UEXT

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UEXT is a universal extension connector created by Olimex. It provides power and three serial buses (UART, I2C, and SPI) over a 2×5, 10‑pin header. All signals use 3.3‑volt logic.

Physical and cables
- Pin layout: 2×5 male header (BH10S) with 0.1 inch (2.54 mm) spacing.
- Mates with a 2×5 IDC female connector and a ribbon cable.
- Cables are cheap and widely available.

Electrical and signals
- Three buses: UART (asynchronous), I2C, and SPI.
- Pins are 3.3V by default. If you need RS‑232, RS‑422, RS‑485, DMX512, or MIDI, you’ll need extra level‑shifting hardware.
- Many MCU pins are multi‑purpose. If you repurpose UEXT pins, plan carefully to avoid conflicts. For safety, avoid using pins 4 or 7 as outputs and use pin 6 cautiously as an output.
- Consider protection: add over‑voltage protection diodes, separate drivers, or a resettable fuse between power and pin 1 to guard against faults.

Power and voltage
- UEXT normally uses 3.3V logic. Some boards can supply 5V on UEXT if configured, but many UEXT devices aren’t 5V tolerant.
- Some boards (e.g., certain Olimexino boards) have a 3.3V/5V jumper that changes the power on pin 1 and the data signals. If set to 5V, you could damage 3.3V devices—so verify the board is set to 3.3V before connecting hardware.

Multiple devices and compatibility
- You can connect multiple UEXT devices on the same ribbon cable to share power and I2C. However, SPI and UART won’t work properly if chip selects or data lines are tied together for all devices.
- If you need parallel operation, you may need resistors on conflicting lines to limit current and avoid damage.
- Some boards use the UEXT footprint but omit some functions (e.g., the asynchronous serial line). Spare pins may be used for other purposes, like reset lines, so check the exact schematic for each device.

In short, UEXT is a simple, open-layout 10‑pin connector for 3.3V MCU projects, combining power, I2C, SPI, and UART with practical notes on safety and compatibility.