xiao-ble-pcb
A simple example Eagle project for the Xiao BLE + E22-900M30S Meshtastic variant.
UPDATE:
The first round of boards was delivered yesterday (7/25/23) and work well, with two small caveats:
-
I'm an idiot and reversed MISO and MOSI, which fortunately can be fixed easily by swapping the pin mappings for those two in the firmware (
variant.h:98-99). -
The landing pads for the MiniBoost were just a little too close to the XIAO for it to fit. I worked around this by scooting the XIAO up a tiny bit on its pads, which created enough overlap between the MiniBoost's landing pads and its through holes for it to be soldered on.
I revised the board to fix these issues and also added another four-pin breakout to expose the XIAO's underside RST, GND, SWDIO, and SWCLK pins. The RST and GND pins can be used to add a secondary external reset switch, which is handy since the built-in reset button on the XIAO is not particularly easy to access once it's in an enclosure.
This new version of the board is currently untested, so it's still "build at your own risk" for the time being. I'll post another update once I receive them, probably mid-August 2023.
This PCB is set up to use the E22 in 'automatic Tx/Rx switching' mode, with TXEN and DIO2 connected to each other rather than to the Xiao. I created it with two additional breakout boards in mind – the Adafruit Switched JST Breakout and the Adafruit MiniBoost. The switched JST goes in the GND, SW, +, and GND pins, and the MiniBoost goes in the EN, 5V, GND, and VIN pins. You can of course use any other 5V power source, and/or forego the JST board and solder the battery directly to the board, but the breakouts are inexpensive ($6.50 USD before tax and shipping) and ideally suited to the task.
To get the full 30 dBm transmit power out of the E22, a buffering capacitor (ideally at least 100 µF) should be placed between the 5V rail and GND. If using the MiniBoost, you can place a through-hole capacitor across its 5V and GND pins and then use the cap's leads in place of header pins for those two pads. Alternately, you could solder a capacitor across pins 10 (VCC) and 11 (GND) on the E22.
The board breaks out the Xiao's unused GPIO pins (D4, D5, D6, NFC1, NFC2) and also preserves access to its SWCLK, SWDIO, RST, and GND pins on the bottom.
Bill of materials
Note: You can use a Xiao BLE Sense instead if you want. I went with the non-sense version since it's $5 cheaper and I didn't need the extra bells and whistles.
| Part | Source | Cost (USD) | Note |
|---|---|---|---|
| Xiao BLE | DigiKey | $10.38 | |
| Ebyte E22-900M30S | AliExpress | $9.42 | Also available on Amazon for a couple bucks more |
| Adafruit Switched JST Breakout | DigiKey | $2.50 | |
| Adafruit MiniBoost (5V @ 1A) | DigiKey | $3.95 | |
| 100 uF capacitor | DigiKey | $0.13 | Just one example with decent specs, lots of other (and smaller) options available |
| Total | $26.38 | Plus the cost of the PCB – roughly $25 (HASL w/ lead surface finish) or $55 (ENIG surface finish) for 10 boards at pcbway.com; can probably be had for less elsewhere |
TODO
- Route the board manually
- Create a "version 2" for assembly with the boost converter, JST + switch, and maybe also a solar charge controller integrated into the design