I²C and SPI are the two workhorse buses for attaching peripherals. I²C uses two shared lines and addresses; SPI uses more wires but runs faster and simpler. Most component modules you buy speak one of them often both options on the same board.
At a glance: 8 minute guide · part 5 of 10 in the complete Arduino guide track · includes a worked example and a quick-reference table.
I2C: two wires, addressed devices
Here is the working theory in one pass. SDA (data) and SCL (clock) connect every device in parallel, each with a 7-bit address. Pull-up resistors (typically 4.7 kΩ) are required most modules carry them. Speeds run 100-400 kHz, fine for sensors and small displays. Address conflicts are the classic headache: two devices claiming 0x3C collide.
| P | r | o | p | e | r | t | y |
|---|---|---|---|---|---|---|---|
| I | 2 | C | |||||
| S | P | I | |||||
| Wires | 2 (SDA, SCL) + shared pull-ups | 4 + one CS per device | |||||
| Typical speed | 100-400 kHz | 1-20 MHz | |||||
| Device selection | 7-bit address | Dedicated CS line | |||||
| Address conflicts | Possible | None | |||||
| Best for | Sensors, RTC, small OLED | SD cards, TFT displays, flash |
SPI: four wires, selected devices
What this means at the bench: MOSI, MISO, SCK and one chip-select (SS) line per device. No addressing overhead pulling a device's CS low simply selects it. SPI typically clocks at megahertz rates, which is why SD cards and TFT screens prefer it. More devices means more CS pins to budget.
Choosing in practice
Prefer I2C for a handful of low-rate sensors where wiring economy matters. Prefer SPI when throughput matters or several identical devices would clash in address space. Level-shift between 5 V and 3.3 V parts in either case these buses are not forgiving of over-voltage.
How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
- Check the module's bus type and voltage before wiring
- Wire the bus pins plus pull-ups for I2C or CS for SPI
- Scan I2C addresses (scanner sketch) to detect conflicts
- Test each device alone before combining on one bus
Worked example
An OLED at address 0x3C and a BME280 also defaulting to 0x76 coexist fine on I2C. Two OLEDs at 0x3C force one to a jumper-selected alternate address or a move to SPI. Run the numbers yourself with the related calculator and the result should agree to within rounding.
Practical note from the bench. On the Procirel bench, the I2C address scanner sketch lives permanently on a test Arduino it diagnoses half of all "dead sensor" reports in seconds.
Field mistakes we see again and again
- Omitting pull-up resistors on bare I2C parts
- Mixing 5 V and 3.3 V devices without level shifting
- Running long I2C wires capacitance kills the clock
Key takeaways
- I2C: two wires, addressed devices the foundation of this guide; revisit it if any measurement here surprises you.
- SPI: four wires, selected devices the foundation of this guide; revisit it if any measurement here surprises you.
- Choosing in practice the foundation of this guide; revisit it if any measurement here surprises you.
Who this guide is for
Beginners get a single focused topic instead of a whole textbook chapter. It assumes the track’s earlier pages in the complete Arduino guide path. Intermediate readers use it as a reference the table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.
What you need before starting
Nothing exotic: the parts or tools named in the guide, a multimeter. A notebook for the numbers. Check the module's bus type and voltage before wiring before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.
Quick reference card
| Aspect | Where to find it in this guide |
|---|---|
| Core theory | I2C: two wires, addressed devices |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Field mistakes we see again and again |
How this fits the complete Arduino guide track
This guide is one stop in the structured learning path. Start from the complete Arduino guide complete guide for the full map, or continue with serial debugging techniques and driving LCD and OLED displays.
Frequently asked questions
Can I mix I2C and SPI devices on one Arduino? Yes the buses are independent; each keeps its own pins and libraries.
Why does my I2C scanner find nothing? Usually missing pull-ups, swapped SDA/SCL, or wrong voltage check wiring before suspecting the part.
Where do I go next? Back to the complete Arduino guide complete guide it indexes every guide in this track and updates as new ones are published.
Continue this track
- Building a foundation? The arduino & microcontrollers complete guide maps every step in order.
- Next: Arduino GPIO: Pin Capabilities, Limits and Safe Usage
- Next: How I Built an AI Robot with Arduino UNO Q
- Next: Powering Arduino Projects: USB, Battery and Supply Design
- Work the numbers: LED resistor finder · battery runtime estimator · 555 timer frequency tool
Measurement discipline
Keep a lab notebook entry for every build in this track. The measured values, the deviations from the guide and the reason for each. Six months from now, those notes are worth more than any tutorial. They describe your bench and your components rather than a general case.
When a result here disagrees with your expectation, write down both numbers before changing anything. The gap between predicted and measured is where the real engineering lives. It is usually a tolerance, a parasitic or an assumption that was never checked.
Formulas and checks from this guide
Verification checklist for this track. Check pin assignments against the sketch header before wiring, confirm supply polarity twice. Serial-print one variable at a time when debugging. Keep each sketch’s pin map in a comment block so the next build inherits working documentation.
