Mikhail Svarichevsky Puts the Classic Casio F-91W's Silicon Chip Under the Microscope
Inspired by work on Joey Castillo's Sensor Watch firmware, Svarichevsky has decapped and photographed the chip that drives Casio's classic.
Engineer Mikhail Svarichevsky has rung in the new year with a high-resolution die shot of an iconic piece of wristwear: the Casio F-91W digital watch.
"[The] Casio F-91W is an iconic watch worn (among others) by Barack Obama and Osama bin Laden," Svarichevsky explains of the device from which the chip under the microscope came. "While working on precision watch firmware (with quartz temperature compensation, aiming for error of under 10 seconds per year) for SensorWatch which is built around F-91W I was curious to see how [the] original chip looked like."
Released in 1989 as a successor to the F-87W and Ryusuke Moriai's first design for the company, Casio's F-91W has been in continuous production ever since its launch ā and gained popularity for its small size, long battery life, and low price. Its internals aren't much to look at, until you get below the surface ā which, in Svarichevsky's case, means using boiling acid to decap the package of the integrated circuit at the watch's heart and capturing a shot of the silicon inside.
"[The] chip is more complicated than I anticipated," Svarichevsky found, "and [the] digital part is less than ~50% of the die area. F-91W has trim jumpers on the PCB to adjust part-to-part variability of the quartz crystal. Looking at die complexity - it might be that they use capacitive tuning of frequency rather than skipping/adding ticks. Also, it might happen that they even have coarse/cost-efficient temperature compensation. Casio specify accuracy of 30 seconds per month, but some F-91W watches are within 6 seconds per month which requires good adjustment & luck for [a] non-temperature compensated watch."
While the watch has been in continuous production, it's not unchanged: Svarichevsky points out that the launch model managed to eke out its battery for just two years, while modern versions can run for up to a decade. "If someone has [an] original version from 1989/early 1990s," he writes, "it would be interesting to compare if it's a 'die shrink' or re-design with improved performance."
Svarichevsky's interest in the watch came about while working on the Sensor Watch, a drop-in board replacement for the F-91W designed by Joey Castillo to considerably improve the device's capabilities ā including support for multiple alarms, calculation and display of planetary positions, and with modular sensor add-ons should the standard hardware prove too limited. Svarichevsky isn't the only one inspired by Castillo's work, either: Pseudonymous maker "Pegor" has been working on his own F-91W upgrade, which replaces both the board and display to offer traditional smartwatch capabilities.
The full die-shot is available on Svarichevsky's website, Zeptobars.
Main article image courtesy of Ashley Pomeroy, CC-BY-SA 3.0.
