A0 Base Board

A0 Board

The A0 is the base board of the modular IoT gateway. It pushes all additional radio/cellular connectivity onto plug-in expansion modules.

Overview

A0 provides everything needed to power and run the gateway, while expansion slots (BUS1/BUS2) carry the connectivity:

ESP32-C6 module on top — main compute plus Wi-Fi 6, Bluetooth 5 LE, and IEEE 802.15.4 connectivity. See the A0 Gateway overview for full specifications.
Power:
- USB-C power input
- External power supply input (VIN terminal)
- Battery support via JST connector, with charging
Expansion: two module slots, BUS1 and BUS2, where any of the following can be plugged in:
- SIM7080G — NB-IoT/LTE-M cellular + GNSS
- WMBUS — Wireless M-Bus metering radio
- E07-400M10S — Sub-GHz radio (400 MHz, 10 dBm)
- Any MikroE Click™ board, since the expansion slots use the Click footprint

Power

The board can be powered from USB-C, the external VIN terminal, or a single-cell Li-ion battery on the JST connector.

Power chain (see a0/power.kicad_sch):

Battery path: MCP73831 single-cell Li-ion charge-management controller (U5) handles charging from the 5 V input to VBAT.
5 V rail: XL1509-5.0 buck converter (U6, 47 µH/2 A inductor L1) steps the input down to the board FB_5V / +5V rail.
3.3 V rail: a pair of AMS1117-3.3 LDOs (U7/U8) regulate down to the +3V3 rail that feeds the ESP32-C6 module and the expansion slots.

Expansion Slots

BUS1 is the pair of 8-pin headers J1 + J3. BUS2 is the pair J4 + J2. Each slot carries SPI, UART, I2C, five mikroBUS-compatible control lines, and both power rails.

BUS1 Pinout (J1 + J3)

Pin	Signal	ESP32-C6	Notes
J1 pin 1	MiBUS1_1	via U1 switch	mikroBUS AN
J1 pin 2	MiBUS1_2	GPIO1	mikroBUS RST — WMBUS SDN / SIM7080G PWRKEY
J1 pin 3	MiBUS1_3	GPIO2	mikroBUS CS — SPI chip-select
J1 pin 4	SCK	GPIO20	SPI clock (shared BUS1/BUS2)
J1 pin 5	MISO	GPIO19	SPI MISO (shared BUS1/BUS2)
J1 pin 6	MOSI	GPIO18	SPI MOSI (shared BUS1/BUS2)
J1 pin 7	+3V3	—	Module power — use this, not J3 pin 7
J1 pin 8	GND	—	Ground
J3 pin 1	MiBUS1_4	GPIO3	mikroBUS PWM — SIM7080G RING
J3 pin 2	MiBUS1_5	GPIO0	mikroBUS INT — SIM7080G CTS (via JP3, see module page)
J3 pin 3	MiBUS1_RX	GPIO16	UART RX — SIM7080G TX
J3 pin 4	MiBUS1_TX	GPIO17	UART TX — SIM7080G RX
J3 pin 5	SCL	—	I2C clock (shared BUS1/BUS2)
J3 pin 6	SDA	—	I2C data (shared BUS1/BUS2)
J3 pin 7	FB_5V	—	+5 V rail — never power 3.3 V modules from here
J3 pin 8	GND	—	Ground

BUS2 Pinout (J4 + J2)

BUS2 mirrors BUS1 with its own control and UART lines. SPI (SCK/MISO/MOSI) and I2C (SCL/SDA) are shared across both slots; only one module should drive them at a time.

Pin	Signal	Notes
J4 pin 1	MiBUS2_1	mikroBUS AN
J4 pin 2	MiBUS2_2	mikroBUS RST
J4 pin 3	MiBUS2_3	mikroBUS CS — via U1 switch (shared with DTR path)
J4 pin 4	SCK	SPI clock (shared)
J4 pin 5	MISO	SPI MISO (shared)
J4 pin 6	MOSI	SPI MOSI (shared)
J4 pin 7	+3V3	Module power
J4 pin 8	GND	Ground
J2 pin 1	MiBUS2_4	mikroBUS PWM
J2 pin 2	MiBUS2_5	mikroBUS INT
J2 pin 3	MiBUS2_RX	UART RX (via SW5 — mutually exclusive with JTAG)
J2 pin 4	MiBUS2_TX	UART TX (via SW5 — mutually exclusive with JTAG)
J2 pin 5	SCL	I2C clock (shared; see DCD note on the SIM7080G page)
J2 pin 6	SDA	I2C data (shared; via SW5)
J2 pin 7	+5V	+5 V rail — never power 3.3 V modules from here
J2 pin 8	GND	Ground

Use the 3V3 pin, not the 5V pin: on BUS1, J1 pin 7 supplies +3V3 while J3 pin 7 supplies +5V (FB_5V). The same applies on BUS2 (J4 pin 7 = +3V3, J2 pin 7 = +5V). All in-house modules are 1.8–3.6 V parts — power them from the +3V3 pin only.

mikroBUS Mapping

The five MiBUSx_y control lines follow the mikroBUS convention:

Base-board net	mikroBUS name	Function
MiBUS1_1 / MiBUS2_1	AN	Analog
MiBUS1_2 / MiBUS2_2	RST	Reset
MiBUS1_3 / MiBUS2_3	CS	SPI chip-select
MiBUS1_4 / MiBUS2_4	PWM	PWM output
MiBUS1_5 / MiBUS2_5	INT	Interrupt input

UART (RX/TX) and I2C (SCL/SDA) plus SPI (SCK/MISO/MOSI) complete the Click-compatible footprint, so off-the-shelf Click boards plug directly into either slot.

SW5 — JTAG / BUS2 Selector

SW5 (8-position DIP, SMXS-08K-TP) multiplexes GPIO4 through GPIO7 between the JTAG debug header (J10) and the BUS2 UART and I2C lines (see a0/esp32-c6.kicad_sch):

One side of the switch carries GPIO4/GPIO5/GPIO6/GPIO7 from the ESP32-C6 module.
The other side carries the JTAG signals (MTMS, MTDI, MTCK, MTD0) and the BUS2 signals (MiBUS2_TX, MiBUS2_RX, SDA, SCL).

Set SW5 toward the JTAG side to debug via J10, or toward the BUS2 side to use UART/I2C on a BUS2 module. The two uses are mutually exclusive — BUS2 UART or I2C and JTAG cannot be used simultaneously (see Getting Started).