2019-01-20

Too Many USB Devices

The Comvette Project employs a number of USB devices:

• 32 Gigabyte Flash Memory Drive to boot the Raspbian OS.
• 128 Gigabyte Flash Memory Drive to store music and movies.
• 5" Diagonal Touch-Screen.
• In-Dash USB Jack.
• GPS Receiver.
• HDMI Audio Extractor.

Clearly that's a few more devices than the Raspberry Pi with four USB ports can accomodate. Even if the Raspberry Pi had more USB ports available, there would be an issue supplying power to all of those power-hungry devices from the Raspberry Pi's very limited power supply.

The Raspberry Pi provides a maximum of 600mA USB power which is shared between all of it's USB ports. This means that each USB port gets only 150mA if divided evenly among the four ports. The USB specification calls for 500mA per port, so you can see that the Raspberry Pi is severely limited. Attaching power-hungry USB devices to the Raspberry Pi is not recommended. If you do, you can expect random lock-ups or reboots from over taxing the Raspberry Pi's power supply.

What to do?

Well, purchasing an industrial grade USB hub was the answer to this problem. And it addressed another issue as well.

In a Car based system like Comvette, there's the issue of providing clean, regulated power to the Raspberry Pi (which is also a USB powered device) and all the other USB powered devices.

The USB hub I purchased can be powered by anything from 9 volts to 36 volts DC. And it's tolerant of voltage fluxuations and spikes which makes it a good fit to be powered by a car's 12 volt DC system.

The hub powers all of the USB devices in the Comvette system including the Raspberry Pi itself. So, currently the configuration looks like this:

Raspberry Pi USB Ports:

1. 32 Gigabyte Flash Memory Drive to boot the Raspbian OS.
2. Cable to USB Hub.
3. Unused.
4. Unused.

USB Hub Ports:

1. Head unit 5" diagonal touch-screen.
2. Head unit USB port.
3. HDMI Audio Extractor.
4. GPS Receiver.
5. 128 Gigabyte Flash Memory Drive to store music and movies.
6. Unused.
7. Unused.

This leaves 2 USB ports available on the Raspberry Pi (which I will probably never use for the reasons given above), and 2 USB ports available on the hub, for future use.