Bookmark this page against your next build in the track. The checklist above is the same one used across 15 guides in this series.
Hard-won notes
Uninitialised variables and pins left floating. Set every pinMode and initial state in setup.
Anything with motors, servos or many LEDs needs external supply with common ground. USB is for logic only.
Extended Application Notes
This section expands the practical application of i2c vs spi: choosing and using both on arduino beyond the worked example, into the situations builders actually meet. Component substitution: when the exact specified part is unavailable, the substitution logic follows the governing parameter of this design, not the nominal value, and the verification step after any substitution is to re-measure the one quantity this guide identified as critical. Batch variation: components vary, and the design margins recommended in the sections above absorb that variation; if a second build behaves differently, the difference itself is diagnostic and points to the tolerance that dominated. Environmental limits: temperature, supply variation and ageing each push a real circuit away from its bench behaviour, and the recommended practice is to test the extremes deliberately rather than discover them in the field. These notes exist because the bench taught them, repeatedly, and each one was once a real troubleshooting session that ended in understanding.
Failure Analysis in Depth
The mistakes section above lists the traps; this section explains why each trap exists and how to recognize it early. Omitting pull-up resistors on bare I2C parts Mixing 5 V and 3.3 V devices without level shifting Running long I2C wires capacitance kills the clock. Each of these failures has a signature that appears in measurement before it appears in smoke: a reading that drifts, a waveform that differs from the prediction, a temperature that climbs faster than the calculation. The discipline this guide teaches is to measure at the first sign, not at the last, and the sections above give the specific instrument and setting for each check. Failure analysis is not pessimism; it is the fastest curriculum in electronics, because a fault understood once is a fault prevented forever.
Pre-Build Checklist
Before powering any build of this design, run the list: every component value verified against the specification above, the critical measurement points identified and accessible, the instrument modes and ranges chosen in advance, the expected values written down beside the bench, and the power source current-limited for first application. The checklist takes two minutes and replaces the most expensive class of beginner error, which is not ignorance but confidence outrunning verification. Builders who adopt the checklist across the guides in this track report first-apply success rates that feel like cheating, but it is not cheating, it is engineering.
What Comes Next
Having worked through this guide, the natural next steps are the adjacent guides in the track index above, each of which assumes exactly the vocabulary this page built. The calculators linked in the tools section verify every number in seconds, and the complete guide at the head of this track maps the entire curriculum. Read once, build once, measure always: that is the method this site teaches and the method every section above followed before publication.
Theory in Practice, Extended
The theory section of i2c vs spi: choosing and using both on arduino deserves one more pass with the bench in mind, because knowing a relationship and applying it under constraint are different skills. In application, the relationship is never isolated: it interacts with tolerances, with temperature, with the behaviour of adjacent stages, and with the measurement itself. The extended practice is to take the governing formula from the sections above and stress it, deliberately. Push the input to the edge of its specified range and watch the output follow the prediction, then push past it and watch the prediction break, because the edge of the specification is exactly where the formula stops being the whole story. That boundary, found on the bench rather than in the datasheet, is the real knowledge this guide offers beyond the mathematics.
Component Sourcing and Substitution Notes
Real builds meet real supply chains, and this section addresses the practical reality. The specified components in this guide were chosen for the reasons stated in the design sections, but equivalent parts from reputable manufacturers almost always serve, provided the governing parameters match, not merely the nominal ones. The substitution checklist: match the parameter this guide identified as critical, verify the package and pinout against the physical part before layout, check the datasheet revision for silent changes, and re-run the verification measurement after installation. Avoid unbranded surplus and marketplace components for anything this guide treats as safety-relevant; the failure mode of a counterfeit is not degradation, it is unpredictability, and unpredictability defeats every other design decision in the chain.
Instrumentation for This Design
Every measurement recommended in this guide maps to a specific instrument configuration, and this section consolidates them. Voltage checks: DC range selected before probing, leads verified against a known source, meter burden considered when the node is high impedance. Current checks: circuit broken at the defined point, meter inserted with the correct range and fuse status confirmed first. Waveform checks: probe compensated against the reference before any amplitude claim, ground lead kept short, bandwidth sufficient for the edge rather than the repetition rate. The instrumentation discipline matters more than the instrument class, and a modest instrument used correctly outperforms an expensive one used casually, a claim this site demonstrates throughout its measurement guides.
Documentation Template for This Build
Close the loop the way professional builds do: record the design values from this guide, the as-built values including every substitution, the measured results beside the predicted ones, and the deviation notes that explain every gap. The template is short, a single page, and it converts a successful build into a reference that survives component changes, firmware updates and the passage of months. Every guide on this site was built and documented exactly this way before publication, and the discipline is offered here as part of the curriculum rather than an afterthought. A build that is documented is twice built, once in copper and once in confidence.
Last updated 23 August 2026