Issues Connecting Teensy 4.0 to Newer Macs with USB-C


Well-known member
Hi, for some time now I've been using internal USB-micro-B to USB-C adapters with my units that I sell with Teensy 4.0s. That way, externally on the units, users have a nice USB-C connector. It's been working fine.

Lately, however, I've noticed that newer Macs don't work AT ALL when directly plugging these units in. If I plug a USB-C to USB-C cable into the units, directly into the Mac, NOTHING happens. No power, nada. The units are dead to the world.

The workaround is goofy.

I must either connect the USB-C cable to a powered hub that is then connected to the Mac via USB-C, or it also works by connecting the unit with a USB-C to USB-A adapter, then re-adapting back to USB-C with a dongle adapter into the computer.

This, obviously, is a bit crazy, and I'm getting customer complaints at having to use a dongle adapter or a powered hub. When previously I didn't have this issue.

Is this a known problem of some kind? Teensy's simply not powering up when plugged directly into a USB-C connector and then directly into the Mac (when properly adapted internally from USB Micro-B to USB-C)?

Any thoughts?


Hi blackketter, that is super helpful.

It sounds like an ongoing issue for these kinds of units.

Can this be solved with a special cable with the 5.1k resistors on CC lines on both sides? Or is it a lot more involved? I am currently using a USB A-C adapter and this works, but it's kludgy for the product to tell people they can't plug the units directly into their computers.

Thank you again,

Thank you Paul. I'm looking into it with the cable manufacturer. I had custom cables made; perhaps they didn't follow spec. I'll post back what I find out from them.

Thanks again!