Adafruit has teased two STEMMA QT breakout designs, but neither the eight-channel multiplexer nor the SX1503 16-pin GPIO expander was confirmed for production or ordering in the announcement. The proposals target two common I2C problems: connecting several devices that share one fixed address, and adding digital inputs and outputs when a microcontroller has too few pins.
What Adafruit announced
An eight-port STEMMA QT multiplexer
The first concept is an eight-port STEMMA QT implementation of an I2C multiplexer. It would provide one host connection and eight switched downstream connections, allowing a controller to communicate with multiple devices that otherwise use the same I2C address. Hackster.io’s report renders the chip name as TCA9584 in its prose and image caption; that part number should be treated as the report’s attribution until Adafruit publishes an authoritative product document.
Adafruit’s Phillip Torrone described the motivation: “It’s great for when you want to connect eight devices of the same I2C address — but what if you want to do to it without any breadboarding, using STEMMA QT or Qwiic? Someone emailed us about it, and we gotta admit, they’re kinda right.”
The teased design was also described as having a voltage level-shifter/selector. Torrone said, “It [also] has a level shift[er]/selector from Vin or 3.3V,” which would allow a 3.3V Qwiic board to be used with a 5V-logic Arduino Uno, while optionally passing 5V power through. That is a description of the proposal, not independent electrical testing.
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- Built with hot swap protective and 400kHz Fast Mode I2C, this multiplexer ensures stable communication across 4 slave devices while simplifying prototyping
- Resolve I2C address conflicts and manage up to 4 identical sensory simultaneously with the PCA9546 I2C multiplexer board, featuring for STEMMA QT / Qwiic compatibility for seamless integration into existing sensory networking
- Designed for robotics developers and electronics hobbyists seeking to expand I2C capabilities while maintaining compact sensory networking architectures
- connection multiple display environmental sensory, or robotic actuators in smart home automation systems or industrial setups without address limitations
- This 4 channel switching module operating for 2.3V-5.5V with low 1μA standby current, offering plug and play connection via Qwiic interfaces and standard 0.1inch headers for breadboard
A 16-pin SX1503 GPIO expander
The second proposal uses the SX1503 to add 16 digital I/O lines over I2C. It was presented as a possible response to shortages of the MCP23017 used on an existing Adafruit board.
The key trade-off is address configuration. Torrone said: “[This chip] doesn’t have any address select pins (boo!), but it does have two bank VCCIO pins so you can use it for level shifting (yay!).” He also said Adafruit might examine the Semtech SX1509, describing it as more expensive, requiring 3.3V power and logic, and adding PWM and keypad-scanning features. Those comments describe possible design directions, not a commitment to a particular board.
Rank #2
- Includes 12 pcs cables of various lengths and styles (10cm,20cm, 30cm, 50cm. SH1.0 connector and breadboard Jumper and Female Jumper)
- Each Qwiic Cable's wires have been color coded to red, black, blue and yellow
- Uses this cable to quickly hook up sparkfun and development boards with sensors, LCDs, relays and more.
- The female and male dupont connector also suitable for raspberry pi and Arduino development boards
Why an I2C multiplexer matters
I2C devices normally share SDA and SCL, but each device must respond to a usable address. If two identical sensors have a fixed address, both answer the same transaction and the controller cannot select one cleanly. Adafruit’s guide captures the practical problem: “Uh oh, this chip has a fixed I2C address, and from what I know about I2C, you cannot have two devices with the same address on the same SDA/SCL pins!”
A multiplexer solves this by placing each downstream device or group behind a separate switchable channel. Software selects one channel, communicates with the device on that branch, then selects another. The devices can retain the same address because they are not electrically active on the bus at the same time.
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- Resolve I2C address conflicts and manage up to 4 identical sensory simultaneously with the PCA9546 I2C multiplexer board, featuring for STEMMA QT / Qwiic compatibility for seamless integration into existing sensory networking
- This 4 channel switching module operating for 2.3V-5.5V with low 1μA standby current, offering plug and play connection via Qwiic interfaces and standard 0.1inch headers for breadboard
- Built with hot swap protective and 400kHz Fast Mode I2C, this multiplexer ensures stable communication across 4 slave devices while simplifying prototyping
- Designed for robotics developers and electronics hobbyists seeking to expand I2C capabilities while maintaining compact sensory networking architectures
- connection multiple display environmental sensory, or robotic actuators in smart home automation systems or industrial setups without address limitations
What is available now
Adafruit documents a related PCA9548 STEMMA QT/Qwiic multiplexer. It supports eight downstream channels, a host port, and breadboard pins, but the available PCA9548 board is not established as the exact hardware teased in the announcement.
