USB

USB is used for two purposes:

Power supply of 5V to all ECUs and expansions.
Communication between ECU A and USB host.

Designing a device suitable for USB-IF certification requires to follow many guidelines. Since RAMN is not meant to be used as a “proper” USB device (such as a keyboard, mouse, etc.) we do not attempt to fulfill all USB-IF requirements. Concretely, this means there is no “VBUS filter” on the board (ferrite beads, etc.)

The USB connection circuit simply features an RC filter between SHIELD and GROUND. A Shunt resistor can be used for simple power analysis. It is also possible to replace the shunt resistor by a polyfuse.

../_images/usb_2layer.png — USB connection circuitry.

Warning

Unless you replace the shunt resistor by a fuse, RAMN assumes that the USB host is protected against failures of its loads (short-circuits, etc.). There is no special ESD protection except for a simple RC filter.