March 2021

Håkon Robbestad Gylterud

2021-03-14, Sunday

Power consumption. My computer has quite a few USB periferals, and sometimes the way they are plugged into the various ports causa a lack of power to run them all. What is the proper way to deal with that?

Universal Serial Bus

For instance, yesterday I was going to scan a “character sheet” I had made for my eldest’s imaginary robot friends. But for some reason the scanner1 would only show up for a couple of seconds, and then dissappear from lsusb. I tried all the different USB ports, but to no avail. Then I noticed that I had forgotten to connect the external power to my USB hub. Lo and behold – the scanner worked again!

Now, forgetting to connect the external power to the USB hub is obviously a mistake2. But why did none of the front-side ports give enough power? How many amps are actually available? How much does each device draw? Should I put up a budget?

If you go look up USB power, you find that USB power is measured in “units” of power, by which they mean sometimes 100 mA and other times 150 mA. And there are low-power devices (1 unit) and high-power devices (5 or 6 units). But how many units do I have avaiable?

The ideal would be if there was a led somewhere to indicate if the USB was having problems delivering enough power. Would that be possible?

2021-03-25, Thursday

Held a talk today for the HoTTEST seminar. I talked about Myott, which is what I will be working on, together with Elisabeth, once I get back to a normal work rythm.

  1. Fujitsu ScanSnap s1300i, works like a charm if you actually give it power, even on Linux.↩︎

  2. This is however an easily made mistake, since there is no LED to indicate power, and many periferals will work fine on the unpowered hub. Before the scanner, my MIDI controllers were happily doing their job on the hub.↩︎

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