We’ve been recently looking into USB 2.0 – the ubiquitous point-to-point communications standard. USB 2 is completely different from USB 3, the blue-connector next-generation USB standard. For instance, USB 2 is a full-duplex pseudo-differential bus, and it’s not AC-coupled. This makes USB2 notoriously difficult to galvanically isolate, as opposed to USB 3.  On the other hand, USB 2 is a lot easier to incorporate into your projects. And perhaps the best way to do so is to implement a USB hub.

USB 2 hubs are, by now, omnipresent. it doesn’t cost much to add to your board, and you truly have tons of options. The standard option is 4-port hubs – one uplink port to your host, four downlink ports to your devices. If you only have two or three devices, you might be tempted to look for a hub IC with a lower amount of ports, but it’s not worth bothering – just use a 4-port chip, and stock up on them.

What about 7-port chips? You will see those every now and then – but take a close look at the datasheet. Some of them will be two 4-port chips inside a single package, with four of the ports bottlenecked compared to the three other ports – watch out! Desktop 7-port hubs are basically guaranteed to use two 4-port ICs, too, so, again, watch out for bottlenecks. lsusb -t will help you determine the hub’s structure in case you don’t want to crack its case open, thankfully.

Recommendations? I use SL2.1 chips – they’re available in an SO16 package, very unproblematic, to-the-point pinout and easily hand-solderable. CH334 is a close contender, but watch out because there are different variants of this chip that differ by both package and pinout, so if you’re buying a chip with a certain letter, you will want to stick to it. Not just that, be careful – different variants run out at different rates, so if you lock yourself into a CH334 variant, consider stocking up on it.

There’s no shortage of Western-origin chips, either – Texas Instruments is a leader here no doubt. If you ever fear running out of hub ICs in your stock while assembling something, you can prepare for this in advance by leaving zero-ohm footprints under the hub’s package. USB 2 doesn’t care for stubs much, and such a hack is very easy to do with SL2.1 in particular. Got two extra ports left over? Put them on a PC-case style dual USB2 9-pin header – there’s never a shortage of fun accessories compatible with it!

Powering USB2 hub ICs is easy – they tend to include a 5 V to 3.3 V linear regulator inside, so you can power them from a 5 V source directly. On the other hand, if you don’t have any 5 V to spare, the overwhelming majority of hub ICs can be powered from 3.3 V directly – usually, that requires shorting the hub’s 5 V input to 3.3 V, but not necessarily. If the datasheet is unclear on 3.3 V-only operation, leave in some 0R jumpers. And, of course, make sure to add 100 nF or similar capacitors – one per hub IC’s power pin. Remember the disclaimer about built-in RC oscillators in MCUs being imprecise? Same goes for hubs – if your hub boasts an internal RC oscillator, don’t trust it, make sure you have a crystal footprint you can populate if you get stability issues.

Putting some USB port pins to the outside world? You will want to protect them from harm – or, rather, you will want to protect your expensive CPU from harm.

Please, Consider ESD Diodes

Bringing USB somewhere far, or even just using it as your link to the external world? You should really use ESD diodes – or at least plan them in and give yourself the option to populate them later. There’s no shortage of USB2-capable ESD diodes, after all, and ESD problems are closer than you might expect.

For instance, I’ve recently built a pocket device consisting of a battery-powered Pi Zero and a USB soundcard connected to wired headphones, with a pretty standard kind of long cable. I wear a lot of synthetic clothes, in particular, hoodies and jackets, and I kept having the Pi reboot every time I took my jacket off or put it on, through static electricity induced into the headphone wires through the cable insulation, going into the USB port on the Pi Zero.

So, I went and put ESD diodes on the USB 2 pins, using the footprint I previously added to my board “just in case” but didn’t populate, and this failure mode has instantly disappeared for good. Remember, footprints are free, and bodges cost time. Want a recommendation? The four-channel diodes are pretty good for USB 2; look for the SRV-05 footprint in KiCad, in the SOT-23-6 package. It’s a generic enough footprint that there’s no shortage of ESD diode packs in the same footprint, they’re low-capacity enough that you can even use it for purposes like captouch pad protection, and they will also work for applications like Ethernet or externally available GPIOs.

Do you need ESD diodes? Yes, just add the footprint. Same goes for over-current control switches, by the way – I’ve already talked about the SY6820, but it bears repeating. Your entire system doesn’t have to reboot when you short-circuit a USB port on the board, and a cheap current-limited switch IC will let you ensure that’s the case, while also letting you switch the port power on and off, as a nice bonus.

This was just a few tips on and around USB 2 hubs and connectors, but I hope it helps you out with your projects.