At the core of the design is a PIC18F26J50 in a 28 pin SOIC package. It’s capable of running at down to 2.15 volts and consumes extremely little power when running at lower clock speeds. And apart from that it features USB so we can have all the benefits of USB without any external components except, of course, a USB socket.
The PIC has two crystals at its disposal. A 8MHz crystal which will be boosted up to 32MHz by its internal PLL. That’s what the PIC will run on when there is work to do. And then there is a 32.768kHz crystal that will be used to run its real-time clock (RTC). When there is little to no work to do this low-frequency clock will also be used to run the CPU which will greatly reduce power consumption. Power consumption is approximately linear in frequency so this should cut power consumption by a factor of about 1000 compared to full-speed operation.
Now we can run the microcontroller at only a few volts but our power supply is a 12 volt battery. So one of the most straight forward things to do in order to save power was to use a switch mode step-down regulator aka buck.
I looked around and found the Texas TPS62120. It’s only capable of providing 75mA but that’s more than enough for us in this case. It works at a switching frequency of 800kHz and only consumes a bit more than 10 microamps with no load at its output.
It needs a 18uH inductor as well as some ceramic capacitors to work. The output voltage is set via a pair of resistors acting as a voltage divider. I’ve added a n-channel mosfet that allows the PIC to increase the output voltage from 2.2 volts to 3.3 volts when needed. Because while the PIC can run on down to 2.15 volts the display can’t. And even the PIC needs 3.0 to 3.6 volts for USB operation.