LIBRARYDebugging a bus.
When a bus does nothing, the fault is usually simple: a missing pull-up, the wrong mode, an address clash, or a swapped wire. A logic analyzer shows the exact bytes.
When a bus does nothing, the cause is almost always simple and physical: a missing pull-up, the wrong mode, two devices at one address, or a swapped wire. The fastest way to find it is a logic analyzer, which shows you the actual bits on the wire and turns a dead bus into a specific fault you can point at.
Check power and ground first
Before you suspect the protocol, confirm the boring things: both parts are powered, they share a common ground, and the wires go where you think they do. A missing ground between two boards is the single most common reason a bus is silent.
The usual suspects
Match the symptom to the bus. On I2C, no pull-ups means the lines never reach a valid high, so nothing moves, and two devices at the same address answer at once and collide. On SPI, the wrong mode (CPOL/CPHA) makes every byte read wrong even though the wiring is right. On UART, a baud mismatch prints garbage, and swapped TX and RX means nothing arrives at all.
The logic analyzer
A logic analyzer clips onto the bus lines, records their exact high-low timing, then decodes it into bytes, addresses, and acknowledgements. It shows whether the clock is even running, whether a device answered, and where the sequence broke. An inexpensive analyzer with a sigrok/PulseView setup is enough for most on-board buses.
Work from the most common cause outward, so your first probe lands on the likely fault before the rare one.
- Confirm power and a shared ground. Both parts powered, both grounds tied together, before anything else.
- Check the wiring against the schematic.
TXtoRX,SDAtoSDA, no swapped pair. - For I2C, confirm the pull-ups are present and no two devices share an address.
- For SPI, confirm the mode (CPOL/CPHA) matches the peripheral's datasheet.
- For UART, confirm both ends are set to the same baud rate.
- Clip on a logic analyzer and read the actual bits: is the clock running, and did the device answer?
Bringing up a One Thousand Drones board, an analyzer on the I2C lines shows the controller sending an address and then either an acknowledgement from the sensor or silence, which tells you in a single capture whether the part is alive, mis-addressed, or missing its pull-ups.
Checkpoint
Quick check
One Thousand Drones engineering team · verified 2026-07
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