This project is about making the nicey watch out of Flexible OLED made by Futaba. This is the earlier (yet cheaper) version of current Flexible display in Futaba's line up. I got this of an Aliexpress for roughly 15 bucks-ish. In fact, this is the only place that you can get the Futaba Flex OLED (beside Alibaba, which is the same company on Aliexpress). Event other main distributors like M0us3r, D1g1k3y, RS C0mp0n3nt don't have the newer Flex OLED on the shelf.
The brain of this project is my go to choice MCU. It's STM8L151F3. Only 1K of SRAM but this display framebuffer is only 256 Bytes CRAZY! (as I always play with >1Kbytes buffer Display).
Nice Thing about this MCU is the built-in RTC with low power mode. Without requiring any external Oscillator XTAL. Though the time keeping my not precise. But It's cool watch so acceptable xD.
However, I have 2 major concerns,
The first, (some part of) display itself is really fragile. Normally the display will sit on some sort of fabric-like thingy that wrap around my wrist (first prototype is gonna be crappy made D.I.Y Velcro wrist band). Bend radius isn't big problem for this though. But there's a SSD1316 die that really fragile, I can prevent this region not to flex with some rigid plastic supporting underneath.
The second one is battery (physical shape, capacity, max discharge current and low power design),
Juice Sucking when sleep: when deep sleep I estimated the juice sucking to be around (at most) 30uA. That's 10uA from STM8L in Active halt at worst case scenario, normally (according to datasheet) should be right around 2-3uA. And the LTC3459 (boost IC for supplying Positive bias voltage to OLED) sleep current according to datasheet is between .1 to 1uA, my guessing is probably around .2-.5uA. The Display itself according to datasheet is around 1 to 10uA so gonna guess that it's ~ 5uA.
I need to take other factors into an account for example parasitic resistance, Imperfection of silicon chip that might deviate from "normal" operation current usage and more that I can't even thing of.
Juice Sucking when running: In running mode, I estimate that it will running about 20mA, STM8L at around 4-4.5mA, Display at 15mA including LTC3456 current consumption. this would be problem when comes to battery max discharge current which is proportional to the battery size.
Batter Size Shape Capacity and Maximum discharge current: finding the battery to suit this project is really hard. I've spending time about 1 month now and still haven't found the good candidate for powering this project yet. I added the LIR2032 into the list since it's capacity, size, and max discharge current is kinda suitable for this project. but still, there's more smaller size battery out there that might suit for this project as well. I'm looking at the wireless earbud / Airpods battery too. but these are too expensive compare to LIR2032.