saixx / mikoto

Bluetooth LE nRF52840 microcontroller in a pro-micro footprint.

みこと

nRF52840 microcontroller, in a pro-micro footprint, inspired by the nRFMicro and nice!nano.

what

1. VDDH power path (5V USB power, and/or 3.7V Li-ion battery)
2. software controllable battery charge current (off, ~100mA, ~250mA, ~350mA)
3. ESD protection on USB lines, and 500mA fuse protection for VBUS and EXT_5V
4. "Bidirectional" power over EXT_5V

The charger IC is "software controlled"; there are two pins (P0.26 and P1.15) that can be toggled (for a total of 4 combinations) to control the charge current. If they are both high-Z (floated), is disabled.

There are also two additional through-holes (the small ones below the USB connector), marked B and G (on the back side); these are B+ and B- respectively, in case you want to solder the batter leads directly to the board.

There is no official support from ZMK for this board. Note that the project files use kicad nightly (5.99).

ext_5v

The EXT_5V pin acts both as a power input and a power output; normally it is an input. When VBUS is high (ie. USB is connected), it will output VBUS (minus a schottky drop, ~0.3V).

EXT_5V is connected to the battery charger, so if you wire your TRRS (for split keyboards) to it (instead of a 3.3V VCC line), you can charge the battery on both halves by only plugging in one half to USB.

assembly

The PCB should be manufactured with an ENIG surface finish, but it can be done using HASL (in which case you should run a solder wick over all the pads first).

All components are 0402 (1005 metric) or larger, and can be placed by hand without magnification. A stencil is mandatory — get a 100µm (0.1mm) thick one.

There's no "component silkscreen" on the board except 4 lines to align the nRF chip, so using something like KiCad's Interactive BOM plugin is a good idea.

It's a good idea to get an electropolished stencil if you want to paste more than one board at a time, if not you will probably need to completely clean out the stencil before pasting another board.

I used Chipquik SMD291AX T4 paste for all the boards.

problems

There are no known problems from revision 5.17 onwards. 5.19 and 5.20 are just minor tweaks to the inner ground plane and the paste layer, so the functionality remains identical.

license

Licensed under the Apache 2.0 license, see LICENSE.

If you encounter any problems, feel free to open an issue on this repo. However, I am not obligated to provide any support, and you undertake any assembly/manufacturing/purchasing at your own risk.

on an AS IS BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE.

In no event and under no legal theory, ..., shall any Contributor be liable to You for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising as a result of this License or out of the use or inability to use the Work.