Relay24 ESP32

XIAO ESP32C3 adapter for the AiO v4.5 Micro (GPS2 socket → Serial2)

A small board (27.3 × 24.4 mm, 2 layers) that plugs into the second GPS socket ("Dual-Left", U11) of the AiO v4.5 Micro, in place of a simpleRTK2B Micro, and connects a Seeed XIAO ESP32C3 (or ESP32S3) to the Teensy's Serial2 (pins 7 and 8). The XIAO plugs into two female headers on the adapter.

Connections

The plug is two 1x10 male headers: J1 = socket pins 1–10, J2 = socket pins 11–20 (J2 pin 1 = socket pin 11). The XIAO plugs into two 1x7 female headers: J3 = XIAO pins 1–7 (D0–D6), J4 = XIAO pins 8–14 (D7–5V).

The positions of the 20 pins were copied from the official AiO v4.5 Micro PCB file (EasyEDA, 2024-02-05) and checked against the schematic: 0.00 mm difference. The outline stays inside the drawing of the original module on the Micro's silkscreen, with the same cut corner (on the side of LED1 and the RJ45 jack).

Why the ideal diode (U2)

• On the Micro the GPS2 socket only has 3.3 V, so the XIAO is fed through its 3V3 pin. An ordinary diode would drop it to about 3.0 V, too low for the ESP32 WiFi.
• U2 (TI LM66100) is an "ideal diode": current flows only from the AiO to the XIAO, with a drop of a few tens of mV. Its CE pin is tied to its output, which turns it off as soon as the XIAO side is higher than the AiO side.
• So the XIAO's USB-C can now be plugged in with the adapter on the board: the XIAO powers itself from USB and U2 stops that power from going back into the AiO's 3.3 V (GPS1 F9P, LEDs, radio, K7803M).
• R1 and R2 are 1 kΩ (100 Ω in v1.1). If the XIAO is on USB with the AiO switched off, its TX pin feeds the unpowered Teensy through R1; 1 kΩ keeps that current to about 3 mA. At 460800 baud, 1 kΩ makes no difference.

Files

• AiO_Micro45_XIAO_C3.kicad_pro / .kicad_sch / .kicad_pcb: KiCad 7 project (schematic + PCB), with its own libraries (AiO_XIAO_Adapter.pretty, AiO_XIAO_Adapter.kicad_sym).
• fabrication_JLCPCB/Gerber_AiO_Micro45_XIAO_C3.zip: Gerbers + drill file, ready for JLCPCB.
• fabrication_JLCPCB/BOM_…csv and CPL_…csv: for JLCPCB's assembly (R1, R2, C1, C2, U2 and the XIAO sockets J3, J4 on the top side, the two pin headers J1 and J2 on the bottom side).
• fabrication_JLCPCB/BOM_…_top_only.csv and CPL_…_top_only.csv: the same without J1 and J2, for an assembly on the top side only.
• In this folder: the schematic as a PDF (adapter_schematic.pdf), views of the board (adapter_PCB.png) and its position on the Micro (GPS2_socket_view.png).

Opening it in EasyEDA

EasyEDA Pro: File → Import → KiCad, then choose the project folder (or the .zip). It imports the schematic and the PCB. EasyEDA is not needed to order the board: the Gerber zip goes straight to JLCPCB.

Ordering (JLCPCB)

• Gerber: Gerber_AiO_Micro45_XIAO_C3.zip. 2 layers, 1.6 mm, any colour. The default settings are fine (minimum track 0.25 mm, minimum hole 0.3 mm).
• Assembly: BOM + CPL. JLCPCB solders everything except the XIAO:
  • R1, R2, C1, C2 and U2: SMD parts on the top side;
  • the pin headers J1 and J2: through-hole parts on the bottom side, with the long pins pointing down (away from the board). The CPL already gives them on the bottom layer, at the centre of each header;
  • the two female headers J3 and J4 for the XIAO: through-hole parts on the top side.
• Through-hole assembly costs a little more than SMD only.
• There are parts on both sides, so choose Standard PCBA with assembly on both sides. With Economic PCBA or top side only, JLCPCB leaves out J1 and J2 (they are on the bottom) and stops with "Coordinate file missing data for: J1, J2".
• Cheaper alternative: Economic PCBA, top side only, with BOM_…_top_only.csv and CPL_…_top_only.csv (the same files without J1 and J2), and solder J1 and J2 by hand.
• In JLCPCB's placement preview, check that J1 and J2 are on the bottom side, over the two rows of 10 holes, and J3 and J4 on the top side, over the two rows of 7 holes.
• Also check U2's pin 1 in the preview: pin 1 (VIN, marked with a small triangle on the silkscreen) must be at the corner of U2 nearest to the J4 row and the USB-C side. Chip rotations in the CPL are sometimes off by 90° or 180° at JLCPCB; if the dot of the chip is elsewhere, rotate it in the preview.

The 2.00 mm headers have 4 mm mating pins and a 2 mm insulator.

