There's an Armband, and Then There's an Arm Watch!

With our past previous coverage of anything watch based, you likely don't need me yet again telling you just how much we love a good timepiece project.

While we've looked at of the weird and wonderful ways of working out how to go about building a watch, from wrist-worn Wi-Fi-enabled ESP32 circular display wristwatches, to nano-sized Nixie tube timepieces, there's a great many ways to watch the seconds tick by.

There are a few specific designs of watch that have, rather rightfully, earned somewhat of a cult status, and with more than a few places in the top 10, the retro-tech flavors of the various Casio family of watches have been staple timepieces for techies of years gone, and years to come.

We've covered the sublime and simple modifications to the Casio F91W from NODE, to the gosh darn impressive GoodWatch from Travis Goodspeed, which saw them retrofit a Texas Instruments MSP430 into the original CA-53W watch housing, allowing for some upgraded functionality on the original calculator features.

It seems fitting that such an arm-worn device might feature an Arm processor, and Greg Davill has finally stepped up to that challenge, with his fork of the GoodWatch hardware — that seeks to swap out the lesser seen MSP430, for a processor that is a bit more familiar to many of us — the SAM L22N16A MCU, from Microchip.

Keeping time with a Cortex-M0

While at heart, the SAM L22 is near-as-dangit close enough to the D21 for all intents and purposes — a Cortex-M0+ 32-bit MCU that shares a enough peripherals and registers with the D21 relatives to see that they fall from the same family tree, the L22 has a sneaky extra peripheral up its sleeve — a built-in Segment LCD controller.

This makes interfacing to the 30 elements found within the stock CA-53W LCD screen a matter of simply wiring them to the individual output channels of the Segment LCD display peripheral, available on the various I/O pins of the SAM L22.

It's a pretty neat routing job, with almost a clean 1:1 fan out — save for the pesky COM0-COM3 signals that see themselves snaking up and around the row of pads that make up the PCB mating face of the Z-axis/Zebra strip elastomer connection system, that is used to attach to the LCD glass itself.

If you've not come across Z-axis strip, it is an interesting elastomer connection system that relies upon a strip of finely spaced conductive elements, sandwiched within an elastomer "shell."

The pitch of the conductive elements is such that you might end up with several of the conductive elements within the tape landing on one of the PCB pads — which can be seen in the image above, shown as an example of this connection system.

It's a solderless flexible mounting system that while doesn't support huge amounts of current, is more than suitable for the low profile, low power requirements of a polarized LCD display, such as the ones used in many a digital wristwatch,

If we look at the above render of the watch, it's clear to see the horizontal area where the Z-strip is intended to be mounted, above the L22 MCU.

Keeping true to form...

With the stock display suitably sorted out, thanks to the Segment LCD controller of the L22, the bulk remainder of the I/O is dedicated to digitizing the demands of your digits — I'm talking of course about buttons.

With some carefully crafted, edge plated cutouts, the board is able to interface with the stock "finger contact" style switches, found on within the original watch body.

Where as interfacing to the keypad found on the CA-53W is a matter of scanning in the key matrix — the connection to which can be seen at the bottom of the PCB — yet another contact strip row, designed to seat the connector used for the tiny tactile switch keyboard.

This is a great little no-messing-about demonstration of what a low power MCU with the right peripherals can do. With nearly no I/O pin left unused (See if you can find where the SWDCLK signal is routed to... SWDIO is pretty easy!), it's a spartan build, but designed for a simple application.

It certainly doesn't take a Cortex-M0+ to keep time and drive a Segment LCD, but having a platform with such a familiar MCU core behind it is sure to make code development an absolute breeze, once Davill gets around to it, that is.

For now, we imagine he's taking a well earned break, what with this low power build being one of the 31 individual projects he's designed in the month of December, as an alternative to the conventional advent calendar for 2020.

If you want to perhaps take up that gauntlet of code development, get your fill of the project sources over at Davill's GitHub and get cracking. Alternatively, make sure you are keeping tabs on this timepiece build by keeping an eye on his Twitter feed — @GregDavill.