Comment by mikeayles

Yep, exactly. I'll do another blog when I have a little more to show, but the concept is, everything is on a grid of 12.7mm squares, where there is a common bus running through north-south. The boards need to be 4 layer and feature size is pretty small, 0402 passives for example, maybe even 0201 if I'm really optimising for size.

SCH: https://github.com/MichaelAyles/heph/blob/main/blogs/0029-im...

3D bottomside: https://github.com/MichaelAyles/heph/blob/main/blogs/0029-im...

In the Phaestus workflow, it chooses modules, for example the main cpu block just has an ESP32C6-XIAO board on it, since it has massive compute, and radio with wifi6 and zigbee, which covers 90% of IOT. Since this is larger than 0.5" it sits in the middle of a 2x2, which bridges both sets of north-south buses, with all common pins.

Bidged pinouts: https://github.com/MichaelAyles/heph/blob/main/blogs/0029-im...

Early XIAO: https://github.com/MichaelAyles/heph/blob/main/blogs/0029-im...

It's been a pain in the butt to design for, since the vias need enough clearance, and we are almost maxed out on our bottom side, so on the 2x2's i've given it as much room as I can in the middle to allow for topside routing with a few vias.

Then, when assembling the board, it does a 0.1mm overlap n/s, which merges the nets, whilst the system keeps track of what signals are used, so I2C is all multiparticipant, spi1 gets the default CS line, otherwise it can use the aux pins via a resistor selector, the AUX5/AUX6 aren't connected to the XIAO, so if you wanted to do something like a USB power monitor, the power can run isolated at higher voltages, as long as they get fed through a current/voltage sensing block, which could be 1x1, 1x2 or 2x2.

As for component placing in the enclosure, buttons are sub-boards with a wire-to-board connector, and the 1x1 module is a block that contains a connector and an PCA9570, which allows the button to be placed anywhere with defined mounting features.

Similar for LCD's, the pre-designed block has a FFC connector, and comes with constraints, e.g. place at end of design, so the screen can fold back on itself, whereas if it was mounted in the middle, it wouldn't be able to go where it needs to be.