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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →I²C uses two shared signals—SDA for data and SCL for clock—but the right pins and safe voltage depend on the exact board. For the standard Raspberry Pi header, use GPIO2 for SDA and GPIO3 for SCL; on an Arduino Uno R3, use A4/SDA and A5/SCL. Before connecting them, check the bus pull-up voltage: Raspberry Pi GPIO is 3.3 V, while Uno R3 IOREF is connected to 5 V. A bus pulled up to 5 V may need a bidirectional level shifter to protect 3.3 V devices.
How I²C works across these boards
SDA carries data and SCL carries the clock. Devices share the two signal lines and a common ground; pull-up resistors bring the lines high. Each active device on the same bus needs a unique address, and every device must tolerate the bus’s electrical levels. Espressif’s I²C documentation explains the signals, pull-ups, and addressing.
I²C does not imply one universal pinout or voltage. Some boards have dedicated pins; others let software select pins. The board’s labels, the MCU’s GPIO voltage limits, and any pull-ups on attached modules all matter.
Which pins to use
| Platform | SDA and SCL | Implementation and cautions |
|---|---|---|
| Raspberry Pi | GPIO2 = SDA; GPIO3 = SCL on the documented main header | The GPIO header uses 3.3 V signals. Confirm the Pi generation and any attached module’s pull-ups. Raspberry Pi GPIO documentation |
| Arduino Uno R3 | A4 = SDA; A5 = SCL | IOREF is connected to 5 V. Check the voltage of every bus pull-up before connecting a 3.3 V board. Uno R3 datasheet |
| NodeMCU ESP8266 | Software-selectable GPIO pins; verify the exact board’s D-label-to-GPIO mapping | Firmware uses software I²C. GPIO16 and board reset wiring have specific caveats. NodeMCU I²C documentation |
| NodeMCU ESP32 / ESP32 | Configurable SDA and SCL pins, subject to chip and board availability | Hardware I²C interfaces are available. Internal pull-ups are weak; check external pull-ups and the module’s requirements. NodeMCU ESP32 I²C documentation and ESP32 Arduino I²C API |
| STM32 | Depends on the exact MCU and board | Consult the specific part datasheet and board schematic; STM32 electrical characteristics are not uniform across the family. ST AN4899 and the STM32F313 datasheet |
NodeMCU labels are not GPIO numbers
On NodeMCU ESP8266 boards, a label such as D1 is a board designation, not necessarily the raw GPIO number expected by code. Consult the mapping for the exact board before wiring or selecting software I²C pins. The firmware documentation also notes pin-specific limitations, including GPIO16 and reset wiring.
#1 Best Overall
- Three Connector Types In One Kit: Includes 40 male-to-male, 40 male-to-female and 40 female-to-female jumper wires for connecting breadboards, female sockets, male headers, sensors, displays and controller modules during temporary prototyping
- 20 cm Length With Separable Ribbons: Each 8 in lead reaches across breadboards and nearby modules without excessive slack; use the 40-wire ribbons as grouped buses or peel off smaller sections and individual wires to match your project layout
- Color-Code Circuits & Troubleshoot Faster: Multicolored insulation makes power, ground, clock, data and control paths easier to identify during LED projects, sensor tests, classroom labs and repeated electronics experiments
- 2.54 mm Connections For Common Headers: Male pins fit compatible 0.1 in female sockets and breadboards, while female ends fit compatible 0.1 in male headers; insert connectors straight and check continuity if a signal becomes intermittent
- Copper-Clad Aluminum With PVC Insulation: The leads are designed for temporary low-voltage signal prototyping rather than mains or high-current wiring; they are not pure-copper wire, automotive jumper cables or a crimp-connector kit
Do you need an I²C level shifter?
Decide based on the voltage that the pull-ups place on SDA and SCL—not only the voltage used to power a board. Raspberry Pi GPIO is 3.3 V, while Uno R3 IOREF is connected to 5 V. If a bus is pulled up to 5 V, a connected 3.3 V GPIO may be exposed to a level outside its permitted range. Check each participant’s datasheet and the breakout boards’ pull-ups. If direct connection is not within every device’s specifications, use a suitable bidirectional I²C level converter or redesign the pull-up arrangement within those limits.
For a mixed-voltage bus, establish which side supplies the pull-ups and what high voltage each device tolerates. A converter is conditional—not automatically required just because the boards use different supply voltages. The bus wiring, pull-up arrangement, and device limits determine the answer.
Rank #2
- ✔【Multiple Length Options】Includes 28pcs cables of various lengths and styles (100mm,200mm, 300mm, 500mm, SH1.0 connector and breadboard Jumper and Female Jumper) that will get your Qwiic boards connected together.
- ✔【Color Scheme】Each Qwiic Cable's wires have been color coded to red, black, blue and yellow. All Qwiic cables have the following color scheme and arrangement: Black = GND; Red = 3.3V; Blue = SDA; Yellow = SCL
- ✔【Easy to Connect】Uses SH1.0-4P connector kits to quickly hook up sparkfun and development boards with sensors, LCDs, relays and more
- ✔【Application】In addition to being suitable for SparkFun development boards and sensor boards, the female and male dupont connector also suitable for Raspberry pi, ESP32 and Arduino development boards
- ✔【Packing List】100mm cable x 6; 200mm cable x 6; 300mm cable x 4; 500mm cable x 4; 150mm Breadboard Jumper (4-pin) cable x 4; 150mm female Jumper (4-pin) cable x 4.
