Some information from inspection and discovery on non-destructive tear-down I did quite a while ago (on a watch with piezo speaker). Note that the chip marked with 'Mag?' in the main shot on https://www.espruino.com/Bangle.js+Technical is the port expander. The Mag(netometer sensor) is the very small chip to its left with very few connections. Zoom into some of the pics, especially the one with the mirrored back, in which I marked some traces, mainly with + for Vcc, - for Ground, C for clock and D for data for the I2S. Port expander has still some unused extensions ;-) ... The first two pic shows IC identifications. The third one I used to follow the PCB traces on front and back. In any graphics editor take a line / dashed double arrow line and move it around to identify the same location on the front and back. I used that technique to follow the traces between the ICs as they go between front and back via the vias.
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