Non-solder bootloader

This doohickey can be used to flash a bootloader or a binary onto a SAMD21E without soldering it. This is useful when the final board for the SAMD21E is too small to afford having a swd connector.

This is still experimental and not 100% reliable, but the board simply consists of copper traces onto which the QFP32 chip is pressed with a 3D printed jig. The 3D printed part is compliant and presses on each leg adequately.

This board features a SAMD11 as a permanent USB-to-swd programmer so that flashing is as simple as plugging the board in and launching edbg or the Arduino core.

Design

Schematic

The schematic is the bare minimum, with a SAMD11 as a USB-to-swd programmer connected to the target SAMD21. The fact that the SAMD21 will not be soldered is not visible in the schematic, or the board design for that matter.

An LED is connected to pin PA00 of the target SAMD21, in case you want to flash an LED blinking code onto it to make sure it's functional.

Board layout

The board design is split in two: the USB and D11 programmer on top, and the non-solder area on the bottom. Note that only a few pins are connected on the target, the bare minimum plus an LED for testing.

M3 holes allow for future improvement on the pressing method, the spacing is:

• horizontal: 1.2in
• vertical: 0.8in

The png images for fabrication can be found here and the kicad files here.

CAD

The press has a lip on its bottom side to apply load directly on the legs of the QFP32 target. There is still some improvement needed there but it works for now.

The 3D files can be found here.

Making

The board can be milled with a 1/64" flat end mill:

After removing all of the extra copper with a 1/32" end mill (for speed), the board looks very simple:

After flashing the free-dap on the SAMD11, you can attach a SAMD21 target with M3 screws and launch a flashing like you normally would when using a programmer.