Alright, back at it again. I'm working on this project pretty consistently (about an hour or two a day) and the electronics are 90% of the way there. I'm just working out the finer details of the layout for the board. 

I hate to admit it but it looks like the first revision of the board may be slightly larger than the screen. It's not exactly ideal but it does give me a platform to experiment with and write software for. I do plan for a revision two of the project maybe with some extra features but for now I'd like to just get some stuff on order.

Either way the main purpose of this log is to talk about some of the decisions that have been made for this project in regards to component selection. 

For the main micro-controller this project uses the esp32 pico d4, with integrated wifi and bluetooth it has more than enough processing power and features to make this project do everything I want it to do. Also since it's an ESP32 there's a great amount of additional functionality I can add later as the needs arise. Power consumption is a bit of a problem with this chip, but I've done some looking into the sleep modes available on the esp32 and running with a 450mAh battery the lifetime should be good enough provided average use for a watch. 

Now obviously power management is a big part of this project and there are 3 IC's used here that will serve to generate a stable (kinda) 3.3v. 

• MAX9065 - window comparator for voltage monitoring of the battery
• mcp73811T - lithium ion battery charger (i just love this IC) 
• S-13A1A33-E8T1U3 - 3.3V LDO regulator

Picking the LDO reg was one of the more difficult parts of the design process for this project. The ESP32 pico can handle voltages up to 3.6V easily and operate down to 3.0V. So the voltage dropout of the regulator needed to be less than .2v at operating current. Luckily I found the S-13A1A33 which fits the bill pretty well and requires a minimum of external components. I thought hard about using a buck-boost converter for this project, but the board space considerations put the nail in the coffin for that one. 

In addition to the power management IC's the current design also includes an ADXL337 accelerometer which uses a 3-wire analog interface for each axis and should allow for the easiest interfacing. Power consumption this chip is also fairly low and overall should reduce the power consumption of the board. The current plan is to only activate the tft backlight when the watch is pointing upwards or when a button is pressed on the watch. This way the LED backlight can't suck too much of the battery life away. In tests so far this results in some fairly good power savings but as always its hard to tell without the physical prototype in hand. 

Either way that's the design as it stands currently, I'll be releasing the rev 1 files as soon as I feel they're done.