UEXT

Key takeaways

UEXT belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect UEXT to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of UEXT from memory before moving on to harder problems.

Reference excerpt

Universal EXTension (UEXT) is a connector layout which includes power and three serial buses: Asynchronous, I2C, and SPI separately over 10 pins in a 2×5 layout. The connector layout was specified by Olimex Ltd and declared an open-project that is royalty-free in 2011, and was used in all their boards after 2004.

Physical characteristics The UEXT connector consists of 10 pins, in a two row by five male pin configuration, with a plastic keyed-shrouded or box header (specifically BH10S). All pins have a spacing of 0.1 inch (2.54 mm). The socket is mated with a 2×5 (10-pin) IDC female connector, and typically connected to a ribbon cable. The UEXT cable assembly is referred to as a 10-pin IDC Connector Ribbon Cable. These cables are readily available and cheap.

Electrical characteristics

The UEXT connector presents power and three serial buses: Asynchronous, I2C, SPI. All pins conform to 3.3 volt digital logic. The asynchronous serial bus requires additional level-shifting circuits and connectors to support RS-232, RS-422, RS-485, DMX512, or MIDI.

Notes:

Direction is relative to host board. I = Input, O = Output. S = Power Supply, PP = Push-Pull logic, OD = Open-Drain logic, all pins conform to 3.3 volt digital logic

Alternate pin uses The functionality of most microcontroller pins are multipurpose, thus allowing the engineer to redefine the purpose of the pin. It is fairly common that a pin will have a choice to be either a general purpose I/O or a peripheral. If a microcontroller pin is connected to the UEXT connector and redefined to be something other than Asynchronous Serial Bus / I2C Bus / SPI Bus, then some thought should be given to the design before making the changes. To minimize the chance of damaging various UEXT boards or the microcontroller, redefined pins should continue to adhere to the direction of the data in this table or alternately redefined as an input. For a safe design, it is recommended that you don't redefine pins 4 or 7 to be outputs, and use pin 6 as an output with caution.

User concerns

If a person is concerned about damaging the data lines of the microcontroller, additional over-voltage protection diodes and/or separate drivers should be added between the microcontroller and UEXT connector. Additionally, a resettable fuse might be added between the host power and pin 1 to protect against over-current conditions. Open source UART dongles are available that are explicitly designed to be pin compatible with UEXT, and may be used to interface a UEXT device directly with a computer over USB.

5 V warning Some boards might provide 5 V output on UEXT pins if they are configured for operation at 5V. For example, OLIMEXINO-328 and OLIMEXINO-32U4 have a 3.3 V/5 V PTH jumper that changes the power mode of the board. If the 3.3 V/5 V jumper is set to 5 V, this jumper also changes the voltage available at pin #1 of UEXT as well as the voltage levels of all data signals available there (UART, SPI, I2C). As the UEXT standard only defines 3.3 V, many UEXT modules may not be 5 V tolerant, and thus get damaged if the board has such jumper set to 5V. Users are advised to verify their boards are configured for 3.3 V before connecting UEXT hardware to prevent damaging said hardware.

Parallel cables It is possible to connect multiple UEXT devices in parallel by clamping additional IDC connectors onto the same ribbon cable. This will result in a correct distribution of power (3.3 V and ground) and allow data communication over I²C. However, a multidrop SPI bus will fail since all the chip selects are tied together, thus all peripherals will be addressed simultaneously resulting in their simultaneous response data on the MISO line being scrambled. UART will also fail due to multiple devices transmitting and receiving on the same lines. Series resistors are necessary on conflicting SPI and UART pins to limit short circuit currents due to conflicts, if compatibility with other UEXT devices is desired.

Similar connectors Some devices use a layout based on the UEXT connector but have chosen not to implement all of the functionality. In particular the asynchronous serial connection may have been omitted. In these cases the spare pins are usually assigned other purposes, such as connecting to a device reset line, so care should be taken to check the exact connector layout on the schematic of a particular device.

Implementations UEXT I/O Boards Olimex UEXT Boards Host Boards Olimex Olimexino-328, 16 MHz ATMEGA328 (Atmel AVR) Olimex Olimexino-STM32, 72 MHz STM32F103RBT6 (ARM Cortex-M3) Olimex LPC-P1114, 50 MHz NXP LPC1114 (ARM Cortex-M0) Olimex DuinoMite, 80 MHz MicroChip PIC32MX795 (PIC32) Olimex STM32-P207, 120 MHz ST STM32F207 (ARM Cortex-M3) Olimex Pinguino 3rd-party LibreSolar

See also IDC connector and Ribbon Cable. Asynchronous serial bus, using a UART, such as RS-232 and RS-422. I2C serial bus. SPI serial bus. Pmod Interface, a similar universal connector format for UART, I2C, SPI, but which carries them over shared wires so it only uses 6 pins

References

External links UEXT connector webpage UEXT specification OLIMEX Ltd

Illustrations

Worked examples

Example 1 — a first encounter with UEXT

Start with the simplest possible case. Write down what UEXT claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to UEXT before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about UEXT ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of UEXT

In research: UEXT appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses UEXT in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: UEXT is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrical signal connectors, so understanding it makes those chapters shorter.
In everyday life: Look for UEXT outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study UEXT in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what UEXT means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain UEXT out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is UEXT in simple terms?

Universal EXTension (UEXT) is a connector layout which includes power and three serial buses: Asynchronous, I2C, and SPI separately over 10 pins in a 2×5 layout. The connector layout was specified by Olimex Ltd and declared an open-project that is royalty-free in 2011, and was used in all their boa…

Why does UEXT matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study UEXT?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on UEXT.