SMA-Q2_replacement_board_v2

nRF5840-based replacement board for the SMA-Q2 smartwatch. Uses the MDBT50Q-1MV2 (and similar) module. Based on the Adafruit Feather nRF52840. Has to be fabricated on 0,8mm FR4 material. Main changes are: no external RTC, all mosfets in SOT-323 package (at least in v2.2), optional barometer and power latching circuit, everything controlled by the nRF52840.

2 versions have been created: v2.2 without MS5611 barometer, but with a simple power latching circuit. v2.1.1 does not have a power latching circuit, but has a footprint for a MS5611 barometer. Both versions use both interrupts of the accelerometer.
During testing and everyday use RF performance turned out to be poor. The reason is the ceramic antenna which is shielded by the close proximity of the lipo battery. This problem was quite easy to solve in retrospective. I just desoldered the chip antenna with 2 soldering irons and connected a simple 30AWG wire which I shortened until reception started to get worse. Ended up with about 20mm of wire bent around the top left corner of the device where least interfering metal is located. Quite surprising that the antenna is quite a bit shorter than one would expect for 2.4GHz (lambda/4 = ~31mm). Let me tell you, at 5dbm TX power the wireless range and link reliability is now really good! FCC compliance is now goneski of course.
This got me motivated to create a 3rd version of the board using the bare chip instead of an integrated module. It makes use of the inbuilt antenna which is also surprisingly short (about 20mm). The feedline was designed to be about 50 ohm (although it probably doesn't have to be, in the original version it's about 65 ohm at 0,7mm trace width and 0.8mm FR4 substrate). This version 2.3 has the barometer, which has proven to be very accurate and useful, the power latching circuit and even a fuel gauge which can measure the supply current (albeit in 1mA steps).
Another feature that proved to be very useful if you like to wear a dev platform on your wrist is a normally closed reed switch in series with the battery so you can power cycle the device if bricked with a magnet (the charging cable f.e.) or just keep the battey disconnected by attaching a magnet to it if the device is not in use and has to be stored for longer periods of time. The pads on the bottom of the board are there to keep the reed switch in place. The battery therefore connects to the leftmost contact (one arm of the reed switch) and GND. I used the smallest SPDT (3 pins) reed switch I could find. Its glass body is 15mm long. Just trim the unused leg and bend the other two in shape carefully making sure the delicate glass body stays intact.

Note: 2.3 has not been tested yet. The Feather bootloader should still work, the pinout for the button, I2C, SPI, QSPI and UART is still the same.