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A hackable ARM Cortex M0+ upgrade for a classic Casio wristwatch

Sensor Watch is a bridge between eras. It takes an iconic 30-year-old design from a golden age of digital watches, and pairs it with a modern, powerful microcontroller and state-of-the-art sensing capabilities. This small circuit board, less than an inch in diameter, replaces the original quartz movement in a Casio F-91W watch to put the capabilities of an ultra-low-power ARM Cortex M0+ microcontroller on your wrist.

Sensor Watch is not like most smart watches. It makes a different set of engineering tradeoffs, to achieve a different set of goals:

  • Instead of a high-resolution TFT LCD, Sensor Watch repurposes the Casio F-91W’s monochrome segment LCD. This gives it an always-on display that consumes mere microamperes of power.
  • By avoiding power hungry features like WiFi and Bluetooth, Sensor Watch can run for a year or more on a single 100 mAh coin cell, eliminating the need for frequent recharging.
  • The lack of an external charging port, coupled with reuse of the F-91W’s famously water resistant enclosure, makes this a hackable wristwatch that can handle depths up to 30 meters.

What Can It Do?

Sensor Watch is designed to be useful right off the bat. The community Sensor Watch firmware, called Movement, has several useful watch faces already:

  • The Clock face allows Sensor Watch to function like, y’know, a watch, displaying the time and date.
  • The World Clock face allows you add a time display for any number of time zones around the world.
  • The Beat Time face displays Swatch Internet Time, a decimal time system that divides the day into 1,000 beats.
  • The TOTP Face displays time-based one-time passwords, the kind that you need for two-factor auth on many websites.
  • The Temperature face displays readings from the thermistor sensor board.
  • The Temperature Log face logs and displays up to 36 hours of timestamped temperature data. Toss this outside of your tent while camping, and you’ve got a weather station you can deploy from your wrist.
  • The Day One face lets you count days from your birth date, to watch the days of your life tick by.

There are others (stopwatch, pulsometer, battery voltage), but the important thing isn’t the apps that come with Sensor Watch. The important thing is that Sensor Watch is open source and easily hackable, which means you can write the apps that make sense for YOU. Do you want an astronomy face that can show you moon phase and predict satellite passes? A transit face programmed with train arrivals for your nearest subway station? An astrology face that can tell you if Mercury is in retrograde? These are all apps that you could write for Sensor Watch.

In addition, the sensor board in Sensor Watch is modular and swappable, so if you want a different kind of sensor than the one that’s on the watch? You can build a sensor board of your own, and sense the things that matter to you.

Style, Meet Sensors

The Casio F-91W is iconic for any number of reasons: it’s affordable and durable, and it’s come in many colors over the years, which means you’ll almost certainly find one that matches your style. It’s also small! At just 34 mm in diameter, it’s smaller than any smart watch, and can feel at home on any wrist. This small size, though, leads to a challenge when repurposing the watch to turn it into a Sensor Watch: how can we fit useful sensor technology into such a small package?

The answer lies in the nine-pin flex PCB connector at the bottom of the Sensor Watch board. By plugging a miniature sensor board in to the Sensor Watch main board, you can add whichever sensors make the most sense for your use case. This makes Sensor Watch far more versatile than a fitness tracker that can only track motion, or a compass watch that can only sense direction.

While Sensor Watch comes with a temperature sensor board, you could instead replace it with a phototransistor board for light metering, a microphone for tracking noise exposure, or, yes, an accelerometer for activity tracking. Sensor Watch leaves a 5.7 × 5.7 × 1 mm area free for whatever sensors you want to add. It’s a small volume, but modern MEMS technology allows smaller sensors to do more than ever before.

In the spirit of open source hardware, several of these sensor boards are already posted in the Sensor Watch repository, and the hope is that as we build more of a community around Sensor Watch, folks will design additional sensor boards to give their Sensor Watch even more interesting capabilities.

The Microchip SAM L22: Big Power in a Small Package

The SAM L22 microcontroller at the heart of Sensor Watch is an ARM Cortex M0+ chip with 256 KB of Flash and 32 KB of RAM, running at up to 32 MHz. It’s similar in many ways to the SAM D21 you’d find in a Feather M0 or Arduino Zero, with many of the same versatile peripherals:

  • An integrated USB peripheral and UF2 bootloader let you plug the board into your computer and program it by dragging firmware onto it, just like a thumb drive.
  • The real-time clock peripheral, paired with a 32.768 KHz crystal, allow for accurate timekeeping and configurable wake-up options.
  • The integrated 12-bit ADC lets you read analog values from any of the five GPIO pins on a sensor board, with oversampling to 16 bits of resolution.
  • The SERCOM peripheral lets you easily communicate with I2C, SPI or UART-oriented devices on a sensor board.
  • Four timer/counter peripherals allow for versatile use cases like pulse-width modulation, frequency counting and configurable periodic callbacks. This is in addition to the TCC peripheral that drives the red/green backlight ¹ and piezo buzzer ².

