30 Sep 2016: EOMA68 Libre 15.6in Laptop PCB2 Revision 2
After reworking PCB1 and PCB2, work on a complete redesign of PCB3 began. Jakub kindly did an enormous amount of research into appropriate components, here, and his company mlab.cz actually releases all their CAD designs as Libre-licensed KiCAD. From his work and the research he's done, he's been able to steer the direction and component selection to near exclusive use of Texas Instruments parts. Texas Instruments has a reputation for reliability and the comprehensiveness of their documentation, so in retrospect this outcome could easily have been predicted.
This last PCB is the one where the most care has to be taken, as mistakes could result in fires or explosions. Many engineers simply will not work with such designs as public ongoing releases as a result, and some even refuse to sell such boards to individuals. Users are therefore expected to take responsibility for helping out and for making their own decisions.
The new revision is 5in long (which increases heat dissipation) and contains a buck/boost converter to bring the battery output voltage up to 5.0 volts. There are actually two 5V outputs: one for the main electronics (USB, speakers, LCD backlight) and one for the EOMA68 Card. The latter is two-way reversible which is why the bq24193 has to be used, as it is a fully "OTG" compliant charger IC. These parts have been selected because the output can be up to 4A @ 5V (20 watts), so a lot of care and attention is needed here.
The video below contains a walkthrough from the schematics to the PCB layout through to the 3D CAD indicating the relationship to surrounding parts. This complex inter-dependence serves to once again emphasise how critical it is that the entire laptop be developed holistically (and why re-using existing laptop casework is totally unrealistic). Changing the length of PCB3 required a change to 3 casework parts and a re-order of the cables to the other 2 PCBs. Using different ICs requires different pinouts (different functions available) such that the GPIO changes, possibly even the size of the connectors required, such that the other PCBs now need to be changed, and the STM32F072 firmware also needs changing. It's a massive chain of knock-on effects, any one of which, amazingly, could result in a near-total internal redesign.