Ultra-low power microcontroller
This is the most important part of the circuit because it is going to be the major cause of power drain, but also because it is the only active component!
The microcontroller for the watch needs to be able to drive 16 discrete LEDs and measure the capacitance of the touch sensor. I chose to use the MSP430G2452 from Texas Instruments for several reasons. It is a ultra-low power MCU with different sleep modes, it can operate from a Lithium coin cell (3.3V down to 1.8V), it has a built-in excitation circuit for the touch sensor and a crystal input for a 32.768kHz crystal for the real-time clock (RTC). It has enough GPIO (10 for the 16QFN version) to drive the 16 LEDs using Charlieplexing.
The real-time clock circuit is simply a 32.768kHz crystal directly connected to the MSP430. No need for two extra caps, they are integrated into the microcontroller. I am using a reference from MicroCrystal the MS3V-T1R as it is thinner than a regular PCB (less than 1.6mm high).
Because it is operating from a coin cell, the LEDs must have a low forward voltage. This limits our choice to red, orange, yellow and some green ones. I’m using cheap 0603 SMD red and orange LEDs I had laying around, bought on eBay.
The number of passives is kept to a bare minimum (4 + 5 resistors for the LEDs), most of them are decoupling caps (2.2µF and 220nF). I chose to keep them because the connection with the coin cell is not going to be perfect. It also helps reduce the fluctuations caused in the supply voltage by the excitation circuit, fluctuations that might destabilize the RTC crystal.
The watch is powered by a single Lithium coin cell, more precisely a CR1612 (12mm diameter and 1.6mm high). It has the perfect height as standard PCB height is 1.6mm. The bottom and top PCBs can act as the battery holder. This coin cells have a limited capacity of 25mAh from 3V to 1.8V.