.
  • Some functionality (like menus) is in JavaScript which is built into the firmware. Those JavaScript files are here
  • describes the build
    • The SPIFLASH device causes SPIFLASH_BASE to be defined, which enables external SPI flash memory support in jshardware.c


The watch body is made of ABS plastic, with the LCD+touchscreen assembly glued to the front.

Want to get inside? Check out the Bangle.js 2 Disassembly page

There are CAD files for the Bangle.js 2 stand, as well as an STL of the watch body available here


Below are shots of the Bangle.js 2 PCB:

The components on the PCB are marked here:


SWD - the 4 pads on the rear of the watch are GND, SWDIO/SWDCLK, VCC. The charge cable has wires for all 4 leads brought out to the USB connector, so the easiest method is just to attach wires from a USB socket to an nRF52DK

This can be connected to an SWD programmer. We'd recommend using an nRF52 DK (or nRF52840DK). When using the nRF52DK you'll need to short the GND DETECT pin to GND and short the VTG pin to VDD to tell it you're programming the Bangle.js and not the on-board nRF52. See the example schematic below.

The easiest way to connect to these is to use the provided USB charge cable and to wire up a USB type A socket:

Pin # Connect
1 5v (not required)

Be sure to fully remove the tape and adhesive covering the SWDIO and SWDCLK pins.

See Advanced Debug for more information on programming via SWD.


The GPS is a AT6558 (or AT6558R on newest Bangle.js devices)

For more information on common usage, check out the Bangle.js 2 GPS page

Espruino handles reception and parsing of NMEA data from the GPS UART when Bangle.setGPSPower(1) is called.

On startup the GPS reports information about itself - this can be viewed with the following (assuming GPS was off before):

setTimeout(function() {
}, 1000);

The first few batches of Bangle.js devices report the following data upon startup, using AT6558 with V5.1.0.0 firmware:

$GPTXT,01,01,02,MA=CASIC*27 false                   # MA = Manufacturer
$GPTXT,01,01,02,IC=AT6558-5N-32-1C510800*48 false   # IC = Chip
$GPTXT,01,01,02,SW=URANUS5,V5.1.0.0*1F false        # SW = Firmware version
$GPTXT,01,01,02,TB=2018-04-18,10:28:16*40 false     # TB = Firmware compile date
$GPTXT,01,01,02,MO=GB*77 false                      # MO = working mode

Newer ones use a AT6558R with V5.3.0.0 firmware.

$GPTXT,01,01,02,MA=CASIC*27 false
$GPTXT,01,01,02,IC=AT6558R-5N-32-1C580901*13 false
$GPTXT,01,01,02,SW=URANUS5,V5.3.0.0*1D false
$GPTXT,01,01,02,TB=2020-03-26,13:25:12*4B false
$GPTXT,01,01,02,MO=GR*67 false

The receiver can be configured with $PCAS commands. It's hard to find decent documentation on these - the best we have found is a Chinese Language datasheet here.

You need to calculate a checksum when sending, which can be done with the following:

function CASIC_CMD(cmd) {
  var cs = 0;
  for (var i=1;i<cmd.length;i++)
    cs = cs ^ cmd.charCodeAt(i);
  Serial1.println(cmd+"*"+cs.toString(16).toUpperCase().padStart(2, '0'));

Here are some example commands that work;

CASIC_CMD("$PCAS03,1,0,0,1,0,0,0,0"); // send only 'GGA+GSV' NMEA data (minimum for Bangle.js GPS event)
CASIC_CMD("$PCAS03,1,0,0,1,1,0,0,0"); // send the NMEA packets Bangle.js expects
CASIC_CMD("$PCAS04,1"); // Set to GPS-only mode
CASIC_CMD("$PCAS02,500"); // Change output speed from default 1000ms to 500ms
// The valid range is 100->1000ms, but to get below 500ms you must disable un-needed packets with PCAS03

CASIC_CMD("$PCAS00"); // Save all changes to flash memory (be careful!)

The receiver also accepts a binary protocol that begins with the characters "\xBA\xCA", but you'll need to consult the datasheet for more information on that.


Unlike Bangle.js 1, all IO is connected direct to the nRF52840 - there's no need for an IO expander

Pin Connected State
0 32876 oscillator
1 32876 oscillator
3 Battery voltage analog GND-[1M]-D3-[3M]-VBATT
5 MEMLCD_CS (active high) out
6 MEMLCD_EXTCOMIN Needs to be toggled at 5ms or 250ms in sleep all the time
7 MEMLCD_DISP 1=disp on, 0=disp off out, pulsed high
8 Backlight out
13 (0x0d) Flash_MISO input
14 (0x0e) Flash_CS out
15 (0x0f) Flash_MOSI out
16 (0x10) Flash_SCK in_pullup
17 (0x11) Button In_pullup, edge detect
19 (0x13) Vibrate, active=1 out
21 (0x15) HRM_POWER, active=1 out
22 (0x16) HRM_INT
23 (0x17) Charge port in_pullup
24 (0x18) HRM_SDA
25 (0x19) Charging complete
26 (0x1a) MEMLCD_SCK
27 (0x1b) MEMLCD_MOSI
29 (0x1d) GPS power, active=1 out
30 (0x1e) GPS TXD
31 (0x1f) GPS RXD
32 (0x20) HRM_SCL
33 (0x21) TOUCH_SDA
34 (0x22) TOUCH_SCL
35 (0x23) TOUCH_RESET (active low) out
36 (0x24) TOUCH_INT (active low)
37 (0x25) ACCEL_SDA
38 (0x26) ACCEL_SCL
39 (0x27) ACCEL_INT input
42 (0x2a) UARX unused debug pin
43 (0x2b) UATX unused debug pin
44 (0x2c) COMPASS_SDA
45 (0x2d) COMPASS_SCL
47 (0x2f) PRESSURE_SDA

To access these pins from Espruino, all you need to do is add D to the pin number. For example to read the analog battery voltage (pin 3), use the command analogRead(D3).


Bangle.js 2 has the following I2C devices. These are connected via their own pins and don't share an I2C bus - see above for the connections.

  • PRESSURE - BMP280, I2C 0x3C(0x1E)
  • TOUCH - Hynitron CST816S, I2C 0x2A(0x15)
  • HRM - VCare VC31, I2C 0x66(0x33)
  • ACCEL - Kionix KX022, I2C 0x3C(0x1E)
  • COMPASS - I2C 0x18(0x0C)

Unused pins (adding hardware)

Most of the pins on the nRF52 chip in Bangle.js 2 are either used or not connected. However there are two exceptions - UARX (D42 in Espruino) and UATX (D43 in Espruino). These pins are not connected to any hardware, but are available as test points so can be connected to whatever you want. They are accessible on the rear of the Bangle.js PCB, under the battery:

There are also test pads for the touchscreen I2C TOUCH_SDA and TOUCH_SCL lines (see GPIO table above) that could be used to connect other I2C devices.

Note: The battery is attached to the back of the Bangle.js PCB with double-sided sticky tape. Take care when removing it, as it's almost impossible to remove the battery without bending it slightly.

If you wish to add your own hardware to Bangle.js, we'd suggest sourcing and fitting a smaller battery, which would then leave room in the rear of the case for your circuitry.

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