¹ Red/Green backlight is Red/Blue on limited-edition boards.
² The piezo buzzer is the only piece that requires soldering. You will need to remove a metal piece from your donor F-91W and solder it to the Sensor Watch board. If you don’t feel comfortable doing this, all other features of Sensor Watch will function identically; the watch just won’t beep.

The Segment LCD: a Low-Power Hero

In addition to these familiar peripherals, the SAM L22 packs one less familiar one: a segment LCD controller. This controller speaks the native language of the F-91W’s display glass, and it’s the key to the unique low power capabilities of Sensor Watch:

  • Unlike a TFT, with its layers of color filters, the segment LCD glass is readable without backlighting.
  • Unlike an OLED, which relies on light-emitting diodes, segments do not consume significantly more power when on versus off.
  • Unlike an e-paper display, which requires current to move ink particles, updating the segment LCD glass does not consume significantly more current than keeping an image on the screen.

This is not to say there aren’t tradeoffs: the segment LCD in the F-91W was only designed to display the time, so not every number or letter works in every position. It also cannot display arbitrary images like an e-paper display or LCD matrix. Still, with a bit of creativity, you can create a watch face that displays quite a lot of information, and carry it with you all year long.

Features & Specifications

  • ARM Cortex M0+ microcontroller running at up to 32 MHz
  • 256 KB of on-chip Flash, with up to 16 KB EEPROM emulation area
  • 32 KB of RAM with full retention in low-power standby mode
  • 32.768 kHz crystal for real-time clock functionality with alarm support
  • Red & green PWM’able LED backlight (red & blue on limited-edition boards)
  • Thermistor for temperature sensing
  • On-board USB Micro B connector
  • Reset button with double-tap UF2 bootloader
  • Nine-pin flex PCB connector for sensor boards
  • Controller for ten digit segment LCD, plus five indicator segments
  • Edge-plated contacts for three interrupt-capable buttons
  • Connection pad for piezo buzzer (requires light soldering)
  • Open Source

The 9-pin sensor board connector offers a lot of additional functionality that you can make use of on a sensor board:

  • 3V power (nominal voltage from a CR2016 coin cell, can drop to ~2.5V near end of life)
  • An I²C interface with built-in pull-up resistors
  • Five general purpose IO pins, which can be configured as:
    • Five analog inputs
    • Five interrupt-capable digital inputs, with internal pull-up or pull-down resistors
    • Five digital outputs
    • SPI controller (with one spare analog / GPIO pin leftover)
    • One UART TX/RX pair (with three GPIO leftover)
    • Up to four PWM pins on two independent TC instances
    • Two external wake inputs that can wake from the ultra-low-power BACKUP mode


³ $35 + the cost of an F-91W

Support & Documentation

And please feel free to reach out using using the Ask a technical question link below!

Manufacturing Plan

We have already produced the first run of about 85 limited-edition boards, thereby proving out the viability of our manufacturing process. When the campaign ends, we will place orders for all of the components required to manufacture the standard-edition Sensor Watch boards. (We have quotes from several different contract manufacturers here in the States.) The limited-edition boards should ship out a month or two after the campaign ends, with the standard-edition boards following along several months later.

We will assemble the temperature-sensor boards in house; these small flex PCB’s have only two components each, making them easy to hand-assemble and test here in our workshop.

Fulfillment & Logistics

As soon as all the boards are assembled and tested, we will ship them to Mouser Electronics, Crowd Supply’s fulfillment partner, who will distribute them to backers. You can learn more about Crowd Supply’s fulfillment service under Ordering, Paying, and Shipping in their guide.

Risks & Challenges

The ATSAML22J18A-MUT remains in short supply due to the global semiconductor shortage. This is without question the biggest risk to the project: this chip is at the heart of Sensor Watch, and we can’t make the boards without it. As of this writing, these chips are expected to be available next summer, but that availability could slip as other customers preorder this inventory between now and the end of the campaign. We plan to handle this risk by working with Crowd Supply, Mouser and Microchip to ensure that we place the best possible order for these chips as quickly as possible after the campaign ends.

Sensor Watch is part of the Microchip Get Launched design competition!

Absolulty LOL

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