Nov 17, 2023

USB hub per-port power switching done right with a couple wires

Like probably most folks who are surrounded by tech, I have too many USB devices plugged into the usual desktop, to the point that it kinda bothers me.

For one thing, some of those doohickeys always draw current and noticeably heat up in the process, which can't be good on the either side of the port. Good examples of this are WiFi dongles (with iface left in UP state), a cheap NFC reader I have (draws 300mA idling on the table 99.99% of the time), or anything with "battery" or "charging" in the description.

Other issue is that I don't want some devices to always be connected. Dual-booting into gaming Windows for instance, there's nothing good that comes from it poking at and spinning-up USB-HDDs, Yubikeys or various connectivity dongles' firmware, as well as jerking power on-and-off on those for reboots and whenever random apps/games probe those (yeah, not sure why either).

Unplugging stuff by hand is work, and leads to replacing usb cables/ports/devices eventually (more work), so toggling power on/off at USB hubs seems like an easy fix.

USB Hubs sometimes support that in one of two ways - either physical switches next to ports, or using USB Per-Port-Power-Switching (PPPS) protocol.

Problem with physical switches is that relying on yourself not to forget to do some on/off sequence manually for devices each time doesn't work well, and kinda silly when it can be automated - i.e. if you want to run ad-hoc AP, let the script running hostapd turn the power on-and-off around it as well.

But sadly, at least in my experience with it, USB Hub PPPS is also a bad solution, broken by two major issues, which are likely unfixable:

  • USB Hubs supporting per-port power toggling are impossible to find or identify.

    Vendors don't seem to care about and don't advertise this feature anywhere, its presence/support changes between hardware revisions (probably as a consequence of "don't care"), and is often half-implemented and dodgy.

    uhubctl project has a list of Compatible USB hubs for example, and note how hubs there have remarks like "DUB-H7 rev D,E (black). Rev B,C,F,G not supported" - shops and even product boxes mostly don't specify these revisions anywhere, or even list the wrong one.

    So good luck finding the right revision of one model even when you know it works, within a brief window while it's still in stock. And knowing which one works is pretty much only possible through testing - same list above is full of old devices that are not on the market, and that market seem to be too large and dynamic to track models/revisions accurately.

    On top of that, sometimes hubs toggle data lines and not power (VBUS), making feature marginally less useful for cases above, but further confusing the matter when reading specifications or even relying on reports from users.

    Pretty sure that hubs with support for this are usually higher-end vendors/models too, so it's expensive to buy a bunch of them to see what works, and kinda silly to overpay for even one of them anyway.

  • PPPS in USB Hubs has no memory and defaults to ON state.

    This is almost certainly by design - when someone plugs hub without obvious buttons, they might not care about power switching on ports, and just want it to work, so ports have to be turned-on by default.

    But that's also the opposite of what I want for all cases mentioned above - turning on all power-hungry devices on reboot (incl. USB-HDDs that can draw like 1A on spin-up!), all at once, in the "I'm starting up" max-power mode, is like the worst thing such hub can do!

    I.e. you disable these ports for a reason, maybe a power-related reason, which "per-port power switching" name might even hint at, and yet here you go, on every reboot or driver/hw/cable hiccup, this use-case gets thrown out of the window completely, in the dumbest and most destructive way possible.

    It also negates the other use-cases for the feature of course - when you simply don't want devices to be exposed, aside from power concerns - hub does the opposite of that and gives them all up whenever it bloody wants to.

In summary - even if controlling hub port power via PPPS USB control requests worked, and was easy to find (which it very much is not), it's pretty much useless anyway.

My simple solution, which I can emphatically recommend:

  • Grab robust USB Hub with switches next to ports, e.g. 4-port USB3 ones like that seem to be under $10 these days.

  • Get a couple of <$1 direct-current solid-state relays or mosfets, one per port.

    I use locally-made К293КП12АП ones, rated for toggling 0-60V 2A DC via 1.1-1.5V optocoupler input, just sandwitched together at the end - they don't heat up at all and easy to solder wires to.

  • Some $3-5 microcontroller with the usual USB-TTY, like any Arduino or RP2040 (e.g. Waveshare RP2040-Zero from aliexpress).

  • Couple copper wires pulled from an ethernet cable for power, and M/F jumper pin wires to easily plug into an MCU board headers.

  • An hour or few with a soldering iron, multimeter and a nice podcast.

Open up USB Hub - cheap one probably doesn't even have any screws - probe which contacts switches connect in there, solder short thick-ish copper ethernet wires from their legs to mosfets/relays, and jumper wires from input pins of the latter to plug into a tiny rp2040/arduino control board on the other end.

I like SSRs instead of mosfets here to not worry about controller and hub being plugged into same power supply that way, and they're cheap and foolproof - pretty much can't connect them disastorously wrong, as they've diodes on both circuits. Optocoupler LED in such relays needs one 360R resistor on shared GND of control pins to drop 5V -> 1.3V input voltage there.

This approach solves both issues above - components are easy to find, dirt-common and dirt-cheap, and are wired into default-OFF state, to only be toggled into ON via whatever code conditions you put into that controller.

Simplest way, with an RP2040 running the usual micropython firmware, would be to upload a file of literally this:

import sys, machine

pins = dict(
  (str(n), machine.Pin(n, machine.Pin.OUT, value=0))
  for n in range(4) )

while True:
  try: port, state = sys.stdin.readline().strip()
  except ValueError: continue # not a 2-character line
  if port_pin := pins.get(port):
    print(f'Setting port {port} state = {state}')
    if state == '0':
    elif state == '1': port_pin.on()
    else: print('ERROR: Port state value must be "0" or "1"')
  else: print(f'ERROR: Port {port} is out of range')

And now sending trivial "<port><0-or-1>" lines to /dev/ttyACM0 will toggle the corresponding pins 0-3 on the board to 0 (off) or 1 (on) state, along with USB hub ports connected to those, while otherwise leaving ports default-disabled.

From a linux machine, serial terminal is easy to talk to by running mpremote used with micropython fw (note - "mpremote run ..." won't connect stdin to tty), screen /dev/ttyACM0 or many other tools, incl. just "echo" from shell scripts:

stty -F /dev/ttyACM0 raw speed 115200 # only needed once for device
echo 01 >/dev/ttyACM0 # pin/port-0 enabled
echo 30 >/dev/ttyACM0 # pin/port-3 disabled
echo 21 >/dev/ttyACM0 # pin/port-2 enabled

I've started with finding a D-Link PPPS hub, quickly bumped into above limitations, and have been using this kind of solution instead for about a year now, migrating from old arduino uno to rp2040 mcu and hooking up a second 4-port hub recently, as this kind of control over USB peripherals from bash scripts that actually use those devices turns out to be very convenient.

So can highly recommend to not even bother with PPPS hubs from the start, and wire your own solution with whatever simple logic for controlling these ports that you need, instead of a silly braindead way in that USB PPPS works.

An example of a bit more complicated control firmware that I use, with watchdog timeout/pings logic on a controller (to keep device up only while script using it is alive) and some other tricks can be found in mk-fg/hwctl repository (github/codeberg or a local mirror).