Playing with Teensy LC - making it more usable

Projects / Project Swiftlet 

The Teensy LC has arrived. Canada Post + custom (out for processing part) only took 2 days from arriving in Canada to my mailbox. Hasn't see it this fast for ages. It was sent USPS First Class and delivered as Canada Post Xpresspost (TM).

Let's say I am less the thrill to see this implemented here because of their targeted market.  This has no place for serious developers.

Shame that first thing I have to do is to disable the smaller MKL02 chip that is providing the bootloader function. It is an ARM with 32kB FLASH housing Paul's bootloader code that is copied to the MKL26. It is in the way of using SWD debugging. I would rather not destroy the board as I have something else in mind once I get my proto board up and running.

Pin 15 is the reset pin. I can probably put the chip in reset by pulling it low and its I/O pins in tristate. I can then access the SWD on the MKL26. The left side of that cap between the micro USB and the MKL02 is a ground. I'll also need to replace the 16MHz crystal with a 32.768kHz for my design. That would leave 2 pads that could be used for the breakout of the SWD connection.

Update: Asserting the reset won't work if the firmware disable the reset on its I/O. According to my multimeter measurements, it seems to be internal pull up/pull downs internal resistance of around 40k ohms on the SWD lines. So may be those lines can be connected to a debugger anyway.

That has to be put on hold a bit until I get my hot air tool situation sorted out.

Desoldered the Crystal off the Teensy LC with my new hot air toy. I taped up the small 0402 and other small parts around it with Kapton tape. It takes 45 seconds at 275C - not even challenging.

I was hoping that the other two corners were "no connects" so I could use them as anchors for the SWD debugger, but Paul has grounded them. Could have used test pads on the SWD pins too. щ(ºДºщ)

Really don't want to pull the small QFN just to bridge its SWD to the MKL26 SWD. (I can access it via test pads near label 1 & 2 on the back side.) I have done half a dozen of QFN transplants already with my old butane hot air tool, so that's a no brainer with my new toy.

I don't think my grounding the Reset pin the bootstrap ARM chip would do anything, but too lazy to remove it.

Notice how the spacing of the diagonal pins sort of lines up with the perfboard. The perfboard has plated through holes and you can make castellated pads by cutting it along the holes.

perf board reflowed onto the ground pads on the crystal footprint. Solder magnetic wires to the SWD pins (7, 8 on the MKL02)

I insulated the 32kHz crystal with kapton tape, soldered it to the pads, solder in the Vcc to the small decoupling cap.

I use first generation old dollar store headphone cables (when they still use litz wires) to connect the SWD to a 0.1" connector. The litz wires as well as the twisted cable are paired like this: SWD_CLK/GND, SWD_IO/VCC, so the signals have a power rail as return path.