Time to talk about some electronics. The schematics are in what seems to be their final iteration (hopefully) so let’s check out the results.
Right here you can see both SoM connectors and their signals. The leftmost connector has the FPGA signals and the other one has the HPS signals. This page also contains the JTAG connector, pads for VCC12 and GND connections, an UART connector for the SoM’s boot log and the wifi mode selection switch. You will also find the reset switch and stabilization module for Reset signal propagation as well as the SoM’s decoupling capacitors.
This page contains the power supply rails. There is a 12V->5V and a 12V->3.3V rail based respectively on the LT8612 module and the PTH12040 module. In order to make sure the power from the bearings is stable, we use big bulk capacitors on the input and output sides of the regulators.
There, you will see the SD card reader, photoelectric and hall sensor connectors and the Mainboard to Highway PCB connectors. We had to connect the SD card in SPI mode as the SDMMC pins on the HPS are used by the 4GB NAND flash on the SoM.
For the board to board connections, we used three 28 pin connectors with two being used for signal and one being used for power. On the signal connectors, we used every other pin for GND for signal integrity purposes.
This last page for the Mainboard contains the USB connector for the wifi dongle as well as the USB chip converting it to ULPI as it is what the HPS uses. In order for this to work, we had to use a small 3.3V->1.8V regulator and a 5V->5V power distribution device in to control the USB power.
That’s it for the Mainboard schematics. Placement and routing is well under way and should be done soon. The LED and highway PCB should follow quickly.