| Option | Channels or GPIO | Same-address devices | Address selection | Voltage and level shifting | Connectors and accessories | Status |
|---|---|---|---|---|---|---|
| Teased STEMMA QT multiplexer | Eight mux ports proposed | Intended for up to eight same-address devices, one selected branch at a time | Not stated | Teaser describes a Vin/3.3V selector and level shifting | STEMMA QT/Qwiic concept; included accessories not stated | Production and order date not confirmed |
| Adafruit PCA9548 board | Eight mux channels | Routes commands to a selected downstream port | 0x70 by default; jumpers allow 0x70 through 0x77 | Onboard 3.3V, 500mA regulator and level shifting for 5V microcontroller contexts | Eight downstream JST-SH ports, host port and breadboard pins; cables and sensors are not included | Documented product; price and stock vary |
| Teased SX1503 expander | 16 GPIO proposed | Not a mux; adds digital I/O over I2C | No address-select pins, according to the announcement | Two bank VCCIO pins were cited as useful for level shifting | STEMMA QT board details not stated | Production and order date not confirmed |
| Adafruit MCP23017 breakout | 16 digital I/O | Not a mux; expands GPIO | Three address pins; up to eight devices on one I2C bus | 3V or 5V logic and power | STEMMA QT/Qwiic-compatible breakout; cable inclusion depends on the package | Documented alternative, not an SX1503 board |
| Adafruit GPIO Expander Bonnet | 16 additional I/O | Not a mux; expands GPIO | Not stated here | Raspberry Pi-oriented design; detailed electrical limits belong to its documentation | STEMMA QT port added in the October 31, 2023 revision | Documented alternative, not the teased SX1503 design |
How to choose between a mux and a GPIO expander
Choose a multiplexer for duplicate I2C addresses
- Use a mux when identical sensors or peripherals have fixed, colliding addresses.
- Check that the target board supports the required voltage and pull-up arrangement.
- Plan software to select a channel before every transaction and to avoid addressing inactive branches.
- Remember that a mux does not create extra GPIO pins and does not convert an I2C device into an analog input.
Choose a GPIO expander for more digital pins
- Use an expander for switches, LEDs, relays and other digital inputs or outputs.
- The MCP23017 is the documented 16-I/O option with three address pins and support for up to eight devices on one bus.
- The proposed SX1503 design would trade away address-select pins for two bank VCCIO pins that could help with voltage-domain interfacing.
- Confirm interrupt behavior, output-current limits and library support for the exact chip before designing a production board.
Voltage and connector details to verify
The teaser’s Vin/3.3V selector is intended to make mixed-voltage projects easier, but it does not remove the need to check every board’s supply, logic thresholds and pull-up voltage. A 5V Arduino host, a 3.3V STEMMA QT peripheral and a pass-through 5V power rail can have different requirements.
Rank #4
- Designed for robotics developers and electronics hobbyists seeking to expand I2C capabilities while maintaining compact sensory networking architectures
- connection multiple display environmental sensory, or robotic actuators in smart home automation systems or industrial setups without address limitations
- Resolve I2C address conflicts and manage up to 4 identical sensory simultaneously with the PCA9546 I2C multiplexer board, featuring for STEMMA QT / Qwiic compatibility for seamless integration into existing sensory networking
- Built with hot swap protective and 400kHz Fast Mode I2C, this multiplexer ensures stable communication across 4 slave devices while simplifying prototyping
- This 4 channel switching module operating for 2.3V-5.5V with low 1μA standby current, offering plug and play connection via Qwiic interfaces and standard 0.1inch headers for breadboard
The documented PCA9548 uses JST-SH STEMMA QT/Qwiic connectors plus header pins. Its guide says the assembled PCB does not include cables or sensors, so budget for compatible four-pin JST-SH cables and verify pinout before connecting another vendor’s Qwiic or STEMMA QT board.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Availability and uncertainty
The announcement did not provide a production schedule, order date or confirmation that either teased design entered Adafruit’s catalog. The PCA9548 and MCP23017 are related documented products, not proof that the teased multiplexer or SX1503 board shipped. Adafruit’s maintained STEMMA QT inventory also lists a PCA9546 four-channel mux, a PCA9548 eight-channel mux, a SparkFun eight-channel Qwiic/STEMMA QT mux and several GPIO expanders; availability and exact compatibility should be checked on the current manufacturer pages.
Best Value
- 4-Channel I2C Expansion: Enables a single I2C host to control four independent channels, solving for address conflicts for identical sensors on board.
- 2. for 3v-5.5V Wide Voltage: Fully compatible with 2. for 3v to 5.5V for logic levels, making it for ideal for Arduino and for Raspberry Pi microcontrollers.
- for stemma QT & for qwiic Interface: Equipped with standard for stemma QT / for qwiic 4-pin connectors and pin headers for fast, plug-and-play wiring setup.
- Hot Swap Capable: Supports hot-swapping functionality to add or remove slave devices dynamically without resetting system power.
- Compact PCB Size: Measures just 2.6 x 2.1 x 1 cm for easy integration into space-constrained DIY robotics and multi-sensor acquisition systems.
Practical buying checklist
- Decide whether the problem is duplicate I2C addresses or insufficient digital pins.
- For duplicate addresses, count the isolated branches you need and confirm that the device library tolerates channel switching.
- For GPIO expansion, count inputs, outputs and interrupts, then check address pins and voltage domains.
- Confirm whether your chosen board includes JST-SH cables, headers or only the assembled PCB.
- Do not substitute a shipping PCA9548 or MCP23017 for the teased designs when a project specifically depends on SX1503 features or the proposed board layout.
- Recheck production status, documentation and stock immediately before committing to a build.
Bottom line
Adafruit’s two designs address real maker needs, but they remain teasers in the cited announcement. The PCA9548 is the documented eight-channel option for same-address I2C devices today, while the MCP23017 breakout and GPIO Expander Bonnet are documented ways to add 16 digital I/O. Neither should be described as the confirmed SX1503 board or as the exact multiplexer design Adafruit previewed.
Quick Recap
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