Assembly

1. Only if the board did not come assembled: U2, R1, R2, C1 and C2, then the headers. U2 is very small (SC-70-6): pin 1 goes at the triangle mark.
2. The 2.00 mm male headers go underneath, with the pins pointing down. When soldering them by hand, to get them well aligned: push them into the Micro's GPS2 socket, lay the adapter on top and solder them from above.
3. XIAO: solder the two 1x7 male headers that come with it underneath the XIAO, long pins pointing down. To keep them straight, push them into J3 and J4 first, lay the XIAO on top and solder from above.
4. Plug the XIAO into J3 and J4 with its USB-C at the "USB-C" mark. The two rows have 7 pins each, so the XIAO also goes in turned 180°: that way the 3.3 V goes into D4 instead of 3V3 and the XIAO can be damaged. Check the USB-C mark before powering the AiO.
5. Under J3 (the D0–D6 row), on the Micro, there is resistor R31: check that the solder joints of J3 underneath the adapter do not touch it.
6. Orientation on the Micro: the cut corner of the adapter matches the cut corner drawn on the silkscreen of the GPS2 socket, and pin 1 (square pad, dot on the silkscreen) sits next to pin 1 of the socket.

Cautions

• The XIAO is powered through its 3V3 pin, from the board's 3.3 V through U2. From v1.2 the XIAO's USB-C can be plugged in with the adapter on the Micro (on v1.1 boards it must not be: unplug the XIAO to program it by USB, or use WiFi OTA).
• The USB-C plug goes in from the right edge of the adapter (towards LED3 and the mini-USB connector of the Micro): check that the cable fits in your box.
• On the sockets the XIAO sits about 11 mm higher than soldered flat (8.5 mm socket + 2.5 mm of the male header). Check that the box lid closes.
• Tractor vibration: hold the XIAO in its sockets, for example with a cable tie around the XIAO and the adapter, or a dot of neutral-cure silicone at the corners (not on the U.FL connector or the buttons).
• Hammond 1455 boxes are aluminium: use an external WiFi antenna on the XIAO's U.FL connector, taken out through the lid.
• With the adapter in place the GPS2 socket cannot hold a second F9P (dual F9P).

Firmware

• Teensy: firmware v4.5_AiO_WEB, Serial2 (pins 7 RX2 and 8 TX2) at 460800. On its web page, Settings → "GPS2 port" must be on automatic (the default): the Teensy finds the XIAO by its hello.
• XIAO: the program v4.5_XIAO_ESP32 that comes in the firmware zip (already compiled in v4.5_XIAO_ESP32_bin). It uses UART1 on D7 (RX, GPIO20) and D6 (TX, GPIO21) at 460800 and empties the receive FIFO at 64 bytes (setRxFIFOFull(64)).
• The XIAO ESP32S3 fits too: same size and pins (D6 = GPIO43 TX, D7 = GPIO44 RX).
• When it starts, the ESP32-C3 writes a few boot lines at 115200 on GPIO21: the Teensy receives garbage for half a second, and the firmware ignores it.
• The official firmware v26 (Firmware_Teensy_AiO-New-Dawn) uses this same link (Serial2) for its ESP32, also at 460800. See docs/ESP32_BRIDGE_INTERFACE.md in that repository.

Checks done

• The schematic and the PCB have the same connections (netlist exported from the schematic = PCB connections).
• KiCad 7 DRC: 0 errors, 0 unconnected items (v1.2).
• Pins and connections checked against the official AiO v4.5 Micro schematic and PCB (AgOpenGPS-Official/Boards).
• The XIAO sockets J3 and J4 are on the XIAO pin positions of Seeed's official KiCad library (XIAO-ESP32-C3-DIP-SMD): 2 × 7 pins, 2.54 mm, rows 15.24 mm apart.
• J1 and J2 are on the bottom side and J3, J4 on the top side, in the PCB and in the CPL; the drill file has the same 44 holes as version 1.0, plus 2 vias for U2 in v1.2.
• U2 pins used: 1 VIN, 2 GND, 3 CE, 6 VOUT (pins 4 and 5 are left unconnected). Check against the TI datasheet before the first order.
• Not yet made or tested on a real board.

Versions

• 1.2 (20 Oct 2026): U2 (LM66100 ideal diode) between the AiO 3.3 V and the XIAO 3V3 pin, with C2 22 µF on the XIAO side, so the XIAO's USB-C can be used with the adapter on the board. R1 and R2 changed from 100 Ω to 1 kΩ. Same outline, same header holes, same XIAO position.
• 1.1 (19 Oct 2026): the plug is now two 1x10 2.00 mm headers (J1, J2) with LCSC codes in the BOM and the CPL (bottom side), so JLCPCB solders them too. The XIAO plugs into two 1x7 female headers (J3, J4, top side, also soldered by JLCPCB) instead of being soldered to the board. Same holes and tracks as 1.0.
• 1.0 (14 Oct 2026): first version.

Credits

• Design: renatorgr, with Claude (Anthropic), 2026.
• Pin positions: official AiO v4.5 Micro board files (EasyEDA PCB 2024-02-05 and schematic), AgOpenGPS-Official/Boards.
• XIAO pin positions: Seeed Studio OPL KiCad library (XIAO-ESP32-C3-DIP-SMD). 0805 and SOT-363 (SC-70-6) footprints: KiCad standard library.