Pull-ups, bus speed, and wiring
I²C needs pull-ups on both SDA and SCL. Some sensor or display breakout boards already include them, so count those before adding more. Multiple pull-ups in parallel reduce the total resistance; if the total becomes too low, a device may be unable to sink the required current. If resistance is too high for the bus capacitance and speed, signal rise times may be too slow.
Select pull-ups using the bus voltage, total capacitance, required rise time, and the devices’ sink-current limits. ST’s AN4899 discusses these factors. There is no single resistor value suitable for every bus.
The Tool Desk
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- Package includes--This breadboard Jumper Wires set contains 840pcs U-Shape male to male jumper wires. Wire length 2 mm/0.08" 5 mm/0.2" 7 mm/0.28" 10 mm/0.39" 12 mm/0.47" 15 mm/0.59" 17 mm/0.67" 20 mm/0.79" 22 mm/0.87" 25 mm/0.98" 50 mm/1.97" 75 mm/2.95" 100 mm/3.94" 125 mm/4.92",60 pieces of each. & 34cm (13.4 inch) Alligator clip test leads & 1 pcs Tweezer
- Easy to use -- Different color-code, insulated, flexible, rigid jumpers in different lengths and can be used repeatedly The jumper wire kit is used in conjunction with any breadboard or any place where a jumper is needed, which is very suitable for breadboard projects.All jumpers are pre-stripped.14 kinds of jumpers with different lengths meet your basic needs, with good ductility, flexibility and easy operation, give a pair of tweezers for easy operation.
- Comes with Storage Box -- This assortment kit is easily to be carried and you won't have to worry about the storage of these small parts. And you'll never be short of the assorted length breadboard jumper wires.
- Easy to distinguish - -products are distinguished by red, orange, yellow, green, blue, purple, coffee, white, and various colors, the same color means the same length
- Wide range of applications-- Breadboard projects,computers, electronic communications, instruments, meters, industrial program control, digital cameras, MP3, card readers, DVDs, LCM/LCD displays, electronic toys，PCB project, pc motherboard etc
Start at a conservative speed supported by every device. Increase it only if the devices, wiring, capacitance, and timing requirements allow it. NodeMCU documents standard and faster modes for its interfaces; STM32 timing capabilities depend on the specific part. Longer or more capacitive wiring can make reliable operation harder, so reducing speed is a useful diagnostic.
Wire the bus and diagnose missing devices
- Identify the exact hardware. Confirm the board variant and MCU, then find SDA and SCL in its official pinout or datasheet. For configurable pins, make sure the software settings match the wires.
- Connect the signals and ground. Wire SDA to SDA, SCL to SCL, and ground to ground. Power each peripheral according to its own specifications.
- Check existing pull-ups. Look for resistors on the controller board and every attached module. Account for their combined resistance before adding more.
- Verify the bus high voltage. Determine which rail pulls SDA and SCL high, then compare that voltage with every participant’s permitted input levels. Add a suitable bidirectional level shifter if needed.
- Begin at a supported, conservative speed. If communication is unreliable, lower the speed before investigating more complex timing issues.
- Check common failure points. Verify power, shared ground, SDA/SCL orientation, pull-ups, voltage compatibility, address conflicts, and the selected software pins. Where supported, an address scan can help determine whether a device responds on the bus.
What to verify for an STM32 board
STM32 is a broad MCU family, not one fixed pinout or electrical specification. Identify the exact MCU on the board, consult its datasheet for I²C pin options and voltage tolerance, and check the board schematic for routing and pull-ups. The cited STM32F313 documentation is an example for that part only; its characteristics should not be applied to other STM32 devices.
Quick Recap
Best Value
- Includes 14 pcs cables of various lengths and styles (10cm/3.9",20cm/7.9", 30cm/11.8", 50cm/19.7". SH1.0 connector and breadboard Jumper and Female Jumper and Alligator clip)
- 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
- The female and male dupont connector also suitable for raspberry pi and Arduino development boards
Rank #4
- Parameters: U shape, 24 AWG, male to male, wire length 2 mm, 5 mm, 7 mm, 10 mm, 12 mm, 15 mm, 17 mm, 20 mm, 22 mm, 25 mm, 50 mm, 75 mm, 100 mm, 125 mm
- Wide applications: designed to work well with any breadboards or anywhere jumper wires are needed, perfect for breadboard projects
- Easy to use: all the colored jumper wire are pre-stripped and preformed right-angled, easy to insert it into breadboard
- Extra bonus: come with a free plastic box, keep your jumper wire kit organized, convenient for store and fetch
- Note: the breadboard wire is a little soft so that convenient for changing its shape to do connect test; if you find that it can't be inserted, please check whether the cable in the pin hole of the breadboard is in good condition or not
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