Teensy4.1: How to use as sub-board and access USB device

A

avsteele

Member
I am planning to integrate the Teensy4.1 onto a larger PCB. (a Eurocard-like 'motherboard') I would like to be able to communicate and program it via the virtual com port as usual but do not want to use the built-in micro-USB. What are my options?

Thanks for any help/thoughts.

Thoughts:
In other similar projects I mount a USB type B port to the motherboard[1] then run the USB lines to the micro-controller board device, but it isn't clear how I can get signals to the D+,D- Device pins.

One option would be to simply mount it near the front of the motherboard and plug in directly. This is less desirable because I don't want to have a USB micro connection (flimsy, and no other boards in my system use this). I also prefer to use my own ESD and overvoltage protection. Finally, in the future this and other boards will share a USB hub on a backplane, I don't want to run usb cables to individual boards.

I don't see any USB-UART bridge either. I guess the main chip has this built in?

Other Details:
The board will be externally powered at the Vin by 5V by the board, it won't be relying on the USB 5v for power at any time. (I will cut the bridge from VIN/VUSB)


[1] https://www.sameskydevices.com/product/resource/uj2-bh-th.pdf
 
USB functionality is built into the chip NXP processor chip.

If you want to keep the USB conversion on the baseboard, you can pick up the D-/D+ USB lines on pads under the bottom of the Teensy as shown here.

USB Pads.jpg


Pogo pins can be used on the baseboard to make contact with those pads.

Pogo Pins.jpg


From there you can run the D-/D+ lines to whatever type of USB connector you want. In this particular case, it happens to be going to a USB C connector.

The traces can be fairly long, but you want to make sure you length match the traces and keep them close together as much as possible and away from other signals since the signals are differential.

If you are routing on the PCB this way, don't connect power on your USB connector and then there is no need to cut the VIN/VUSB trace.

Another option is to use a panel mount cable. One end plugs into the Teensy USB Micro-B connector and the other end mounts on a panel somewhere. This is a micro-B to micro-B cable, but there are cables that also adapt to different USB connector types. In this case you would want to cut the VIN/VUSB trace.

I'm not a big fan of this approach unless you can strain-relief the cable connecting to the Teensy so it's not flopping around.

USB 2.0 Micro-B Panel Mount Extention Cable on Teensy 4.1.JPG
 
Great thanks for the suggestions Ken. I hadn't thought of the pogo pins.

One question about the Vbus connection. Is it truly not needed to route these to the Teensy board at all? I don't know much about the specifics, but I know with the USB-UART bridges I'd used in the past (e.g. CP2012) this was used to detect a connection.

There is a 'front panel' which sits in front of the PCB, so the panel mount cable is also a reasonable option.

I've some experience with routing the D+- differential pair in the past so i think that shouldn't be a problem.
 
Since you will be powering the Teensy off VIN, there is no need to route the USB power to it since you aren't dealing with an intermediate chip like a CP2102. In my setup, I just route the D-/D+ and it works fine.
 
I'm thinking about using pogo pins for the connection. Do you have any recommendations for pogo pins that work well with low‑profile pin headers?
 
The POGO pins shown above in the picture with the low profile headers are 7.5mm tall. They are individual small pins and a little tricky to install for that reason. I use a fixture, but you can also put the POGO pins in the board after the sockets are installed and use the Teensy to hold them in place during soldering since they are spring loaded.

They can be ordered from RTLECS on Aliexpress. RTLECS have a range of sizes on that page. Use 7.5mm for use with low profile headers or use 12.5mm for standard height headers.

Another option that is a little easier to install because they have the pins mounted together into a housing are these 7.67mm Mill-Max parts PN:825-22-002-10-001101. They are available from places like Digikey.
 
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