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I'd love to see your design and improvements whenever you can share them!

You can see the design here:

https://github.com/blackketter/teensy4_esp32_breakout

It uses teensy digital pins 24, 25, 26, & 28 for a serial connection and reset for serial programming.

If you haven't seen it yet, check out the Teensy 4.0 breakout board and USB host board I made in this thread:

https://forum.pjrc.com/threads/57672-Another-Teensy-4-0-Breakout-Board

It copies...

What are looking to do with the combination?

I've built an ESP32 add-on to my Teensy 4 breakout board and have successfully gotten firmware updates to the ESP32 to work passing through the...

I totally understand, those pins are tricky to access! And your board looks excellent.

Sorry, but the USB pads on the bottom of Teensy 4.0 are for an additional USB host port, they aren't connected to the existing USB device port on the board used for connecting to the host computer. ...

Any reason to not use the SD pins available on the bottom of the T4? There are a few breakout board designs that ease connection to those pads:
...

They do appear to be identical, same datasheet, etc. I can't think of a reason why the new part number wouldn't work exactly the same.

Yes, by cycling the power or having the teensy reboot itself.

Another data point: I use platformio on macOS with T4 and have never had a system crash.

Also, can you reproduce the problem _without_ reprogramming the teensy?

For the software, check out KiCad (https://kicad-pcb.org). It's free and open source and can handle some very complex designs. While it's not trivial to get started, it's gotten a lot easier to use...

I've put together a design for a combination USB Host and SD breakout board. The idea is that it straddles atop the teensy and connects via a tall-ish header to the original Teensy 4 breakout board...

The extra circuitry in the Teensy 3.6 (copied into the Teensy 4 USB Host Breakout) provides power overload protection, power control, and USB ESD protection.

If you are willing to bet that you...

I'm not on that specific hardware, but I've been on a MacBook Pro on that version of the OS and have not seen any system crashes.

I'll throw out a few things to try toward narrowing it down:

1....

The ESP32-S2 looks really interesting and could be used as you described.

Indeed, if the ESP32-S2 has a robust USB host implementation, it could give the Teensy 3.6 and 4.0 a good bit of...

Thanks, Paul, I realize that this is a long-term project and am just glad that you are thinking about it for the future. And totally agree about making sure it's rock solid to avoid dead Teensies.
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I’d care about macOS and Linux.

I do remember seeing this before, it's excellent work. Well done!

I was throwing this out there to start a discussion about a way for Paul to be able to keep his business model of bootloader...

One of the frequent requests on this forum is for a way to update teensy firmware from a source other than USB.

I wonder if it would be possible in a future version to the bootloader chip to...

I'd love to have that capability too.

That's a really clear bit of documentation. Well done. Can I ask how you made it?

Do you have any third-party virus, networking or firewall software installed on your computer?

I meant to suggest this earlier: pads on the bottom to provide access to the USB connector data pins. This allows for a design that has its own USB port for power/programming (USB-C anyone?)

Can't reproduce here with Teensy loader 1.47 on High Sierra 10.13.6 with firewall on or off.

Paul: is there any way to get the error result that prints that message?

I can try to reproduce this here, but first I assume you checked to see that this isn't an issue with the firewall configuration?

Excellent, @Simmic, thanks! I've tried importing this model into the KiCad footprints for Teensy 4. Here's what a 3d render looks like for the breakout board I've been working on:

18097

I...

Sounds great, @Dog-One! Can you give a little background on what changes you had to make to get it to build? I'd like to get it working under Arduino and PlatformIO.

Still an issue. Filed bug here https://github.com/PaulStoffregen/cores/issues/401 so this doesn't get lost.

Just saw this post about the next version of PlatformIO which has some VERY slick tools for investigating firmware symbols, memory allocations, static code analysis, etc.
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Thanks, Paul, that's super useful and clear.

A couple of thoughts:

Would it be possible to add the default values for the various fixed addresses?

One note on the graphic: I puzzled over...

Another good suggestion. I'll add that to my next version!

Good point, I've added it to the OSHPark project page for the latest design.

No good reason, they were like that in the original default footprint for the header that I used. Rounded them out:

17971

Good suggestion. I'll see what I can fit in there! How's this?

17970

And some more updates this morning to add additional ground connections and a ground plane, as well as some more silkscreen improvements:

https://github.com/blackketter/teensy4_header_breakout
...

I've updated the design a bit to improve the silkscreen and tweaked the edge cuts to not cut through two pins that didn't need cutting (teensy pins 12 & 13). Updated here:
...

+1 for this! I really like these hybrid castellated / through-hole pads on the edge. I do understand that there will be parts on the bottoms of teensy 4.x board requiring a cutout in the base board.

They were pretty clear with me that the castellated designs were not guaranteed. Their tips are here: https://docs.oshpark.com/tips+tricks/castellation/

My guess is that the thinness of the board...

Another datapoint: I got my 0.8mm boards back today, finally, and only one of the three of them is usable. The cuts were all made on all the boards but two of the three boards each had the plating...

@Paul: Any clue where I could dig to see if I could find where the lockup is happening? Happy to dig in here a bit...

Ah, I see from the schematic that the POR_B pin is used by the boot loader chip. Would it be feasible to add a pin that forced a reset by disabling the 3.3v regulator?

I'm only really missing one specific pin from the Teensy 4.0: RESET

Apart from that, I'd really love to be able to attach inexpensive LCD displays and take advantage of the 2D graphics...

Also, I should have noted earlier that after the USB serial lockup my larger application continued to work.

In fact, USB serial writes from Teensy to host continue to work and loop() continues to...

Alright! Thanks! With that test the ISR blinking continues to work even after the USB serial is locked up. And no HardFault blinking.

I'm not following. Something to test?

Thanks defroster, but alas, no. A single blink when it start, then it locks up as above without any blinks.

Once it locks up I can't unlock it with further transfers. With your example code, shorter sends to the Teensy work fine (led goes on, then goes off a couple of seconds later), but once a large...

macOS Mojave 10.14.6

Thanks, defragster. I did try your changes to no avail.

I suspect that the issue outlined at https://forum.pjrc.com/threads/57325-Teensy-4-0-Serial-available()-stops-incrementing is the same...

I think I'm seeing a bug in usb_serial on Teensy 4.0. When I try sending a block of text larger than 2413 bytes via the serial port, it locks up. Here's the sketch:

#include <Arduino.h>

void...